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Fun Projects 28 Chapter

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               LEDBARGRAPH(E/P)
Thiscircuit     ten
          drives LEDsin response an analog
                                 to           voltage      to
                                                     applied its
inputandcan be usedto provide visualindication the instantaneous
                             a                of
power being developed an audio
                    by                lt canalsobe used provide
                              amplifier.               to
     indication variable
visual        of            in
                       events science experiments.
Youcanextend project using chips you'llbe ableto continu-
            the    by     two     and
    monitor audio
ously     the   outputpower both
                          of    channels yourstereo
                                       of         system.
Thecircuit be used
         can      withamplifiers     from
                               ranging 0.1to 100watts.
In some      you
       cases should a small
                     use      transformer, as in Figurc1,to
                                         wired                     l
isolate inputandmatch
      the             impedances. ls necessary
                                This           whentheampli-       j
fierhasa low-output
                 impedance.
                          Resistor is chosen
                                 R,         according theout-
                                                    to             I
putpower theamplifier.
          of         Values given Table
                          are     in     1.

                              Table1
          Amplifier    Power
                  Output                    Rx
                                            -
             0.1 to 1 watt

              1 to 5 watts         10 ohms, ll2watl

             5 to 20 watts             22 ohms,1 watt

             20 to 50 watts            47 ohms,1 watt

            50 to 100 watts            100ohms,2watts



A schematic     of              is
         diagram the LEDbargraph showninFigure2.Thecircuit
             lC
usesan LM3914 to drivethe LEDs,        voltages oneof ten
                              converting      into
outputs.
Thecomponents placed a printed-circuit
                 are       on                         in
                                         board shown Figure3.
                                              as
lf youd liketo buildyourcircuit same
                              the       you
                                     way, canetchyourownboard
      the
using solder component-side
                and               templates    in
                                          shown thisfigure.
                                          F!|ffi 2t llDlrlrruh
I




    (-)SEETABLE        8:1000o
                     TRANSF'RMER




                     Figure I

                                                 +6112Y




     18   't7   16    15   14   13   12     11 . 10
                       tcl
                      1M3914
                         5
                             D1   R1
                            1N914 10K




                     Figure2


                                                             fn
HmHdcctslrrilcHpilmmu




                               Figure 3


P2 adjusts dynamic
          the       rangein whichthe circuit       P1
                                           operates. adjustssensi-
tivityanddepends the application. cansetthevalueto a fixedpointby
               on               You
usinga trimmerpotentiometer.

Audiocomesfrom the loudspeaker                          Any
                                  outputof the amplifier. smalltrans-
                                                       coil
formerwith a high-impedance and a low-impedance can be used.
                             coil
Any smalltransformer with a 117VAC primary,   secondary  ratedfrom 5 to
12.6volts,and currentratingsbetween  100and 500 mA can be used.

Position the polarized
       of                      suchas LEDs,diode,and electrolytic
                     cornponents,
         shouldbe observed.
capacitor,

Bass response provided C1.Youcan alter this component
              is       by                            withinthe
rangegivenin the schematic
                         diagram achieve
                                to      bestperformance.

Remember the highest
         that       applied       to                exceed
                           voltage the inputshouldn't     the
powersupplyvoltage.



ltf
                                              FililGct ltB hntiln
                                                     28

                   PartsList - LEDBargraph
      lC1   - LM3914         circuit(National)
                    Integrated

      LEDs - Common
                  red,yellow green
                            or

      D1    - 1N914 1N4148
                  or           purpose
                         general           diode
                                     silicon

      Rl    - 10,000          resistol
                   ohm, 114W,5%

      R2    - 470ohm,114W,5%
                           resistor

      P1    - 10,000
                   ohm potentiometer

      P2    - 4,700ohmtrimmer
                            potentiometer

      C1    - 1 to 10 uF,12 WVDCelectrolytic       -
                                           capacitor see text

                         ldeasto Explore
To learnmoreaboutthe circuitor to get betterperformance:
      o Determine voltagelevelthat turns on each LED usinga DC
                 the
      powersupplyplugged the inputof the device.
                        to

      . By wiringprobesat the positivepowersupplyline and the input
                                                  or          meter.
      you can usethis circuitas a digitalresistance conductance

SciencepQects:

      o Thiscircuitcan replace common
                              the      multimeterto         voltage
                                                     indicate
      variations experiments. visualetfectis better.
               in            The

                                                      or
      o lt can alsobe usedin the outputof a lb detector bio-feedback.

      o Conectan LDRbetween inputand (+)for a digitalphotometer.
                              the
                                            (+)
      Fora digitalthermometer an NTCbetween and the input.
                            wire

                                                  in
      o Thiscircuitcan also replace LEDand resistor the Continuity
                                  the
      Testerand sameexperiments, advantages, be performed.
               the                  with         can


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