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Kumpulan skematik elektronika 1

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5-BAND GRAPHIC EQUALISER                                                                           R. SUNDAR
                                                                                                            A   KUMAR

  SOMEN GHOSH                             bandwidth product greater than 3          plifier. Proper quality factor (Q) needs
                                          MHz. The NE5532 or LM833 used in          to be selected to avoid overlap in ad-

       his equaliser uses low-cost        this circuit meets these requirements.    jacent bands as this introduces
       op-amps. Good-quality op-              Equaliser circuits typically divide   colouration into the audio signal.
       amps powered by a single volt-     the audio spectrum into separate fre-         We have used the multiple-feed-
age supply are readily available in the   quency bands and have independent         back bandpass filter topology shown
market. The op-amp should have a          gain control for each band. The out-      in left-most corner at the bottom of the
noise density of less than 24nV/√Hz,      put of each band is mixed at IC4(A)       figure. This is a circuit for single-chan-
slew rate of more than 5V/µs and gain-    and then fed to an audio power am-        nel bandpass filter. If the capacitors are

WWW.EFYMAG.COM                                                                      ELECTRONICS FOR YOU • MAY 2007 • 87
                                                                                              amplifier with a gain of ‘2.’ The input
                   Component Values for 5-band Equaliser                                      signal is divided by ‘2’ by the resistive
Centre      C                           Ra        Rb        Rc         Gain (A) Quality (Q)   network comprising R3 and R4. Hence
frequency   (μF)                        (kilo-    (kilo-    (kilo-
                                                                                              the net gain of this amplifier is unity.
fo (Hz)                                 ohms)     ohms)     ohms)
                                                                                              Two 100k resistors (R1 and R2) are
60          C4=C5=0.1                   R9=11     R11=27    R10=91     4.1      1.7           used as a voltage divider and the junc-
250         C7=C8=0.1                   R14=2.7   R15=6.3   R13=22     4.1      1.7           tion voltage is fed to its
1000        C10=C11=0.047               R18=1.5   R19=3.3   R17=11     3.7      1.6           positive input through R6. This
4000        C13=C14=0.0022              R22=7.5   R23=18    R21=63     4.2      1.7           divider has enough power to feed all
16000       C16=C17=0.0022              R26=2     R27=4.3   R25=15     4.2      1.7           other op-amps directly. Resistor Ro
of the same value, the calculations are           the octave division, application and        has the dual function of noise reduc-
fairly simple. For calculating the com-           some degree of manufacturers’ prefer-       tion and resistive isolation of capaci-
ponent values, use the following for-             ence, but nearly all share the basic oc-    tive load. It may be varied between 50
mulae:                                            tave boundaries that are based on a         and 150 ohms depending on the noise
 Centre frequency   (fo)   :   1/2πC√(Ra||Rb)Rc   centre frequency of 1000 Hz.                in the circuit.
 Bandwidth          (B)    :   1/πCRc                  A balance between the number of            The potmeters (VR1 through VR5)
 Quality factor     (Q)    :   fo/B = πfoCRc      filters and bandwidth need to be ob-        are in the signal path and hence should
 Gain               (A)    :   –Rc/2Ra            served. It is possible to use a wider       be of the best quality possible. Wrap
    These can be combined to give the             bandwidth and fewer filters, or nar-        the body of the pots with bare copper
following formulae:                               rower bandwidth and more filters.           wire and solder the other end of the
 Ra = Q/2πfoAC                                    Anything narrower than 1/3 octave is        wire to ground. Since the filters are
 Rb = Q/2πfoC (2Q2–A)                             rare, since the complexity of the filters   very sensitive, all resistances should
 Rc = Q/πfoC                                      increases for higher values of ‘Q.’ This    be metal-film type and the capacitors
    Begin the calculations by choosing            can get rather expensive and in reality     should be polyester type.
a large value of capacitance (~0.1F) and          is of limited use for most applications         Each stage of the op-amp needs to
smaller value of resistances. Increasing          in audio systems.                           be capacitively coupled to the next
the capacitance decreases resistances                  National Semiconductor lists the       stage so that the DC does not get
(Ra, Rb and Rc). Care must be taken to            following mid-frequencies for a 10-         propagated and amplified. For a good
avoid overloading on the input buffer             band graphic equaliser: 32, 64, 125, 250,   low-frequency response, this coupling
op-amp. Note that stray capacitances              500, 1k, 2k, 4k, 8k and 16k. It also rec-   capacitor should be greater than 1 µF.
on the board reduces the value of ‘C.’            ommends a ‘Q’ of 1.7 for equalisers.        A 10µF, 16V capacitor is used in each
The bandwidth and gain do not de-                      The table lists the component val-     stage of the circuit here.
pend on Rb. Hence, Rb can be used to              ues for different centre frequencies of         The circuit is powered by a 12V
modify the mid-frequency without af-              the equaliser. We used ‘Q’ of 1.7 and       DC regulated supply. A well-regulated
fecting the bandwidth and gain.                   gain (A) of 4.                              supply using 7812 is recommended.
    For equalisers, there are standard                 The circuit for the 5-band equaliser   Ground the Vcc pin of each op-amp
mid-frequencies that are normally                 uses IC1 (A) LM833 as the buffer stage      with a 0.1µF ceramic disk capacitor to
used. The exact frequencies depend on             for the equaliser. It is a non-inverting    bypass the noise.

88 • MAY 2007 • ELECTRONICS FOR YOU                                                                              WWW.EFYMAG.COM

ANTI-COLLISION REAR LIGHT                                                                                S.C. DW

  ASHOK K. DOCTOR                           NAND schmitt trigger CD4093, 8-stage          of LEDs flash one by one.
                                            shift-and-store bus register CD4094               All the LEDs will then glow for

          uring poor visibility, i.e.,      and some descrete components.                 some time and switch off for some
          when there is fog, or at dawn         An oscillator is built around gate A,     time, and the cycle will repeat. Input
          or dusk, or when your ve-         whose frequency can be varied through         pins 12 and 13 of the unused gate D
hicle gets stalled on a lonely stretch of   preset VR1 when required. The output          must be tied to ground and pin 11 left
a highway, this flashing light will pro-    of the oscillator is fed to IC1 and IC3.      open. Preset VR1 should be of cermet
vide safety and attract the attention of    When the circuit is switched on, the          type and used to change the flashing
people to help you out. It uses high-       oscillator starts oscillating, the counter    rate of each group of LEDs.
brightness yellow LEDs.                     starts counting through IC1 and the               The circuit works off regulated
    The circuit uses a dual binary          data is shifted on positive-going clock       12V. Assemble it on a general-purpose
counter CD4520, quadruple 2-input           through IC3. As a result, the four groups     PCB and house suitably.

WWW.EFYMAG.COM                                                                           ELECTRONICS FOR YOU • JUNE 2007 • 83

antiSleep alarm for StudentS
   SureSh Kumar K.B.                                       BC547, relay RL1 and buzzer.
                                                                The Schmitt-trigger NAND gate

         his circuit saves both time and                    (IC1) is configured as an astable multi-              s.c. dwiv
         electricity for students. It helps                 vibrator to generate clock for the timer
         to prevent them from dozing                        (IC2). The time period can be calcu-
off while studying, by sounding a beep                      lated as T=1.38×R×C. If R=R1+VR1=15
at a fixed time interval, say, 30 minutes.                  kilo-ohms and C=C2=10 µF, you’ll get       D3, further counting stops and relay
If the student is awake during the beep,                    ‘T’ as 0.21 second. Timer IC CD4020        RL1 energises to deactivate all the ap-
he can reset the circuit to beep in the                     (IC2) is a 14-stage ripple counter.        pliances. This state changes only when
next 30 minutes. If the timer is not reset                      Around half an hour after the reset    IC1 is reset by pressing switch S1.
during this time, it means the student                      of IC1, transistors T1, T2 and T3 drive        Assemble the circuit on a general-
is in deep sleep or not in the room, and                    the buzzer to sound an intermediate        purpose PCB and enclose it in a suit-
the circuit switches off the light and fan                  beep. If IC2 is not reset through S1       able cabinet. Mount switch S1 and the
in the room, thus preventing the wast-                      at that time, around one minute later      buzzer on the front panel and the relay
age of electricity.                                         the output of gate N4 goes high and        at the back side of the box. Place the
     The circuit is built around Schmitt-                   transistor T4 conducts. As the output      12V battery in the cabinet for powering
trigger NAND gate IC CD4093 (IC1),                          of gate N4 is connected to the clock       the circuit. In place of the battery, you
timer IC CD4020 (IC2), transistors                          input (pin 10) of IC2 through diode        can also use a 12V DC adaptor. 

9 8 • M a r c h 2 0 1 0 • e l e c t ro n i c s f o r yo u                                                                 w w w. e f y M ag . co M
                                                                                                         The output from IC1 is fed to
                                                                                                     display driver LM3915 (IC2) through

aquariuM Probe                                                                  s.c. dwiv
                                                                                           edi       preset VR3 (50-kilo-ohm). With
                                                                                                     careful adjustments, the wiper of
                                                                                                     VR3 can provide 0-400 millivolts
                                                                                                     to the input of IC2. The highly sensi-
   D. Mohan KuMar                                       temperature, the diode generates 2mV        tive input of IC2 accepts as low as
                                                         output voltage. That is, at 5°C, it is 10   50 mV if the reference voltage at its

         number of environmental                         mV, which rises to 70 mV when the           pin 7 is adjusted using a variable
         factors including light and                     temperature is 35°C. This property          resistor. To increase the sensitivity
         temperature affect fish culture.                is exploited in the circuit to sense the    of IC2, preset VR4 is connected at
The temperature of water has profound                    temperature variation in aquarium           one end to ‘reference voltage end’
effect because fish cannot breed above                   water. Fig. 1 shows the circuit diagram     pin 7 and its wiper is connected to
or below the critical temperature lim-                   of the aquarium probe.                      ‘high end’ pin 6 of the internal resis-
its. Temperature between 24°C and                            Since the output from the diode         tor chain.
33°C is found to be the best to induce                   sensor is too low, a high-gain invert-          When approximately 70 mV is
spawning in fishes. This particular                      ing DC amplifier is used to amplify         provided to the input of IC2 by adjust-
temperature range is also necessary                      the voltage. CA3140 (IC1) is the CMOS       ing preset VR3, LED1 (green) lights
for the healthy growth of nursery fish                   version op-amp that can operate down        up to indicate that the temperature is
fries (young fishes). Rise of water tem-                 to zero-volt output. The highest output     approximately 35°C, which is the cross-
perature due to sunlight may adversely                   available from IC1 is 2.25V less than       ing point. When the input receives 100
affect the fish rearing process.                         the input voltage at pin 7. With resistor   mV, LED2 (red) lights up to indicate
                                                                                                                       approximately 50°C.
                                                                                                                       Finally, the buzzer
                                                                                                                       starts beeping if the
                                                                                                                       input receives 130
                                                                                                                       mV corresponding
                                                                                                                       to a temperature of
                                                                                                                           In short, LEDs
                                                                                                                       and the buzzer re-
                                                                                                                       main standby when
                                                                                                                       the temperature of the
                                                                                                                       water is below 35°C
                                                                                                                       (normal). With each
                                                                                                                       step increase of 30
                                                                                                                       mV in the input (cor-
                                                                                                                       responding to 15°C
                                                                                                                       rise in temperature),
                                                                                                                       LEDs and the buzzer
                                                                                                                       become active.
Fig. 1: Circuit for aquarium probe                                                                                         Pin 16 of IC2 is
                                                                                                                       used to drive the pi-
    The circuit of aquatic probe de-                                                   R4 and        ezobuzzer through transistor T1. When
scribed here can monitor the tem-                                                      VR2, the      pin 16 of IC2 becomes low, T1 conducts
perature of water and indicate the                                                     variation     to beep the piezobuzzer. Resistor R7
rise in temperature through audio-                                                     in diode      keeps the base of transistor T1 high to
visual indicators. A readily available                   Fig. 2: Diode sensor assembly voltage       avoid false alarm. IC4 provides regu-
signal diode 1N34 is used in the                                                       can be        lated 9V DC to the circuit.
circuit as the temperature sensing                       amplified to the required level. Resis-         Assemble the circuit on a common
probe. The resistance of the diode                       tor R1 restricts current flow through       PCB and enclose in a suitable case.
depends on the temperature in its                        diode D1 and preset VR1 (1-kilo-ohm)        Glass signal diode D1 is immersed
vicinity.                                                sets the input voltage at pin 3. IC3        in water to sense the temperature
    Typically, the diode can gener-                      (7805) provides regulated 5 volts to the    of water. Its leads should be coated
ate around 600 mV when a potential                       inputs of IC1, so that the input voltage    with enamel paint to avoid shorting
difference is applied to its terminals.                  is stable for accurate measurement of       in water. Alternatively, enclose the
For each degree centigrade rise in                       temperature.                                diode in a small glass tube or test

9 2 • j u ly 2 0 0 8 • e l e c t ro n i c s f o r yo u                                                                  w w w. e f y m ag . co m
tube having sufficient internal space   mV, so that the input of IC2 (pin 5)    hot water. Now red LED2 will glow.
to fit the diode as shown in Fig. 2.    receives 70 mV corresponding to the     At this position, the voltage at pin 6
Make the sensor assembly waterproof     diode output voltage at 35°C. At this   of IC1 will be around 100 mV. When
using wax.                              stage, green LED1 should turn on.       the temperature of water increases
    Take care while calibrating and     If it doesn’t, adjust VR4 until LED1    further to 65°C, the buzzer starts
setting the circuit. With 5V DC sup-    just lights up. Immerse the diode       beeping. After calibration, immerse
ply to diode D1 and an ambient tem-     in temperature-adjusted hot water       the diode assembly in the aquarium
perature of about 35°C, D1 generates    (35°C) and adjust VR3 and VR4 until     tank just below the water surface
around 70 mV. Adjust VR3 until the      green LED1 lights up. Increase the      and fix it permanently to avoid float-
voltage in its wiper increases to 70    water temperature to 50°C by adding     ing. 

w w w. e f y m ag . co m                                                        e l e c t ro n i c s f o r yo u • j u ly 2 0 0 8 • 9 3

automated alarm CirCuitS                                                                                                            s.c. dwiv

Pallabi Sarkar and                                          Normally, when the door is closed,
anirban SenguPta                                          reed switch S1 is closed, transistor T1
                                                          conducts and the monostable multivi-                        When the door is opened, reed

         wo alarm circuits are presented                  brator (IC1) remains in standby mode                     switch S1 gets disconnected, T1 stops
         here. One produces bird-chirp-                   with ‘low’ output at pin 10.                             conducting and low-to-high pulse at pin
         ing sound and the other British
police siren tone.
     Fig. 1 shows the circuit of the bird-
chirping-sound alarm unit along with
the circuit of the control unit. Fig. 2
shows the circuit of only the British
police siren tone generator, which has
to be integrated with the control circuit
portion of Fig. 1 at points A and B to
complete the circuit diagram of auto-
mated alarm.
     The control unit is built around ICs
CD4047 and CD4027 (as shown on the
left side of the dotted line in Fig. 1). As
mentioned earlier, it is common to both
the alarm circuits. IC CD4047 (IC1)
is wired in positive-edge-triggering
monostable multivibrator mode to set
and reset IC CD4027 (IC2). The output
pulse width of IC1 depends on the
values of capacitor C2 and resistor R3
connected to its pins 1, 2 and 3.                         Fig. 2: Alarm circuit that generates police siren tone

Fig. 1: Alarm circuit that generates bird-chirping sound

9 6 • F e b r ua ry 2 0 0 9 • e l e c t ro n i c s f o r yo u                                                                        w w w. e F y m ag . co m
8 of IC1 triggers the monostable and a    T2 (BC548), which enables the alarm            ing sound.
short-duration positive pulse of about    circuit.                                           For the chirping-sound alarm gen-
10 seconds is available as Q output at        The output at point A is used to           erator, assemble the circuit shown in
pin 10. At the same time, complementary   enable the alarm tone generator circuit        Fig. 1 on a separate general-purpose
output Q goes low at pin 11. The output   (on the right side of the dotted line)         PCB and enclose in a small box. And if
from IC1 is used to set and reset IC2.    consisting of two 555 timer ICs marked         you want an alarm circuit with British
    IC2 is a low-power, dual J-K mas-     as IC3 and IC4. The R-C network de-            police siren tone, assemble the circuit
ter/slave flip-flop having independ-      termines the frequency of the sound            shown in Fig. 2 on another general-
ent J, K, set, reset and clock inputs.    produced. The triangular waveform              purpose PCB and connect it to points
The flip-flops change states on the       of the astable multivibrator is taken          A and B of the control unit shown in
positive-going transition of the clock    out from the junction of pins 2 and            Fig. 1 after removing the circuit on the
pulses. IC2 is wired such that its Q      6 of IC3. This waveform is fed as the          right side of the dotted line. Use a 9V,
output turns ‘high’ when reset pin 4      control voltage at pin 5 of IC4 through        500mA standard adaptor to power the
receives a high pulse. When set pin 7     resistor R18. The output received from         circuit.
receives a high pulse, Q output goes      pin 3 of IC4 is fed to the base of transis-        This circuit may be used as a secu-
low and Q output goes high. This          tor T3 to drive an 8-ohm loudspeaker           rity alarm in banks, households and
lights up LED2 and drives transistor      (LS1), which generates the bird-chirp-         motorcars. 

w w w. e F y m ag . co m                                                          e l e c t ro n i c s f o r yo u • F e b r ua ry 2 0 0 9 • 9 7

autoMatiC light Controller                                                                                                     s.c. dwiv

using 7806
   M.K. Chandra Mouleeswaran                               of the transistor. In this way, the volt-       Normally, the resistance of LDR1
                                                            age regulator is able to operate a light    is low during daytime and high dur-

         oltage regulator ICs (78xx se-                     bulb automatically as per the ambient       ing nighttime. During daytime, when
         ries) provide a steady output                      light.                                      light falls on LDR1, pnp transistor T1
         voltage, as against a widely                            To derive the power supply for         conducts. The common terminal of IC1
fluctuating input supply, when the                          the circuit, the 50Hz, 230V AC mains        connects to the ground and IC1 outputs
common terminal is grounded. Any                            is stepped down by transformer X1           6V. As a result, transistor T2 does not
voltage about zero volt (ground) con-                       to deliver a secondary output of 12V,       conduct and the relay remains de-en-
nected in the common terminal is add-                       250 mA. The secondary output of the         ergised. The light bulb remains ‘off’ as
ed to the output voltage. That means                        transformer is applied to a bridge rec-     the mains connection is not completed
the increase in the common terminal                         tifier comprising diodes D1 through         through the relay contacts.
voltage is reflected at the output. On                      D4, filtered by capacitor C1 and fed            During nighttime, when no light
the other hand, if the common terminal                      to the input terminal of the regulator      falls on LDR1, it offers a high resist-
is disconnected from the ground, the                        (IC1).                                      ance at the base junction of transistor
full input voltage is available at the                           The common terminal (pin 2) of IC1     T1. So the bias is greatly reduced and
output.                                                     is connected to the ground line of the      T1 doesn’t conduct. Effectively, this
    This characteristic is utilised in the                  circuit through transistor BC557 (T1).      removes the common terminal of IC1
present circuit. When the common                            The transistor is biased by R2, R3, VR1     from ground and it directs the full
terminal is connected to the ground,                        and LDR1. The grounding of IC1 is           input DC to the output. Transistor T2
the regulator output is equivalent to                       controlled by transistor T1, while light    conducts and the relay energises to
the rated voltage, and as soon as the                       is sensed by LDR1. Using preset VR1,        light up the bulb as mains connection
terminal is disconnected from the                           you can adjust the light-sensing level      completes through the relay contacts.
ground, the output increases up to the                      of transistor T1.                               As LDR1 is in parallel to VR1+R3
input voltage.                                                   The output of IC1 is fed to the base   combination, it effectively applies
    The common terminal is control-                         of transistor T2 (through resistor R4       only half of the total resistance of
led by a transistor, which works as a                       and zener diode ZD1) and relay RL1.         the network formed by R3, VR1 and
switch on the terminal. For automatic                       LED1 connected across the positive          LDR1 to the junction at T1 in total
control of light, a light-dependent re-                     and ground supply lines acts as a           darkness. In bright light, it greatly
sistor (LDR1) is connected to the base                      power-‘on’ indicator.                       reduces the total effective resistance
                                                                                                                           at the junction.
                                                                                                                               The circuit is
                                                                                                                           simple and can be
                                                                                                                           assembled on a small
                                                                                                                           PCB. Use a heat-sink
                                                                                                                           for IC1. Make sure
                                                                                                                           that LDR1 and the
                                                                                                                           light bulb are well
                                                                                                                               The circuit can be
                                                                                                                           used for streetlights,
                                                                                                                           tubelights or any
                                                                                                                           other home electri-
                                                                                                                           cal lighting system
                                                                                                                           that needs to be au-
                                                                                                                           tomated. 

9 2 • A p r i l 2 0 1 0 • e l e c t ro n i c s f o r yo u                                                                  w w w. e f y m Ag . co m

AUTOMATIC PHASE CHANGER                                                                                      S.C. DW

  MUHAMMAD AJMAL P.                              The mains power supply phase R
                                             is stepped down by transformer X1 to

     n three-phase applications, if low      deliver 12V, 300 mA, which is recti-            As soon as phase-R voltage goes
     voltage is available in any one or      fied by diode D1 and filtered by ca-        below 200V, the voltage at inverting
     two phases, and you want your           pacitor C1 to produce the operating         pin 2 of IC1 goes below reference volt-
equipment to work on normal voltage,         voltage for the operational amplifier       age of 5.1V, and its output goes low.
this circuit will solve your problem.        (IC1). The voltage at inverting pin 2 of    As a result, transistor T1 conducts and
However, a proper-rating fuse needs          oprational amplifier IC1 is taken from      relay RL1 energises and load L1 is
to be used in the input lines (R, Y and      the voltage divider circuit of resistor     disconnected from phase ‘R’ and
B) of each phase. The circuit provides       R1 and preset resistor VR1. VR1 is used     connected to phase ‘Y’ through relay
correct voltage in the same power sup-       to set the reference voltage according      RL2.
ply lines through relays from the other      to the requirement. The reference volt-         Similarly, the auto phase-change of
phase where correct voltage is avail-        age at non-inverting pin 3 is fixed to      the remaining two phases, viz, phase
able. Using it you can operate all your      5.1V through zener diode ZD1.               ‘Y’ and phase ‘B,’ can be explained.
equipment even when correct voltage              Till the supply voltage available in    Switch S1 is mains power ‘on’/’off’
is available on a single phase in the        phase R is in the range of 200V-230V,       switch.

building.                                    the voltage at inverting pin 2 of IC1           Use relay contacts of proper rating
    The circuit is built around a trans-     remains high, i.e., more than reference     and fuses should be able to take-on
former, comparator, transistor and re-       voltage of 5.1V, and its output pin 6       the load when transferred from other
lay. Three identical sets of this circuit,   also remains high. As a result, transis-    phases. While wiring, assembly and in-
one each for three phases, are used.         tor T1 does not conduct, relay RL1 re-      stallation of the circuit, make sure that
Let us now consider the working of           mains de-energised and phase ‘R’ sup-       you:
the circuit connecting red cable (call it    plies power to load L1 via normally-            1. Use good-quality, multi-strand
‘R’ phase).                                  closed (N/C) contact of relay RL1.          insulated copper wire suitable for your

WWW.EFYMAG.COM                                                                          ELECTRONICS FOR YOU • JULY 2007 • 93
current requirement.                      nections from mains.                         2. If the input voltage is low in two
    2. Use good-quality relays with           EFY Note: 1. During testing in the   phases, loads L1 and L2 may also be
proper contact and current rating.        lab, we used a 12V, 200-ohm, single-     connected to the third phase. In that
    3. Mount the transformer(s) and re-   phase changeover relay with 6A cur-      situation, a high-rating fuse will be re-
lays on a suitable cabinet. Use a Tag     rent rating. Similarly, ampere-rated     quired at the input of the third phase
Block (TB) for incoming/outgoing con-     fuses were used.                         which is taking the total load.

94 • JULY 2007 • ELECTRONICS FOR YOU                                                                  WWW.EFYMAG.COM

Constant-Current                                                                                              s.c. dwiv

Battery Charger
   Monoj Das                                             13.5-14.2V in the case of a 12V battery),   14+5=19V.
                                                          give indication and the charger will            For the sake of simplicity, this con-

         here are many ways of battery                    switch off automatically. You need not      stant-current battery charger circuit is
         charging but constant-current                    remove the battery from the circuit.        divided into three sections: constant-
         charging, in particular, is a                        4. If the battery is discharged be-     current source, overcharge protection
popular method for lead-acid and Ni-                      low a limit, it will give deep-discharge    and deep-discharge protection sec-
Cd batteries. In this circuit, the battery                indication.                                 tions.
is charged with a constant current that                       5. Quiescent current is less than 5         The constant-current source is
is generally one-tenth of the battery                     mA and mostly due to zeners.                built around MOSFET T5, transistor
capacity in ampere-hours. So for a                            6. DC source voltage (VCC) ranges       T1, diodes D1 and D2, resistors R1, R2,
4.5Ah battery, constant charging cur-                     from 9V to 24V.                             R10 and R11, and potmeter VR1. Diode
rent would be 450 mA.                                         7. The charger is short-circuit pro-    D2 is a low-temperature-coefficient,
    This battery charger has the follow-                  tected.                                     highly stable reference diode LM236-5.
ing features:                                                 D1 is a low-forward-drop schottky       LM336-5 can also be used with reduced
    1. It can charge 6V, 9V and 12V bat-                  diode SB560 having peak reverse volt-       operating temperature range of 0 to
teries. Batteries rated at other voltages                 age (PRV) of 60V at 5A or a 1N5822          +70°C. Gate-source voltage (VGS) of T5
can be charged by changing the values                     diode having 40V PRV at 3A. Nor-            is set by adjusting VR1 slightly above
of zener diodes ZD1 and ZD2.                              mally, the minimum DC source volt-          4V. By setting VGS, charging current
    2. Constant current can be set as                     age should be ‘D1 drop+Full charged         can be fixed depending on the battery
per the battery capacity by using a                       battery voltage+VDSS+ R2 drop,’ which       capacity. First, decide the charging
potmeter and multimeter in series with                    is approximately ‘Full charged battery      current (one-tenth of the battery’s Ah
the battery.                                              voltage+5V.’ For example, if we take        capacity) and then calculate the nearest
    3. Once the battery is fully charged,                 full-charge voltage as 14V for a 12V        standard value of R2 as follows:
it will attain certain voltage level (e.g.                battery, the source voltage should be           R2 = 0.7/Safe fault current

1 1 4 • Au g u s t 2 0 0 9 • e l e c t ro n i c s f o r yo u                                                              w w w. e f y m Ag . co m
    R2 and T1 limit the charging cur-       diode ZD1 starts conducting after            LED2 will glow to indicate that the bat-
rent if something fails or battery termi-   its breakdown voltage is reached,            tery voltage is low.
nals get short-circuited accidentally.      i.e., it conducts when the battery               Values of zener diodes ZD1 and
    To set a charging current, while        voltage goes beyond a prefixed high          ZD2 will be the same for 6V, 9V and
a multimeter is connected in series         level. Adjust VR2 when the battery           12V batteries. For other voltages, you
with the battery and source supply is       is fully charged (say, 13.5V in case of      need to suitably change the values of
present, adjust potmeter VR1 slowly         a 12V battery) so that VGS of T5 is set      ZD1 and ZD2. Charging current pro-
until the charging current reaches its      to zero and hence charging current           vided by this circuit is 1 mA to 1 A, and
required value.                             stops flowing to the battery. LED1           no heat-sink is required for T5. If the
    Overcharge and deep-discharge           glows to indicate that the battery is        maximum charging current required is
protection have been shown in dotted        fully charged. When LED1 glows, the          5A, put another LM236-5 in series with
areas of the circuit diagram. All com-      internal LED of the optocoupler also         diode D2, change the value of R11 to 1
ponents in these areas are subjected to     glows and the internal transistor con-       kilo-ohm, replace D1 with two SB560
a maximum of the battery voltage and        ducts. As a result, gate-source voltage      devices in parallel and provide a good
not the DC source voltage. This makes       (VGS) of MOSFET T5 becomes zero and          heat-sink for MOSFET T1. TO-220 pack-
the circuit work under a wide range of      charging stops.                              age of IRF540 can handle up to 50W.
source voltages and without any influ-           Normally, zener diode ZD2 con-              Assemble the circuit on a gen-
ence from the charging current value.       ducts to drive transistor T3 into con-       eral-purpose PCB and enclose in a
Set overcharge and deep-discharge           duction and thus make transistor T4          box after setting the charging current,
voltage of the battery using potmeters      cut-off. If the battery terminal voltage     overcharge voltage and deep-discharge
VR1 and VR2 before charging the bat-        drops to, say, 11V in case of a 12V bat-     voltage. Mount potmeters VR1, VR2
tery.                                       tery, adjust potmeter VR3 such that          and VR3 on the front panel of the
    In overcharge protection, zener         transistor T3 is cut-off and T4 conducts.    box. 

w w w. e f y m Ag . co m                                                           e l e c t ro n i c s f o r yo u • Au g u s t 2 0 0 9 • 1 1 5

BAttery-level indicAtor                                                                                                     edi
                                                                                                                 s.c. dwiv
   Aniruddh K.S.                                          makes it easier to recognise the
                                                           voltage level on the basis of the

          ormally, in mobile phones,                       calibration made. Red LEDs (LED1
          the battery level is shown in                    through LED3) indicate battery           initially set it at 3V. Slowly adjust VR1
          dot or bar form. This lets you                   capacity of less than 40 per cent.       until LED1 glows. Now, increase the
easily recognise the battery level. Here                   Orange LEDs (LED4 through LED6)          input voltage to 15V in steps of 1.2V
we present a circuit that lets you know                    indicate battery capacity of 40 to       until the corresponding LED (LED2
the battery level of a device from the                     less than 70 per cent and green          through LED10) lights up.
number of LEDs that are glowing. It                        LEDs (LED7 through LED10) indi-              Now the circuit is ready to show
uses ten LEDs in all. So if three LEDs                     cate battery capacity of 70 to under     any voltage value with respect to the
glow, it indicates battery capacity of                     100 per cent. The brightness of the      maximum voltage. As the number of

30 per cent. Unlike in mobile phones                       LEDs can be adjusted by varying the      LEDs is ten, we can easily consider one
where the battery-level indicator func-                    value of preset VR2 between pins 6       LED for 10 per cent of the maximum
tion is integrated with other functions,                   and 7.                                   voltage.
here only one comparator IC (LM3914)                           Diode D1 prevents the circuit            Connect the voltage from any
does it all.                                               from reverse-polarity battery con-       battery to be tested at the input
    The LM3914 uses ten comparators,                       nection. The tenth LED glows only        probes of the circuit. By examining
which are internally assembled in the                      when the battery capacity is full,       the number of LEDs glowing you
voltage divider network based on the                       i.e., the battery is fully charged.      can easily know the status of the bat-
current-division rule. So it divides the                   When the battery is fully charged,       tery. Suppose five LEDs are glowing.
battery level into ten parts.                              relay-driver transistor T1 conducts      In this case, the battery capacity is
    The circuit derives the power                          to energise relay RL1. This stops the    50 to 59 per cent of its maximum
supply for its operation from the                          charging through normally-open           value.
battery of the device itself. It uses                      (N/O) contacts of relay RL1.                 Assemble the circuit on a general-
ten LEDs wired in a 10-dot mode.                               For calibration, connect 15V vari-   purpose PCB. Calibrate it and then en-
The use of different coloured LEDs                         able, regulated power supply and         close in a box. 

8 4 • D E C E m b E r 2 0 0 8 • e l e c t ro n i c s f o r yo u                                                        w w w. E f y m ag . Co m

Bicycle indicaTor                                                                                                          edi
                                                                                                              s.c. dwiv
   T.K. Hareendran                          the square-wave oscillators (one built
                                             around T1 and T2 and the other built

         he electronic bicycle signaling     around T3 and T4) drive four red LEDs
         unit described here uses low-       (LED1 and LED2, and LED5 and LED6,                determined by timing capacitors C1
         cost components and is a good       respectively), which blink to indicate            and C2. Resistors R2 and R3 limit the
                                                                                                    operating current of LEDs (LED1
                                                                                                    and LED2). At the same time, the
                                                                                                    green LED (LED3) starts glowing
                                                                                                    to indicate the present direction
                                                                                                        Similar action happens in the
                                                                                                    next oscillator circuit built around
                                                                                                    transistors T3 and T4 when switch
                                                                                                    S2 is flipped to ‘on’ position. Indi-
                                                                                                    cators at the front right (FR) and
                                                                                                    rear right (RR) start blinking, and
                                                                                                    at the same time the green LED
                                                                                                    (LED4) glows to indicate the direc-
                                                                                                    tion status.
                                                                                                        Switch S3 is used for emergency
                                                                                                    indication. When it is flipped to ‘on’
                                                                                                    position, both the oscillators get
                                                                                                    power supply through diodes D1
                                                                                                    and D2. As a result, LED1 through
Fig. 1: Circuit of bi-cycle indicator                                                               LED6 start working simultaneous-
                                                                                                                 ly. In this condition, all
                                                                                                                 the LEDs blink, except
                                                                                                                 LED3 and LED4, which
                                                                                                                 glow steadily.
                                                                                                                     After assembling
                                                                                                                 the circuit on a general-
                                                                                                                 purpose PCB, enclose
                                                                                                                 it in a suitable cabinet
                                                                                                                 as shown in Fig. 2 and
                                                                                                                 mount on the handle
                                                                                                                 bar of the bicycle, pref-
                                                           Fig. 3: Suggested enclosure (indicators)              erably at the mechani-
                                                                                                                 cal centre point. Con-
                                                          the direction of turn. Ad-           nect switch S1 at the left-hand side, S2
                                                          ditional steady-glow LEDs            at the right-hand side and emergency
                                                          (LED3 and LED4) are in-              switch S3 in the middle of the master
                                                          corporated to indicate the           unit. Now place this master unit at
Fig. 2: Suggested enclosure (master unit)
                                                          working status.                      the top of the handle bar and do the
substitute to many commercially avail-           The working of the cir-                       essential interconnections using flex-
able versions. It works in an extremely      cuit is straightforward. When                     ible wires. Connect the front indica-
different manner and is convenient to        switch S1 is flipped to ‘on’ po-                  tors (LED1 and LED5) to the left and
operate.                                     sition, DC supply from                            right side of the handle and similarly
    The circuit works off a 9V PP3           the battery is extended to the oscil-             rear indicators (LED2 and LED6) can
(alkaline-type) battery and is basically     lator circuit formed by transistors               be mounted in the carrier frame of
a set of two independent free-running        T1 and T2. Now the left-side oscil-               the bicycle. For the direction indica-
oscillators (astable multivibrators) built   lator starts oscillating and the visual           tor, you can use the symbol shown in
around four low-power transistors            indicators at the front left (FL) and             Fig. 3 and place it at the centre of the
and a few passive components. Both           rear left (RL) start blinking at a rate           handle. 

w w w. e f y m ag . co m                                                                     e l e c t ro n i c s f o r yo u • j u ly 2 0 0 8 • 9 5
                   blown fuse indicator
        enerally, when an equipment in-      second arm it is only 2V. So current flows    (see Fig. 2). An optocoupler is used to
        dicates no power, the cause may      through the second arm, i.e. through the      trigger the siren. When the fuse blows, red
        be just a blown fuse. Here is a      green LED, causing it to glow; whereas        LED glows. Simultaneously it switches
circuit that shows the condition of fuse     the red LED remains off.                      ‘on’ the siren.
through LEDs. This compact circuit is            When the fuse blows off, the supply            In place of a bicolour LED, two LEDs
very useful and reliable. It uses very few   to green LED gets blocked, and because        of red and green colour can be used.
components, which makes it inexpensive       only one LED is in the circuit, the red LED   Similarly, only one diode in place of D1
too.                                         glows. In case of power failure, both LEDs    and D2 may be used. Two diodes are used
     Under normal conditions (when fuse      remain ‘off’.                                 to increase the voltage drop, since the
is alright), voltage drop in first arm           This circuit can be easily modified to    two LEDs may produce different voltage
is 2V + (2 x 0.7V) = 3.4V, whereas in        produce a siren in fuse-blown condition       drops.


Car anTi-THefT Guard                                                                                       s.c. dwiv

   T.K. Hareendran                        tor R1, which prevents transistor T1
                                           from conducting. In this position, anti-

          ere is an easy-to-build car      theft guard cir-
          anti-theft guard. The circuit,   cuit is in sleep
          shown in Fig. 1, is simple and   mode.
easy to understand. When key-oper-               W h e n
ated switch S2 of the car is turned on,    someone opens
12V DC supply from the car battery is      the car door,
extended to the entire circuit through     switch S1 be-
polarity-guard diode D5. Blinking          comes ‘off’ as
LED1 flashes to indicate that the guard    shown in Fig.
circuit is enabled. It works off 12V       2. As a result,
power supply along with current-limit-     transistor T1
ing resistor R4 in series.                 conducts to fire
    When the car door is closed, door      r e l a y - d r i v e r Fig. 2: Wiring diagram for door switch (S1)
switch S1 is in ‘on’ position and 12V      SCR1 (BT169)
power supply is available across resis-    after a short delay introduced by ca-                    pacitor C1. Electromagnetic relay RL1
                                                                                                                   energises and its N/O
                                                                                                                   contact connects the
                                                                                                                   power supply to pi-
                                                                                                                   ezobuzzer PZ1, which
                                                                                                                   starts sounding to in-
                                                                                                                   dicate that someone
                                                                                                                   is trying to steal your
                                                                                                                   car. To reset the circuit,
                                                                                                                   turn off switch S2 using
                                                                                                                   car key. This will cut-
                                                                                                                   off the power supply to
                                                                                                                   the circuit and stop the
                                                                                                                   buzzer sound.
                                                                                                                       Assemble the cir-
                                                                                                                   cuit on a general-pur-
                                                                                                                   pose PCB and house
                                                                                                                   in a small box. Con-
                                                                                                                   nect switch S1 to the
                                                                                                                   car door and keep pi-
                                                                                                                   ezobuzzer PZ1 at an
                                                                                                                   appropriate place in
Fig. 1: Circuit of car anti-theft guard                                                                            the car. 

w w w. e f y m ag . co m                                                                     e l e c t ro n i c s f o r yo u • m ay 2 0 0 8 • 7 5
                                                                                     via switch S1.
                                                                 IVEDI                   The circuit works off a 9V bat-
                                                          S.C. DW
CLOCK TIMER                                                                          tery. Assemble it on a general-pur-
                                                                                     pose PCB and enclose in a suitable
                                                                                     cabinet. Provide an AC outlet in the
  D. MOHAN KUMAR                           set VR1. The inverting and non-invert-    cabinet to switch on the appliance us-
                                           ing inputs of LM311 are different from    ing the circuit. As mentioned earlier,

             ith this simple clock-        other op-amps and it outputs sink cur-    the input signal is obtained from the
             controlled timer, you will    rent through pin 7 or source current      buzzer terminals of the clock. Remove
             never again miss your         through pin 1.                            the small buzzer of the clock and con-
favourite TV or radio programme. The           When pin 3 of IC1 is at a higher      nect point ‘A’ to the positive termi-
TV or radio will switch on automati-       voltage than pin 2, its output sinks      nal and point ‘B’ to the negative ter-
cally at the time preset by you and        as indicated by LED1. This gives a        minal of the buzzer. Connect the
will remain ‘on’ until the power sup-      short negative pulse to the monostable    mains AC terminal outlet to the nor-
ply fails or is disconnected.              wired around timer NE555. Resistor        mally-opened (N/O) contact of relay
    The circuit uses the AC signals gen-   R5 keeps trigger pin 2 of IC2 high.       RL1. So when the relay energises,
erated at the buzzer terminals of an       The short-interval monostable outputs     230V AC operates the connected ap-
alarm clock. The AC signals are am-        a high signal for a brief period to the   pliance.
plified by transistors T1 and T2 and       gate of SCR1 (BT169) and relay RL1            Set the desired time in the clock by
the amplified output from the emitter      energises. The latching action of SCR1    adjusting the alarm set-up and switch
of T2 is fed to the inverting input of     keeps the relay pulled even when the      on the circuit. When the set time
negative-voltage comparator IC LM311       output of the monostable turns low.       reaches, the appliance will switch on
(IC1). The non-inverting input of IC1      The relay can be de-energised by dis-     automatically. The circuit can also be
gets a presettable voltage through pre-    connecting the supply to the circuit      connected to digital clocks.

96 • FEBRUARY 2007 • ELECTRONICS FOR YOU                                                                WWW.EFYMAG.COM

CURRENT SENSOR                                                                                   S.C. DW

  D. MOHAN KUMAR                            transistors in the input to provide very
                                            high input impedance (1.5 T-ohms),

          igh-wattage appliances like       very low input current (10 pA) and         by feeding Q9 output to the
          electric irons, ovens and heat-   high-speed switching performance.          piezobuzzer for aural alarm through
          ers result in unnecessary             The inverting input of IC1 is pre-     the intermediate circuitry. Resistors R5
power loss if left ‘on’ for hours unno-     set with VR1. In the standby mode,         and R6 along with capacitor C1 main-
ticed. Here is a circuit that senses the    the primary of the transformer accepts     tain the oscillations in IC2 as indicated
flow of current through the appliances      e.m.f. from the instrument or sur-         by blinking LED1. The high output
and gives audible beeps every fifteen       rounding atmosphere, which results in      from IC2 is used to activate a simple
minutes to remind you of power-’on’         low-voltage input to IC1. This low         oscillator comprising transistors T2
status.                                     voltage at the non-inverting input         and T3, resistors R8 and R10, and ca-
    This is a non-contact version of cur-   keeps the output of IC1 low. Thus tran-    pacitor C2.
rent monitor and can sense the flow         sistor T1 doesn’t conduct and pin 12           When the Q9 output of IC2 be-
of current in high-current appliances       of IC2 goes high to disable IC2. As a      comes high, zener diode ZD1 provides
from a distance of up to 30 cm . It uses    result, the remaining part of the cir-     3.1 volts to the base of transitor T2.

a standard step-down transformer (0-        cuit gets inactivated.                     Since transistor T2 is biased by a high-
9V, 500mA) as the current sensor. Its           When a high-current appliance is       value resistor (R8), it will not conduct
secondary winding is left open, while       switched on, there will be a current       immediately. Capacitor C2 slowly
the primary winding ends are used to        drain in the primary of the transformer    charges and when the voltage at the
detect the current. The primary ends        to the negative rail due to an increase    base of T2 increases above 0.6 volt, it
of the transformer are connected to a       in the e.m.f. caused by the flow of cur-   conducts. When T2 conducts, the base
full-wave bridge rectifier comprising       rent through the appliance. This results   of T3 turns low and it also conducts.
diodes D1 through D4. The rectified         in voltage rise at the non-inverting in-   The piezobuzzer connected to the col-
output is connected to the non-invert-      put and the output of IC1 becomes high.    lector of T3 gives a short beep as ca-
ing input of IC CA3140 (IC1).               This high output drives transistor T1      pacitor C2 discharges. This sequence
    IC CA3140 is a 4.5MHz BIMOS op-         into conduction and the reset pin of       of IC2 output at Q9 becoming high and
erational amplifier with MOSFET in-         IC2 becomes low, which enables IC2.        conduction of transistors T2 and T3 re-
put and bipolar transistor output. It           IC CD4060 (IC2) is a 14-stage ripple   sulting in beep sound repeats at short
has gate-protected MOSFET (PMOS)            counter. It is used as a 15-minute timer   intervals.

94 • DECEMBER 2007 • ELECTRONICS FOR YOU                                                                  WWW.EFYMAG.COM
                               DARKROOM TIMER
       he timer circuit described here pro-    built around transistor T2, turning it on         sistor T2 is coupled to a small speaker
        vides a pleasant musical tone in       and off. As capacitor C1 is charged               through a transistor-radio type output
        your darkroom at 1-second inter-       through preset VR1 and resistor R1, the           transformer.
vals. The circuit takes up very little space   emitter voltage of UJT rises toward the               The 22-kilo-ohm value of resistor R3
and can be easily converted into a metro-      supply voltage.                                   represents a compromise between tone
nome.                                               When the emitter voltage becomes suf-        duration and intensity. You can use re-
    Unijunction transistor (UJT) T1 func-      ficiently positive, the emitter becomes for-      sistors having a value anywhere between
tioning as a relaxation oscillator triggers    ward biased and discharges capacitor C1           10 kilo-ohms and 25 kilo-ohms for differ-
the phase-shift audio oscillator circuit       through the emitter-base 1 (B1) junction          ent durations and intensities of the out-
                                                                         and resistor R2.        put signals.
                                                                         The voltage drop            Since the unijunction transistor is
                                                                         across R2 forward       functioning as the oscillator trigger,
                                                                         biases transistor       changing the values of one or more com-
                                                                         T2 and turns it on.     ponents in the UJT circuit will change
                                                                         As capacitor C1         the rate of the tone burst. The tone fre-
                                                                         becomes         dis-    quency can be varied by changing the
                                                                         charged, the cur-       value of any or more of capacitors C2
                                                                         rent through re-        through C4 and resistors R5 and R6 in
                                                                         sistor R2 drops         the phase-shift network.
                                                                         and transistor T2           The primary winding of transformer
                                                                         is cut off.             X1 can be tuned for a slight increase in
                                                                              A tone signal      the output, using capacitor values between
                                                                         is generated by         0.05 and 0.25 μF for C5 by trial-and-error
                                                                         transistor T2 and       method. Tone pulses should begin about
                                                                         R-C         coupled     ten seconds after the unit is turned on.
                                                                         phase-shift oscilla-    After a minute or so, adjust preset VR1
                                                                         tor. Part of the sig-   for 1-second beats by comparing the tim-
                                                                         nal taken from the      ing of the beats with the seconds needle
                                                                         collector of tran-      on your wristwatch.


DESKTOP POWER SUPPLY                                                                               S.C. DW

  T.K. HAREENDRAN                                             top power supply.
                                                              Regulator IC LM317T         tive load is

          seful for electronics hobbyists,                    is arranged in its stan-    connected at
          this linear workbench power                         dard application. Di-       the     output.
          supply converts a high input                        ode D1 guards against       Similarly, ca-
voltage (12V) from the SMPS of a PC                           polarity reversal and       pacitor C3 sup-
into low output voltage (1.25 to 9                            capacitor C1 is an ad-      presses any re-
volts). An adjustable three-pin voltage                       ditional buffer. The        sidual ripple.
regulator chip LM317T (IC1) is used                           green LED (LED1) in-            Connect a Fig. 3: Suggested power
here to provide the required voltages.       Fig. 2: Pin      dicates the status of the
                                             configuration of                             standard digi- supply box
The LM317T regulator, in TO-220 pack,        LM317            power input. Diode D2       tal voltmeter in
can handle current of up to 1 amp in                          prevents the output         parallel with the output leads to accu-
practice.                                    voltage from rising above the input          rately set the desired voltage with the
    Fig. 1 shows the circuit of the desk-    voltage when a capacitive or induc-          help of variable resistor VR1. You can
                                                                                                  also use your digital multimeter
                                                                                                  if the digital voltmeter is not
                                                                                                  available. Switch on S1 and set
                                                                                                  the required voltage through
                                                                                                  preset VR1 and read it on the
                                                                                                  digital voltmeter. Now the
                                                                                                  power supply is ready for use.
                                                                                                      The circuit can be wired on
                                                                                                  a common PCB. Refer Fig. 2 for
                                                                                                  pin configuration of LM317 be-
                                                                                                  fore soldering it on the PCB. Af-
                                                                                                  ter fabrication, enclose the cir-
                                                                                                  cuit in a metallic cover as
                                                                                                  shown in Fig. 3. Then open the
                                                                                                  cabinet of your PC and connect
                                                                                                  the input line of the gadget to a
                                                                                                  free (hanging) four-pin drive
                                                                                                  power connector of the SMPS
Fig. 1: Circuit of desktop power supply                                                           carefully.

98 • SEPTEMBER 2007 • ELECTRONICS FOR YOU                                                                    WWW.EFYMAG.COM
  EFY LAB                                   CD4026, pin 14 (cascading output) is
                                            to be left unused (open), but in case of                       IVEDI
                                                                                                    S.C. DW

         digital dice circuit can be eas-   CD4033, pin 14 serves as lamp test pin
         ily realised using an astable      and the same is to be grounded.
         oscillator circuit followed by          The circuit uses only a handful of
a counter, display driver and a dis-        components. Its power consumption is
play.                                       also quite low because of use of CMOS       astable oscillator configured around
   Here we have used a timer NE555          ICs, and hence it is well suited for bat-   IC1 as well as capacitor C1 (through
as an astable oscillator with a fre-        tery operation. In this circuit two tac-    resistor R1), which charges to the bat-
quency of about 100 Hz. Decade              tile switches S1 and S2 have been pro-      tery voltage. Thus even after switch
counter IC CD4026 or CD4033 (which-         vided. While switch S2 is used for ini-     S1 is released, the astable circuit
ever available) can be used as counter-     tial resetting of the display to ‘0,’ de-   around IC1 keeps producing the clock
cum-display driver. When using              pression of S1 simulates throwing of        until capacitor C1 discharges suffi-
                                                                                                           ciently. Thus for du-
                                                                                                           ration of depression
                                                                                                           of switch S1 and dis-
                                                                                                           charge of capacitor
                                                                                                           C1 thereafter, clock
                                                                                                           pulses are produced
                                                                                                           by IC1 and applied to
                                                                                                           clock pin 1 of counter
                                                                                                           IC2, whose count ad-
                                                                                                           vances at a frequency
                                                                                                           of 100 Hz until C1
                                                                                                           discharges       suffi-
                                                                                                           ciently to deactivate
                                                                                                               When the oscilla-
                                                                                                           tions from IC1 stop,
                                                                                                           the last (random)
                                                                                                           count in counter IC2
                                                                                                           can be viewed on the
                                                                                                           7-segment display.
                                                                                                           This count would
                                                                                                           normally lie between
                                                                                                           0 and 6, since at the
                                                                                                           leading edge of every
                                                       the dice by a player.            7th clock pulse, the counter is reset to
        Decoded Segment Outputs                            When battery is con-         zero. This is achieved as follows.
          for Counts 0 through 9                       nected to the circuit, the           Observe the behavior of ‘b’ seg-
                                                       counter and display section      ment output in the Table. On reset, at
                                                       around IC2 (CD4026/4033)         count 0 until count 4, the segment ‘b’
                                                       is energised and the display     output is high. At count 5 it changes
                                                       would normally show ‘0’, as      to low level and remains so during
                                                       no clock input is available.     count 6. However, at start of count 7,
                                                       Should the display show          the output goes from low to high state.
                                                       any other decimal digit, you     A differentiated sharp high pulse
                                                       may press re-set switch S2       through C-R combination of C4-R5 is
                                                       so that display shows ‘0’. To    applied to reset pin 15 of IC2 to reset
                                                       simulate throwing of dice,       the output to ‘0’ for a fraction of a
                                                       the player has to press          pulse period (which is not visible on
                                                       switch S1, briefly. This ex-     the 7-segment display). Thus, if the
                                                       tends the supply to the          clock stops at seventh count, the dis-

WWW.EFYMAG.COM                                                                   ELECTRONICS FOR YOU • NOVEMBER 2007 • 97
play will read zero. There is a prob-      other chance until the display is non-    the counter by ‘1,’ the same makes the
ability of one chance in seven that dis-   zero.                                     circuit somewhat complex and there-
play would show ‘0.’ In such a situa-          Note. Although it is quite feasible   fore such a modification has not been
tion, the concerned player is given an-    to inhibit display of ‘0’ and advance     attempted.

98 • NOVEMBER 2007 • ELECTRONICS FOR YOU                                                               WWW.EFYMAG.COM

DiGital theRmometeR                                                                                   s.c. dwiv

   Raj K. GoRKhali                      through the base-emitter junction of
                                         npn transistor BC108 (T1). The volt-

       his digital thermometer can       age across the base-emitter junction        before the temperature is displayed
       measure temperatures up to        of the transistor is proportional to its    on the meter. Preset VR1 is used to
       150°C with an accuracy of ±1°C.   temperature. The transistor used this       set the zero-reading on the meter and
The temperature is read on a 1V full     way makes a low-cost sensor. You can        preset VR2 is used to set the range of
scale-deflection (FSD) moving-coil       use silicon diode instead of transistor.    temperature measurement.
voltmeter or digital voltmeter.          The small variation in voltage across           Operational amplifiers IC3 and IC4
    Operational amplifier IC 741 (IC3)   the base-emitter junction is amplified      operate off regulated ±5V power sup-
provides a constant flow of current      by second operational amplifier (IC4),      ply, which is derived from 3-terminal
                                                                                                     positive voltage regula-
                                                                                                     tor IC 7805 (IC1) and
                                                                                                     negative low-dropout
                                                                                                     regulator IC 7660 (IC2).
                                                                                                     The entire circuit works
                                                                                                     off a 9V battery.
                                                                                                         Assemble the cir-
                                                                                                     cuit on a general-pur-
                                                                                                     pose PCB and enclose
                                                                                                     in a small plastic box.
                                                                                                     Calibrate the thermom-
                                                                                                     eter using presets VR1
                                                                                                     and VR2. After calibra-
                                                                                                     tion, keep the box in
                                                                                                     the vicinity of the object
                                                                                                     whose temperature is
                                                                                                     to be measured. 

w w w. e f y m ag . co m                                                            e l e c t ro n i c s f o r yo u • J u ly 2 0 1 0 • 9 9

DiGital timeR enhancement                                                                                            s.c. dwiv

   Raj K. GoRKhali                                    circuit can be used:
                                                           1. You want an appliance or gadget     and relay RL1 de-energises to discon-

        his simple circuit automatically               to switch on automatically at a preset     nect the load from mains power supply
        activates or deactivates an elec-              time                                       through its contacts. At this time, you
        tronic device at the time of alarm                 2. You switch on an appliance or       need to pause the alarm using pause
preset in a clock. When the alarm rings,               gadget manually at a particular time       switch of the clock.
the tone burst generated at the terminal               and want it to switch off automatically         When you press reset switch S1,
of the buzzer triggers the circuit and the             at a preset time                           LED1 glows to indicate that the circuit
relay energises or de-energises to switch                  Let us see how it works when you       is ready to work. When you press start
on or switch off the load.                             want your appliance to switch on at a      switch S2, LED2 glows to indicate start
    The circuit is built around ICs                    preset time, say, 3 pm. Set the alarm in   mode. Glowing of LED3 means that
CD40106 (IC1) and CD4017 (IC2) and                     your clock to 3 pm and slide switch S3     the counter has stopped counting and
a few discrete components. IC1 is a hex                towards Q1. When the alarm sounds          needs to be reset before use.
Schmitt trigger, while IC2 is a decade                 at 3 pm, Q0 output of IC2 advances to           When the counter is in stop mode,
counter. The circuit works off regulat-                Q1 and relay RL1 energises to connect      Q2 output of IC2 remains high. As this
ed 6V power supply, while the alarm                    the load (appliance) to mains power        pin is connected to the clock-enable
clock runs off its own 1.5V battery.                   supply through its contacts. The load      input (pin 13) of IC2, the clock input
    The tone burst generated at the                    remains ‘on’ until you reset IC2 by        is inhibited. In this condition, any tone

piezobuzzer is tapped from its connec-                 momentarily pressing S1. At this time,     burst signal arriving from the clock
tion points. The positive terminal of the              you need to pause the alarm using          has no effect on IC2 and therefore the
clock buzzer is connected to the base of               pause switch of the clock.                 circuit remains in stop mode. You can
transistor T1 and the negative terminal                     Now suppose you manually start        now set the alarm time in the clock.
is connected to ground of the circuit.                 the load at 3 pm and want it to stop           Assemble the circuit on a general-
    When the alarm clock sounds, the                   automatically at 6 pm. First, reset IC2    purpose PCB and enclose in a small
signal from the clock buzzer makes                     by momentarily pressing S1 and slide       cabinet. Connect the base of transistor
transistor T1 conduct. As a result, pin                switch S3 towards Q2. Set the alarm in     (T1) to positive terminal of the alarm
1 of gate N1 goes low and it outputs                   your clock to 6 pm. To start the load,     clock and negative terminal to ground
high at pin 2. This low-to-high transi-                press switch S2 momentarily at 3 pm.       of the circuit. Put the alarm clock at a
tion clocks the counter (IC2) at pin 14                The Q0 output of IC2 advances to Q1        convenient place. If you do not want to
through diode D1 and gate N2. In this                  and relay RL1 energises to connect the     use a 6V battery, replace it with a 6V
way, IC2 advances by one at each clock                 load to mains power supply through         adaptor to power the circuit. Mount
produced due to the sounding alarm.                    its contacts. When the alarm sounds at     the LEDs and the pushbutton on the
    There are two situations where this                6 pm, Q1 output of IC2 advances to Q2      front panel of the cabinet. 

9 8 • M ay 2 0 1 0 • e l e c t ro n i c s f o r yo u                                                                 w w w. e f y M ag . co M

Drinking Water alarm                                                                                     s.c. dwiv

   Dr C.H. VitHalani

        he State Jal Boards supply
        water for limited duration in a
        day. Time of water supply is
decided by the management and the
public does not know the same. In such
a situation, this water alarm circuit will
save the people from long wait as it
will inform them as soon as the water
supply starts.
    At the heart of this circuit is a
small water sensor. For fabricating
this water sensor, you need two
foils—an aluminium foil and a plastic
foil. You can assemble the sensor by
rolling aluminium and plastic foils
in the shape of a concentric cylinder.
Connect one end of the insulated
flexible wire on the aluminium foil
and the other end to resistor R2. Now        work only when water flows through         from the tap. The circuit works off a 9V
mount this sensor inside the water           the water tap and completes the cir-       battery supply.
tap such that water can flow through         cuit connection. It oscillates at about        Assemble the circuit on any gen-
it uninterrupted. To complete the            1 kHz. The output of the timer at pin      eral-purpose PCB and house in a
circuit, connect another wire from           3 is connected to loudspeaker LS1 via      suitable cabinet. The water sensor is
the junction of pins 2 and 6 of IC1 to       capacitor C3. As soon as water starts      inserted into the water tap. Connect
the water pipeline or the water tap          flowing through the tap, the speaker       the lead coming out from the junc-
itself.                                      starts sounding, which indicates re-       tion of 555 pins 2 and 6 to the body of
    The working of the circuit is sim-       sumption of water supply. It remains       the water tap. Use on/off switch S1
ple. Timer 555 is wired as an astable        ‘on’ until you switch off the circuit      to power the circuit with the 9V PP3
multivibrator. The multivibrator will        with switch S1 or remove the sensor        battery. 

w w w. e f y m ag . co m                                                           e l e c t ro n i c s f o r yo u • m a r c h 2 0 0 8 • 1 0 5
                                                                                                        affect operation of the IC. The two-tone

                                                                                                        frequencies generated are switched by
                                                                                s.c. dwiv               an internal oscillator in a fast sequence,
                                                                                                        which appear at the output amplifier
                                                                                                        and drive the piezo buzzer element
   Raj K. GoRKhali                                        dicating a call on line 2 that is not con-       The hold section is built around
                                                           nected to the telephone receiver. When       IC1 and IC2 . Switch S1 is used to hold

        his simple circuit of a duo-                       you have a call on line 2, the ringer        line 1 and S2 is used to put line 2 on
        phone allows you to access                         will buzz. The telephone receiver can        hold. Since one telephone set is used
        two telephone lines through                        then be connected to line 2 through          for two separate lines, provision is
one telephone set. Each telephone con-                     the telephone changeover switch S4 to        thus made to hold the first call while
versation will remain entirely separate                    receive the call.                            the telephone set is connected to make
unless you choose to combine the two                           The ringer section is built around       or receive the second call.
lines through a conference switch. Its                     IC3 and its associated components. Its           The circuit comprises two identi-
unique feature is a three-party conver-                    circuit uses IC 1240 to detect the ring      cal hold circuits, each with its own
sation/conference facility.                                signal and keeps the buzzer ringing          flashing LED to maintain the holding
    The entire circuit is divided into                     for an incoming call on line 2. The sup-     current. Each hold circuit has a timer
three main sections—the ringer, hold                       ply voltage for the ringer is obtained       LM555 (IC1 or IC2) connected as a
and conferencing. The telephone set                        from the phone line’s AC ring (80V AC        free-running oscillator operating at
is connected to line 1 under normal                        RMS) signal and is regulated inside the      a frequency of 2 Hz. The output pin
conditions. The ringer is used for in-                     IC so that the noise on the line does not    3 of each timer is used for driving an
                                                                                                           LED that flashes twice in a second.
                                                                                                           The hold circuit is powered by the
                                                                                                           telephone lines through manually-
                                                                                                           operated hold switches (S1 and S2).
                                                                                                           Resistors R2 and R6 are placed in
                                                                                                           the hold circuits to ensure that suf-
                                                                                                           ficient current is drawn from the
                                                                                                           telephone line to prevent a discon-
                                                                                                               The conferencing section is built
                                                                                                           around the audio coupling trans-
                                                                                                           former X1. Switch S3 enables three-
                                                                                                           way conversation through both the
                                                                                                           telephone lines. The transformer
                                                                                                           couples the audio signals from one
                                                                                                           telephone line to the other. At the
                                                                                                           same time, complete DC isolation
                                                                                                           is maintained between both the
                                                                                                           telephone lines. Capacitors C1 and
                                                                                                           C3 are used for preventing any DC
                                                                                                           from flowing into the transformer
                                                                                                           windings. Resistor R1 provides a
                                                                                                           holding current on line 1 when the
                                                                                                           telephone set is connected to line
                                                                                                           2 during a conference call. Once
                                                                                                           the three-way conversation is es-
                                                                                                           tablished through the double-pole
                                                                                                           single-throw (DPST) switch S3, the
                                                                                                           hold circuits and flashing LED indi-
                                                                                                           cators are turned off. LED3, which
                                                                                                           gets illuminated by the holding cur-
                                                                                                           rent through R1, provides a visual

9 2 • N o v e m b e r 2 0 0 9 • e l e c t ro n i c s f o r yo u                                                             w w w. e f y m ag . co m
indication of the conferencing.          connect switch S4 to line 1. All other       nect’ plugs, you can easily remove
    The working of the circuit is sim-   switches should be in the ‘off’ mode         the unit from the telephone lines.
ple. To check if the wiring of switch    and all LEDs should be unlit. This           Check the polarity of the telephone
S4 is correct, connect the telephone     permits the telephone ringer to be           lines with a multimeter and connect
set to line 1. Now lift up the handset   activated if a call comes on line 2. For     it to the circuit accordingly.
and dial the number of line 2. The       making calls using line 1 or line 2,              To check the circuit after complet-
ringer would sound. Now discon-          you can simply connect switch S4 to          ing the wiring, connect a 6V regulated
nect line 1 and connect line 2 through   the desired line.                            power supply to line 1. When you
switch S4. You would get the dial            Assemble the circuit on a gen-           switch S1 to the ‘on’ position, LED1
tone from line 2.                        eral purpose PCB and enclose it in           blinks at a rate of 2 Hz. If you flip
    To check a conference call, you      a suitable cabinet. Fix the switches         switch S1 to the ‘off’ position and
would need the help of two friends.      S1 through S4 on the front side of           switch S3 to the ‘on’ position, LED1
First connect switch S4 to line 1 and    the cabinet. Also fix the LEDs on the        stops blinking and LED3 starts glow-
make a call to friend 1. Now flip the    front of the cabinet and the buzzer at       ing, indicating that the conferencing
DPST switch S3 to the ‘on’ position.     the back of the cabinet. It would be         facility is being used. Now disconnect
This puts on hold friend 1 on line 1     better if you use telephone sockets          line 1 from the 6V power supply, con-
and the conference LED3 lights up.       for the telephone lines. Sockets are         nect it to line 2 and flip switch S2 to
Connect switch S4 to line 2 and dial     relatively inexpensive and save time         the ‘on’ position. Now LED2 blinks at
friend 2. When the call on line 2 is     when troubleshooting needs to be             a rate of 2 Hz. Before connecting the
answered, a three-way conversation       done. Use modular plugs to connect           circuit to the telephone lines, flip each
can be made.                             the circuit and the two telephone            hold switch to the ‘off’ position. Now
    When the duophone is not in use,     lines. By using such ‘quick discon-          your circuit is ready to be used. 

w w w. e f y m ag . co m                                                      e l e c t ro n i c s f o r yo u • N o v e m b e r 2 0 0 9 • 9 3

ElEcTRic GuiTAR PREAmPlifiER                                                                                                      sani the

   T.K. HAREENDRAN                                up attached to a guitar headstock is
                                                   shown in Fig. 1. The pickup device has

            ere is the circuit of a guitar         a transducer on one end and a jack on          secure optimal input impedance. With
            preamplifier that would ac-            the other end. The jack can be plugged         the component values shown here, the
            cept any standard guitar               into a preamplifier circuit and then to        input impedance is above 50 kilo-ohms
                                                   a power amplifier system.                      and the peak output voltage is about
                                                       The pickup device captures me-             2V RMS. Master-level-control potmeter
                                                   chanical vibrations, usually from              VR1 should be adjusted for minimal
                                                   stringed instruments such as guitar            distortion.
                                       pickup      or violin, and converts them into an               The input from guitar pickup is fed
                                                   electrical signal, which can then be           to this preamplifier at J1 terminal. The
                                                   amplified by an audio amplifier. It            signal is buffered and processed by the
                                                   is most often mounted on the body              op-amp circuit wired around IC TL071
                                                   of the instrument, but can also be at-         (IC1). Set the gain using preset VR2.
           Headstock                               tached to the bridge, neck, pickguard          The circuit has a master and a slave
                                                   or headstock.                                  control. RCA socket J2 is the master
Fig. 1: A typical example of mounting the guitar       The first part of this preamplifier        signal output socket and socket J3 is
                                                   circuit shown in Fig. 2 is a single-tran-      the slave.
                                                                                                               It is much better to take
                                                                                                           the signal from J2 as the input
                                                                                                           to the power amplifier sys-
                                                                                                           tem or sound mixer. Output
                                                                                                           signals from J3 can be used to
                                                                                                           drive a standard headphone
                                                                                                           amplifier. Using potmeter
                                                                                                           VR3, set the slave output sig-
                                                                                                           nal level at J3.
                                                                                                               House the circuit in a
                                                                                                           metallic case. VR1 and VR3
                                                                                                           should preferably be the types
                                                                                                           with metal enclosures. To
                                                                                                           prevent hum, ground the case
                                                                                                           and the enclosures. A well-
                                                                                                           regulated 9V DC power supply
Fig. 2: Guitar preamplifier circuit
                                                                                                           is crucial for this circuit. How-
                                                                                                           ever, a standard 9V alkaline
pickup. It is also versatile in that it has        sistor common-emitter amplifier with           manganese battery can also be used to
two signal outputs.                                degenerative feedback in the emitter           power the circuit. Switch S1 is a power-
   A typical example of using a pick-              and a boot-strapped bias divider to            on/off switch. 

w w w. e f y m ag . co m                                                               e l e c t ro n i c s f o r yo u • s e p t e m b e r 2 0 1 0 • 1 1 7

                                                            tors R2 and R3, preset VR1 and capaci-
                                                            tor C3. The amplitude of the output                    s.c. dwiv

          ere is the circuit of a simple                    pulse can be varied to some extent by
          electric window charger.                          adjusting variable resistor VR1. You
          With a couple of minor circuit                    can vary the frequency from 100 Hz
variations, it can be used as an electric                   to 150 Hz.                                drives pnp transistor T1 into conduc-
fence charger too. A standard 12V, 7Ah                          X1 is a small, iron-core, step-down   tion for the duration of the time period.
sealed maintenance-free (SMF) UPS                           transformer (230V AC primary to 12V,      The collector of T1 is connected to the
battery is required for powering the                        1A secondary) that must be reverse        base of driver transistor T2 through re-
entire unit.                                                connected, i.e., the secondary winding    sistor R5. When transistor T1 conducts,
    Any component layout and mount-                         terminals of the transformer should       T2 also conducts. When T2 conducts, a
ing plan can be used. However,                              be connected between the emitter and      high-current pulse flows through the
try to keep the output terminals of                         ground and the output taken across the    secondary winding of transformer X1
transformer X1 away from the circuit                        primary winding. Switch S1 is used for    to generate a very high-voltage pulse
board. Timer NE555 (IC1) is wired as                        power ‘on’/‘off’ and LED1 works as a      at the primary winding.
a free-running oscillator with narrow                       power-‘on’ indicator. LED2 is used to         This dangerously high voltage
negative pulse at the output pin 3. The                     indicate the pulse activity.              can be used to charge the window
pulse frequency is determined by resis-                         The output pulse from pin 3 of IC1    rails/fences. Ordinary silicon diode
                                                                                                                       D1 (1N4001) protects
                                                                                                                       T2 against high-voltage
                                                                                                                       peaks generated by X1
                                                                                                                       inductance during the
                                                                                                                       switching time.
                                                                                                                           You can replace
                                                                                                                       X1 with another trans-
                                                                                                                       former rating, and, if
                                                                                                                       necessary, replace T2
                                                                                                                       with another higher-
                                                                                                                       capacity transistor. The
                                                                                                                       circuit can be used to
                                                                                                                       charge a 1km fence
                                                                                                                       with some minor mod-
                                                                                                                       ifications in the output
                                                                                                                           Caution. Take all
                                                                                                                       the relevant electri-
                                                                                                                       cal safety precautions
                                                                                                                       when assembling, test-
                                                                                                                       ing and using this high-
                                                                                                                       voltage generator. 

1 1 0 • S e p t e m b e r 2 0 0 8 • e l e c t ro n i c s f o r yo u                                                      w w w. e f y m ag . co m

   T.K. Hareendran                          J2 are two standard RCA sockets. A
                                             home-made security loop can be used                         s.c. dwiv

          he electronic bicycle lock de-     to link these two input points. Around
          scribed here is a worthwhile       50cm long, standard 14/36 flexible
          alternative for bicycle own-       wire with one RCA plug per end is

                                                                          Fig. 3: Lock fitted on the bicycle

                                                                                          tone generator UM3561 (IC1). IC1
Fig. 1: Circuit of electronic bicycle lock                                                remains enabled until power to the
                                                                                          circuit is turned off using switch S1
                        ers who want         enough for the security loop.                or the loop is re-plugged through J1
                        to make their            Fig. 1 shows the circuit of the          and J2.
                        bicycles ‘intelli-   electronic bicycle lock. It is powered           Assemble the circuit on a general-
                        gent’ at reason-     by a compact 9V battery (6F22).              purpose PCB and house in a small tin-
                        able cost. One       Key lock switch S1 and smoothing             plate enclosure. Fit the system key lock
                        of the benefits      capacitor C2 are used for connect-           switch (S1) on the front side of the en-
                        of building it       ing the power supply. A connected            closure as shown in Fig. 2. Place RCA
                        yourself is that     loop cannot activate IC1 and there-          sockets (J1 and J2) at appropriate posi-
Fig. 2: Lock box        the circuit can      fore the speaker does not sound.             tions. Now, mount the finished unit in
                        be used for vir-     When the loop is broken, zener diode         place of your existing lock (as shown
tually any make of bicycles.                 ZD1 (3.1V) receives operating power          in Fig. 3) by using suitable clamps and
     In the circuit, input jacks J1 and      supply through resistor R2 to enable         screws. 

w w w. e f y M ag . co M                                                                 e l e c t ro n i c s f o r yo u • M ay 2 0 0 9 • 8 5

ElEctRonic candlEs                                                                                                       edi
                                                                                                               s.c. dwiv
   Raj K. GoRKhali
                                                         gate trigger circuit components. It is

          ere is a simple circuit that                   basically half-wave AC power being
          can produce the effect of                      supplied to the electric bulb.
          candle light in a normal                           The third part is the power sup-      to provide better flickering effect in
electric bulb. A candle light, as we                     ply circuit to generate regulated 5V      the bulb.
all know, resembles a randomly                           DC from 230V AC for random signal             The random signal triggers the
flickering light. So, the objective of                   generator. It comprises a stepdown        gate of SCR1. The electric bulb gets
this project activity is to produce a                    transformer (X1), full-wave rectifier     AC power only for the period for
randomly flickering light effect in an                   (diodes D3 and D4), filter capacitor      which SCR1 is fired. SCR1 is fired
electric bulb.                                           (C9), followed by a regulator (IC5).      only during the positive half cycles.
    To achieve this, the entire circuit                      The random signal generator of        Conduction of SCR1 depends upon
can be divided into three parts. The first               the circuit is built around an 8-bit      the gate triggering pin 3 of IC2, which
part comprises IC1 (555), IC2 (74LS164),                 serial in/parallel out shift register     is random. Thus, we see a flickering
IC3 (74LS86), IC4 (74LS00) and the as-                   (IC2). Different outputs of the shift     effect in the light output.
sociated components. These generate a                    register IC pass through a set of logic       Assemble the circuit on a general-
randomly changing train of pulses.                       gates (N1 through N5) and final out-      purpose PCB and enclose it in a suitable

Fig. 1: Circuit diagram for electronic candle

                                The second               put appearing at pin 6 of gate N5 is      case. Fix bulb and neon bulb on the
                             part of the circuit         fed back to the inputs of pins 1 and      front side of the cabinet. Also, connect
                             consists of SCR1            2 of IC2. The clock signal appears        a power cable for giving AC mains
                             (C106), an electric         at pin 8 of IC2, which is clocked by      supply to the circuit for operation. The
                             bulb connected              an astable multivibrator configured       circuit is ready to use.
                             between anode of            around timer (IC1). The clock fre-            Warning. Since the circuit uses
Fig. 2: Pin configurations
                             SCR1 and mains              quency can be set using preset VR1        230V AC, care must be taken to avoid
of C106 and 7805             live wire, and              and VR2. It can be set around 100 Hz      electric shock. 

1 4 2 • J a n ua ry 2 0 1 0 • e l e c t ro n i c s f o r yo u                                                         w w w. e f y m ag . co m
                            ElEctronic card-
                              lock systEm
        he circuit presented here can       be on and other photo-transistors will        as buffer with Schmitt trigger. All outputs
        be used as a lock for important     be in off state. When transistor T1 is        (Q1 through Q7) of this IC are connected
        electronic/electrical appliances.   on, its collector voltage falls, making       to IC2 (ULN2003) which is used as relay
When card is inserted inside its mecha-     transistor T9 to cut-off. As a result, col-   driver. IC2 consists of seven highcurrent
nism, depending upon the position of        lector voltage of transistor T9 as also       relay drivers having integral diodes. Ex-
punched hole on the card, a particu-        pin 2 of IC1 go logic high. This causes       ternal free-wheeling diodes are therefore
lar appliance would be
switched on.
    The card is inserted just
like a floppy disk inside the
disk drive. This card should
be rectangular in shape
with only one punched hole
on it.
    The circuit uses
eight photo-transistors
(T1 through T8). When
there is no card in the
lock, light from incandes-
cent lamp L1 (40-watt,
230V) falls on all the
photo- transistor detec-
tors. Transistor T8 is
used as enable detector
for IC1 (74LS244). When
light is incident on it, it
conducts and its collector
voltage goes low. This
makes transistor T16 to
cut-off, and its collector
voltage goes high. This logic high on       pin 18 (output Q1) also to go high,           not required.
its collector terminal will inhibit IC1     switching LED1 on. Simultaneously,                When an input of this IC is made logic
as long as light is present on photo-       output Q1 is connected to pin 1 of IC2        high, the corresponding output will go
transistor T8.                              (ULN2003) for driving the relay cor-          logic low and relay connected to that pin
    IC1 will get enabled only when the      responding to appliance 1. Similarly,         gets energised. This switches on a specific
card is completely inserted inside the      if card for appliance 2 is inserted, only     appliance and the corresponding LED.
lock mechanism. This arrangement en-        output pin 16 (Q2) of IC1 will go high-           Once a specific card is inserted to
sures that only the selected appliance is   making LED2 on while at the same              switch on a specific relay, that relay
switched on and prevents false operation    time energising relay for appliance           gets latched through its second pair of
of the system.                              2 via ULN2003. The same is true for           contacts. Thus even when the card is
    You can make these cards using a        other cases/appliances also.                  removed, the specific appliance remains
black, opaque plastic sheet. A small            The time during which card is pres-       on. The same holds true for all other re-
rectangular notch is made on this           ent inside the mechanism, the system          lays/appliances as well. The only way to
card to indicate proper direction for       generates musical tone. This is achieved      deenergise a latched relay after removal
insertion of the card. If an attempt        with the help of diodes D1 through D7         of the corresponding card is to switch off
is made to insert the card wrongly,         which provide a wired-OR connection at        the corresponding switch (S1 through
it will not go completely inside the        their common-cathode junction. When           S7) which would cut-off the supply to the
mechanism and the system will not           any of the outputs of IC1 is logic high,      desired relay.
be enabled.                                 the commoncathode junction of diodes              The +5V and +12V supplies can be
    When card for any appliance (say        D1 through D7 also goes logic high,           obtained with conventional arrangement
appliance 1) is completely inserted in      enabling IC3 (UM66) to generate a mu-         using a step-down transformer followed by
the mechanism, the light will fall only     sical tone.                                   rectifier, filter and regulator (using 7805
on photo-transistor T1. So only T1 will         In this circuit IC1 (74LS244) is used     and 7812 etc).

                                                                                                 ELECTRONICS PROJECTS Vol. 22   13

electronic Dice USing At89c2051
   DebDoot Sheet
                                                      counter variable, which, on being inter-

        his simple circuit demonstrates               rupted by an external trigger, latches                           sani the
        the capability of an AT89C2051                the counter value and displays a ran-
        microcontrol-                                 dom number through its output ports.
ler chip to function                                  This method is similar to the one used
as a random number                                    in PCs or calculators for generating ran-
generator based on                                    dom numbers at any instance.                   and uses an 18MHz crystal to generate
the flying counter                                        The application of this dice is            the clock (refer Fig. 2). Switch S1 con-
principle. The pro- Fig. 1: Suggested                 similar to the one used in a game of           nected at pin 1 is used as a reset switch.
gram in the chip con- LED arrangement                 dice. The random numbers generated             Interrupt occurs at pin 6 of IC1 on logic
                        for electronic dice
stantly updates the display                           are not displayed numerically, but             0. Switch S2 connected to pin 6 (INT0)
                                                                              represented by         of IC1 is used to trigger an external in-
                                                                              the number of          terrupt to make pin 6 low. It is used as
                                                                              glowing LEDs.          input to generate the random number.
                                                                              The LEDs are           The random number is indicated by
                                                                              the dot repre-         glowing of the LEDs (LED1 through
                                                                              sentation on the       LED7) connected to port pins P1.2-P1.7
                                                                              face of a dice.        and P3.7.
                                                                              Suggested LED              TL0 and TH0 act as free-wheeling
                                                                              arrangement for        counters in auto-increment mode and
                                                                              the electronic         constantly count up from the initial
                                                                              dice display is        value. When the interrupt occurs, the
                                                                              shown in Fig. 1.       value from the counter is latched and
                                                                                  The use of         glowing LEDs indicate the random
                                                                              IC AT89C2051           number generated by the microcontrol-
                                                                              (IC1) module           ler chip. Assembly language is used for
                                                                              in the design is       programming the chip. The Assembly
                                                                              quite simple. It       code listing is self-explanatory.
                                                                              operates off 3-            EFY note. The source code is in-
Fig. 2: Circuit for electronic dice using AT89C2051                           5.5V DC supply         cluded in this month’s EFY-CD and

w w w. e f y m ag . co m                                                                          e l e c t ro n i c s f o r yo u • J u n e 2 0 1 0 • 1 0 5

ELECTRONIC HORN                                                                               S.C. DW

  ASHOK K. DOCTOR                        IC LM3900 (IC1). IC LM3900 has four
                                         independent op-amps (A1 through A4)

         ere’s a simple circuit of an    with a large output voltage swing. It    a low-frequency squarewave genera-
         electronic horn that is built   can work at up to 32V DC.                tor. Op-amp A2 works as an integra-
         around quadruple op-amp            The first op-amp (A1) is wired as     tor, while op-amp A3 works as a

comparator. A2 and A3 together               When power is switched on, a ba-     9V. To generate several different
work as a ‘wandering voltage gen-        sic tone is generated by transistor T2   tones, connect its point A1 to pins
erator’ op-amp. Op-amp A4 is wired       and transformer X1, which is fre-        1, 3, 4, 5, 8, 9, 10, 11, 12 and 13 of IC1
as a buffer and its output provides      quency-modulated by the wandering        and point A2 to pins 1, 2, 3, 6, 8, 11
base current to npn transistor T2. npn   voltage generator, which, in turn, is    and 13.
transistor T2 and audio output trans-    influenced by the low-frequency              The circuit can be used as an auto-
former X1 form a voltage-controlled      squarewave generator.                    mobile horn by using about 10W au-
oscillator.                                  The circuit works off regulated      dio amplifier.

WWW.EFYMAG.COM                                                              ELECTRONICS FOR YOU • JANUARY 2007 • 109

AUTOMATIC LOW-POWER                                                                                     SUNIL K

  S.C. DWIVEDI                                   current through diode D5 and limit-
                                                 ing resistor R16. By adjusting preset

            ere is a white-LED-based             VR1, the output voltage can be ad-
            emergency light that offers          justed to deliver the required charg-
            the following advantages:            ing current.
    1. It is highly bright due to the use            When the battery gets charged to
of white LEDs.                                   6.8V, zener diode ZD1 conducts and
    2. The light turns on automatically          charging current from regulator IC1
when mains supply fails, and turns off           finds a path through transistor T1 to      Fig. 2: Pin configurations of LM317, BD140
when mains power resumes.                        ground and it stops charging of the        and BC548
    3. It has its own battery charger.           battery.
When the battery is fully charged,                   The LED driver section uses a to-      off. On the other hand, when mains
charging stops automatically.                    tal of twelve 10mm white LEDs. All         fails, the base of transistor T2 becomes
    The circuit comprises two sections:          the LEDs are connected in parallel         low and it conducts. This makes all
charger power supply and LED driver.             with a 100-ohm resistor in series with     the LEDs (LED1 through LED12) glow.
The charger power supply section is              each. The common-anode junction of             The mains power supply, when
                                                                                                                     charges the bat-
                                                                                                                     tery and keeps
                                                                                                                     the LEDs off as
                                                                                                                     transistor T2 re-
                                                                                                                     mains cut-off.
                                                                                                                     During mains
                                                                                                                     failure,     the
                                                                                                                     charging sec-
                                                                                                                     tion       stops
                                                                                                                     working and
                                                                                                                     the battery sup-
                                                                                                                     ply makes the
                                                                                                                     LEDs glow.
                                                                                                                     the circuit on a
                                                                                                                     pose PCB and
                                                                                                                     enclose in a
                                                                                                                     cabinet with
Fig. 1: Automatic high intensity LED-based emergency light                                                           enough space
                                                                                                                     for battery and
built around 3-terminal adjustable               all the twelve LEDs is connected to        switches. Mount the LEDs on the cabi-
regulator IC LM317 (IC1), while the              the collector of pnp transistor T2 and     net such that they light up the room.
LED driver section is built around               the emitter of transistor T2 is directly   A hole in the cabinet should be drilled
transistor BD140 (T2).                           connected to the positive terminal of      to connect 230V AC input for the pri-
    In the charger power supply sec-             6V battery. The unregulated DC volt-       mary of the transformer.
tion, input AC mains is stepped down             age, produced at the cathode junc-             EFY lab note. We have tested the
by transformer X1 to deliver 9V, 500             tion of diodes D1 and D3, is fed to        circuit with twelve 10mm white LEDs.
mA to the bridge rectifier, which com-           the base of transistor T2 through a 1-     You can use more LEDs provided the
prises diodes D1 through D4. Filter              kilo-ohm resistor.                         total current consumption does not ex-
capacitor C1 eliminates ripples. Un-                 When mains power is available, the     ceed 1.5A. Driver transistor T2 can de-
regulated DC voltage is fed to input             base of transistor T2 remains high and     liver up to 1.5A with proper heat-sink
pin 3 of IC1 and provides charging               T2 does not conduct. Thus LEDs are         arrangement.

126 • JANUARY 2008 • ELECTRONICS FOR YOU                                                                         WWW.EFYMAG.COM

FlaSher For Deepawali                                                                                    s.c. dwiv

   Sunil Kumar                           The output of IC1 drives transistor T1.
                                              Working of the circuit is simple.

         ere is the circuit for a port-   When output pin 3 of IC1 goes high,
         able electric lamp-cum-LED       transistor T1 conducts to fire TrIAC1          currents.
         flasher. It uses a 25W, 230V     and the bulb glows. Bulb L1 turns off              In brief, the bulb and the LEDs
AC bulb and nine LEDs. When the           when output pin 3 of IC1 goes low.             flash alternately depending on the
bulb glows all the LEDs remain ‘off,’         The collector of transistor T1 is con-     frequency of IC1. Flashing rates of the
and when the LEDs glow the bulb           nected to anodes of all the LEDs (LED1         bulb as well as LEDs can be varied by
remains ‘off.’                            through LED9). So when T1 is cut-off           adjusting potmeter Vr1. Connect the
   The circuit is built around timer IC   the LEDs glow, and when T1 conducts            power supply line (L) of mains to bulb
555 (IC1), which is wired as an astable   the LEDs go off. Current-limiting resis-       L1 via switch S1 and neutral (N) to
multivibrator generating square wave.     tor R4 protects the LEDs from higher           MT1 terminal of TrIAC1.
                                                                                                   A 12V, 200mA AC adaptor is
                                                                                               used to power the circuit. Using
                                                                                               switch S1, you can switch off the
                                                                                               bulb permanently if you do not
                                                                                               want it to flash.
                                                                                                   Assemble the circuit on a
                                                                                               general-purpose PCB and en-
                                                                                               close in a circular plastic cabinet
                                                                                               keeping the bulb at the centre
                                                                                               and LEDs at the circumference.
                                                                                               Drill holes for mounting the
                                                                                               ‘on’/‘off’ switch. Use a bulb
                                                                                               holder for bulb L1 and LED
                                                                                               holders for the LEDs. Also use
                                                                                               an IC socket for timer IC 555.
                                                                                                   Warning. While assembling,
                                                                                               testing or repairing, take care to
                                                                                               avoid the lethal electric shock. 

w w w. e f y m ag . co m                                                       e l e c t ro n i c s f o r yo u • N o v e m b e r 2 0 1 0 • 1 1 5


             S.C. DW


         his unidentified flying object
         (UFO) is nothing but an elec-
         tronic toy depicting the fantacy.
It comprises three separate sections,
viz, rim flasher, dome flasher and
sound generator.
    The rim flasher is a simple sequen-
tial circuit built around timer IC 555
                                             Fig. 1: Rim flasher
(IC1) and decade counter IC CD4017
(IC2) as shown in Fig. 1. IC1 is wired
as an astable multivibrator whose out-
put is fed to clock pin 14 of decade
counter IC2. All the eight outputs of
IC2 are connected with two LEDs each.
These 16 LEDs (LED1 through LED16)
are arranged round the rim of a fly-
ing-saucer-like toy. The colour of LEDs
used may be yellow, pink orange or
even white to give a good colour ef-
    The dome flasher circuit is built
around a 14-stage ripple-carry binary
counter and oscillator IC CD4060 (IC3)
as shown in Fig. 2. Three outputs are
used here. Three groups of LEDs with
six LEDs in each are arranged such that
each group flashes at a different rate.
Preset VR1 (47-kilo-ohm) is used to
                                             Fig. 2: Dome flasher
vary the flash cycle.
    These 18 LEDs (LED17 through
LED34) are arranged around the grove
(disk) of a general-purpose PCB or
veroboard, which is covered by a
transparent dome. Use different-
coloured LEDs for each group to cre-
ate the required light effect. Red, blue,
yellow or green LEDs will create a nice
effect. If a transparent dome is not pos-
sible, drill holes around the top to fix
the LEDs.
    The sound generator is built
around two 555 timers, two transis-
tors and some discrete components as
shown in Fig. 3. Timer IC5 is config-        Fig. 3: Sound generator

WWW.EFYMAG.COM                                                         ELECTRONICS FOR YOU • JULY 2007 • 95
                                                                                                         ameter each. Make sure
                                                                                                         that bowls have rims to
                                                                                                         facilitate fixing of LEDs
                                                                                                         with small screws. For
                                                                                                         fixing the LEDs, refer to
                                                                                                         Fig. 4. Assemble the rim
                                                                                                         flasher, dome flasher
                                                                                                         and sound generator cir-
                                                                                                         cuits on separate gen-
                                                                                                         eral-purpose PCBs and
                                                                                                         mount these on the deep
                                                                                                         bowls along with batter-
                                               Fig. 5: Assemble unit of unidentify bird
                                                                                                         ies and speaker. PCB1,
                                               pin 5 of timer IC4. The rectangu-          PCB2 and PCB3 are for rim flasher,
Fig. 4: Fittings of LEDs on rim
                                               lar-wave output at pin 3 of timer          dome flasher and sound generator, re-
ured as an astable multivibrator. The      IC5 is fed to transistor BC548 (T1) to         spectively.
charge-discharge cycle of capacitor C8     operate timer IC4, which is also an                The        assembled           flying
(47µF) generates a sawtooth waveform       asymmetrical multivibrator. If a 75-           saucer is shown in Fig. 5. When you
which rises rapidly but falls slowly.      ohm-impedance speaker is available,            switch on the circuit, rim LEDs and
This waveform is fed to the base of        there is no need to use resistor R16 (68       dome LEDs flash, and at the same
transistor T2 (BC327), which is an emit-   ohms).                                         time, a sound is generated. This gives
ter follower. Its output is used to con-      For assembling the circuit, use two         the simulated effect of an unidentified
trol frequency modulation. It is fed to    deep, plastic bowls of about 20 cm di-         flying object.

96 • JULY 2007 • ELECTRONICS FOR YOU                                                                         WWW.EFYMAG.COM

Four-StaGe FM tranSMitter                                                                                          s.c. dwiv

   PradeeP G.                            the pre-driver stage. You can also use
                                          transistor 2N5109 in place of 2N2219.

        his FM transmitter circuit uses   The preamplifier is a tuned class-A RF       frequency generated. You can also use
        four radio frequency stages:      amplifier and the driver is a class-C        a 12V battery to power the circuit.
        a VHF oscillator built around     amplifier. Signals are finally fed to the        Assemble the circuit on a general-
transistor BF494 (T1), a preamplifier     class-C RF power amplifier, which de-        purpose PCB. Install the antenna prop-

built around transistor BF200 (T2), a     livers RF power to a 50-ohm horizontal       erly for maximum range.
driver built around transistor 2N2219     dipole or ground plane antenna.                   Coils L1 through L5 are made with
(T3) and a power amplifier built              Use a heat-sink with transistor          20 SWG copper-enamelled wire wound
around transistor 2N3866 (T4). A con-     2N3866 for heat dissipation. Carefully       over air-cores having 8mm diameter.
denser microphone is connected at the     adjust trimmer VC1 connected across          They have 4, 6, 6, 5 and 7 turns of wire,
input of the oscillator.                  L1 to generate frequency within 88-          respectively.
    Working of the circuit is simple.     108 MHz. Also adjust trimmers VC2                 EFY note. This transmitter is meant
When you speak near the microphone,       through VC7 to get maximum output            only for educational purposes. use of
frequency-modulated signals are           at maximum range.                            this transmitter with outdoor antenna
obtained at the collector of oscillator       Regulator IC 78C09 provides stable       is illegal in most parts of the world. The
transistor T1. The FM signals are am-     9V supply to the oscillator, so variation    author and EFY will not be responsible
plified by the VHF preamplifier and       in the supply voltage will not affect the    for any misuse of this transmitter. 

w w w. e f y M ag . co M                                                          e l e c t ro n i c s f o r yo u • M a r c h 2 0 1 0 • 1 0 3

  DR C.H. VITHALANI                        built around 3-terminal adjustable
                                           regulator LM317.                                               IVEDI
                                                                                                   S.C. DW

         his simple automatic emer-            In the inverter section, NE555 is
         gency light has the following     wired as an astable multivibrator that
         advantages over conventional      produces a 15kHz squarewave. Out-
emergency lights:                          put pin 3 of IC 555 is connected to the
     1. The charging circuit stops auto-   Darlington pair formed by transistors      minals of each side of the tubelight
matically when the battery is fully        SL100 (T1) and 2N3055 (T2) via resis-      and then connect to the secondary of
charged. So you can leave the emer-        tor R4. The Darlington pair drives fer-    X1. (You can also use a Darlington pair
gency light connected to AC mains          rite transformer X1 to light up the        of transistors BC547 and 2N6292 for a
overnight without any fear.                tubelight.                                 6W tubelight with the same trans-
     2. Emergency light automatically          For fabricating inverter transformer   former.)
turns on when mains fails. So you          X1, use two EE ferrite cores (of               When mains power is available, re-
don’t need a torch to locate it.           25×13×8mm size each) along with plas-      set pin 4 of IC 555 is grounded via
     3. When mains power is available,     tic former. Wind 10 turns of 22 SWG        transistor T4. Thus, IC1 (NE555) does
emergency light automatically turns        on primary and 500 turns of 34 SWG         not produce squarewave and emer-
off.                                       wire on secondary using some insula-       gency light turns off in the presence
     The circuit can be divided into in-   tion between the primary and second-       of mains supply.
verter and charger sections. The in-       ary.                                           When mains fails, transistor T4
verter section is built around timer           To connect the tubelight to ferrite    does not conduct and reset pin 4 gets
NE555, while the charger section is        transformer X1, first short both ter-      positive supply though resistor R3. IC1
                                                                                             (NE555) starts producing square
                                                                                             wave and tubelight turns on via
                                                                                             ferrite transformer X1.
                                                                                                 In the charger section, input
                                                                                             AC mains is stepped down by
                                                                                             transformer X2 to deliver 9V-0-
                                                                                             9V AC at 500 mA. Diodes D1
                                                                                             and D2 rectify the output of the
                                                                                             transformer. Capacitors C3 and
                                                                                             C4 act as filters to eliminate
                                                                                             ripples. The unregulated DC
                                                                                             voltage is fed to IC LM317 (IC2).
                                                                                             By adjusting preset VR1, the
                                                                                             output voltage can be adjusted
                                                                                             to deliver the charging voltage.
                                                                                                 When the battery gets
                                                                                             charged above 6.8V, zener diode
                                                                                             ZD1 conducts and regulator IC2
                                                                                             stops delivering the charging
                                                                                                 Assemble the circuit on a
                                                                                             general-purpose PCB and en-
                                                                                             close in a cabinet with enough
                                                                                             space for the battery and
                                                                                             switches. Connect a 230V AC
                                                                                             power plug to feed charging
                                                                                             voltage to the battery and
                                                                                             make a 20W tube outlet in the
                                                                                             cabinet to switch on the

100 • FEBRUARY 2007 • ELECTRONICS FOR YOU                                                                WWW.EFYMAG.COM

Generator rooM liGht                                                                                               s.c. dwiv

   Manuj Paul                                           erator room light, while Fig. 2 shows
                                                         the battery charger circuit, which is        IC1 remain high, making output pin 3

          t night when power fails, one                  optional and can be omitted if the gen-      of IC1 low and transistor T2 cut-off. So
          finds it difficult to reach the                erator is self-start type and has built-in   lamp L1 connected between the collec-
          generator to start it. Here is                 battery.                                     tor of T1 and the positive terminal of
the circuit for a generator room light                       At the heart of the generator room       12V supply does not glow.
that automatically turns on at night,                    light circuit (Fig.1) is a light-dependent       As the ambient light fades dur-
facilitating easy access to the generator.               resistor (LDR1) that senses the ambi-        ing sunset, the resistance of LDR1
During daytime, the light remains off.                   ent light as well as light from glowing      increases. As a result, the voltage drop
    Fig. 1 shows the circuit for gen-                    LED1.                                        across LDR1 increases and npn transis-
                                                                                                                 tor T1 conducts. Pins 2 and
                                                                                                                 6 of IC1 go low to make its
                                                                                                                 output pin 3 high, and lamp
                                                                                                                 L1 glows.
                                                                                                                     You can replace incan-
                                                                                                                 descent lamp L1 with bright
                                                                                                                 white LEDs using proper
                                                                                                                 current-limiting resistors.
                                                                                                                     Assemble the circuit on
                                                                                                                 a general-purpose PCB and
                                                                                                                 enclose in a suitable cabinet.
                                                                                                                 Install the unit near the gen-
                                                                                                                 erator. Arrange LED1 and
                                                                                                                 LDR1 such that during the
                                                                                                                 availability of mains, light
                                                                                                                 emitted from LED1 falls di-
                                                                                                                 rectly on LDR1. Also, make
                                                                                                                 sure that during daytime
                                                                                                                 the ambient light falls on
                                                                                                                 the LDR.
                                                                                                                     For powering the battery
Fig. 1: Circuit for generator room light                                                                         charger circuit (Fig. 2), 15V
                                                                                                                 AC secondary voltage is
                                                                                       During         derived from step-down transformer
                                                                                   daytime, sun-      X1. For fast charging of the battery,
                                                                                   light or light     you may increase the current rating of
                                                                                   from LED1          transformer X1.
                                                                                   reduces the            The charger charges the battery
                                                                                   resistance         through a thyristor (SCR1) when the
                                                                                   of LDR1. As        battery voltage is low. The thyristor
                                                                                   a result, the      gets a regulated gate voltage from the
                                                                                   voltage drop       zener diode, and goes to tickle charg-
                                                                                   across LDR1        ing mode when the battery voltage
                                                                                   decreases and      nears the zener voltage.
                                                                                   npn transistor         Assemble the charger circuit on a
                                                                                   T1 does not        general-purpose PCB and enclose in
                                                                                   conduct. The       a suitable cabinet. Use two crocodile
                                                                                   collector of T1    clips (red for positive and black for
                                                                                   and therefore      negative) for connecting the battery
Fig. 2: Battery charger circuit (optional)                                         pins 2 and 6 of    terminal to the charger circuit. 

8 4 • J u ly 2 0 0 9 • e l e c t ro n i c s f o r yo u                                                                   w w w. e f y m ag . co m

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