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Fun Projects 23 Chapter

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         lor

                     OFF RELAY (P)
         TOUCHON/TOUCH
                                                                              I
AO-powered  appliances, suchas lamps,smallhomeappliances,     toolsand
otheritems,can be controlled yourfingertips
                             by             usingthis circuit.Also,test
equipment experiments be controlled thiscircuit.You turnon
          and              can            by                 can
                         time sensorX1, then turn it otf by touching
any load by first touching                                           Xl
again.Thereis no shockhazard, the controlcircuitis completely
                               as                               isolated
from the AC powerline.

The controlcircuitis powered  from four or eightAA cells,or, if you prefer,
                                                                          a
powersupplyranging                               to
                      from 6 lo 12 voltsaccording the relaycoil.
                      powersupplies
Don'tusetransformerless            because                   from
                                          theyare not isolated
the AC powerlineand can causesevereshocksand shorts.

The schematic diagram theTouch
                     of          OnffouchOff Relayis givenin Figure 1.
The circuitusestwo lOs.The 555 timerworksas a monostable,   producing
one controlpulsewhenthe sensoris touched. The 4013 is a flip-flopthat
controls relayby a transistor.
        the

Components  placement a homemade
                       on             printed-circuit
                                                   boardis as shownin
Figure2. Component  layout shouldbe altered the relayusedin the project
                                           if
is an equivalent the type indicated partslist.
               of                 in

Relaycoil voltageis determined the powersupplyvoltageand the load
                              by
currentrequirement. can use a mini 1A DPDT(RadioShack275-2491
                   You
12V,280                                                      type
          ohm,43 mA,andwireit as shownin thefigure, useanother
                                                  or
        depending the loadrequirements.
of relay,         on
                    (RadioShack 275-278) suitablefor heavyduty
A 10A SPDT mini-relay                  is
appliances.

As a simplerule,you can use a 6- or 12-volt
                                          relaywith coil currentsranging
from 10 to 100mA andcontacts to 10A,according the taskyou havein
                              up                  to
mind.

Positions the polarized
          of          components,                             capaci-
                                  suchas the diode,electrolytic
                  shouldbe observed.
tors and transistor,


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           Figure 1

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                             Figure 2


Sensor the Same the'Touch
     are       as              pQect described this book.
                         Switch"             in

Don't use metallicbox to housethe device,as there are parts connected
directly the AC powerline.Be sure that thereare no powerline contacts
        to
withthe lowvoltage circuitto avoidshocksand dangerousshorts.

The loadis connected shownin Figure 3, to be ON whenthe relaycoil is
                      as
          You                      closedcontacts turn otf a load
energized. can also use the normally              to
whenthe relaycoil is energized.

Remember currentrequirements highwhenthe relaycoil is ener-
           that                    are
gizedand lowwhenthe transistor is otf (relaycoil nonenergized). is
                               Q2                            That
an importantfactorto consider you are usingbatteries powerthe unit.
                            if                     to



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                                         20
                                    ErleG[ Iffi mtltrEl0rr.trr

After aresure youhave
    you        that      wired parts thecircuit
                             all   of                 plug
                                              correctly, the
power                    You
     cordintothewallsocket. cannowtestthecircuit.
Connect loadto theoutput
       a                (relay                    The
                             contacts) touch sensor. load
                                    and     the
should
     be powered if all is OK.
                ON



           PartsList -Touch On/Touch Relay
                                   Off
     lC1                  circuit- timer
           - 555 Integrated

     lC2                       circuit- dualflip-flop
           - 40138CMOSintegrated

     Q1,2 - 8C548General-purpose silicon
                              NPN      transistors

     D1    - 1N914             silicon
                  General-purpose     diode

     K1    - Relay- see text

     R1,2, 5     - 100,000         resistors
                         ohm,114W,5%

     R3,4 - 47,OOO          resistors
                 ohm,114W,57o

     RO    - 1,000          resistor
                  ohm,114W,5%

     Cl    - 1 uF,25 WVDCelectrolytic
                                    capacitor

     C2    - 0.1 uF ceramic metalfilm capacitor
                           or

     Cg    - 4.7 uF,12 WVDCelectrolytic
                                      capacitor

     C4    - 100 uF,16 WVDCelectrolytic
                                      capacitor

     S1    - SPSTtoggleor slideswitch

     81    - 6 or 12V (according relay)- see text
                               to
                 -
     X1,2 - sensor seetext



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                             Figure3



                          ldeasto ExPlore
                                     performance:
Tolearnmoreabout circuit to get better
               the     or
     o Betterperformance be obtained replacing with a
                          can         by     Ql
              transistor, as a 8C517.
     Darlington        such
     o A 100,000 potentiometer
                 ohm         placedbetween base Q1and
                                          the      of
                                                  adjustment
                           supply givea sensitivity
     thenegative of thepower
                pole            will
     forthecircuit.
               how                           works, thefunc-
     o Explain a flip-flop, usedin thiscircuit,
                           as                      and
     tionof theC3/R5                   Explain thecurrent
                    network thisproject.
                          in                 why         flow-
     ingthrough fingers
                the     when sensor touched
                             the      is             cause
                                                doesn't    any
     shock  hazard.
      o Depending theapplication, canpower theloadwhen
                  upon              you         oN
                        (to
      relay notenergized reduce
          is                     power              is possible
                                      consumption).That
                      closed
      usingthe normally     contacts the relay.
                                   of



                  -
u
                               PililiGr2E lffim/rdclffirffiu

                      uses:
      projects uncommon
Science      and
                                    pointsX1 andX2 you can
    . By connecting reedswitchbetween
                   a
                to
    usethecircuit detectmovement mechanical inexperiments
                               of        items
    or projects. atfix magnet themoving to actontheswitch.
              Just    a     on        item
                                               by
    o Experiments animals beconducted thiscircuit using
                with     can         with
    appropriate               between andX2.
                      connected
             transducers            X1
    o An LDRcan be usedto control circuit
                                  the    witha flashof light.A
    lantern,       lighteletc., beused turnappliances or otf.
           cigarette          can     to             on

				
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