Fun Projects 27 Chapter

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                           (VP)
                      TESTER
             coNTrNUrrY
                    to                    such       resis-
                                 components as diodes,
Thiscircuit be used testelectronic
            can
tors,coils,         lamps,
          transformers,  fuses,      loudspeakers many
                              switches,         and    oth-
ers.          of
    Thestate thecomponent under is indicated an LED.
                               test        by
When probes separated
      the       are                the
                            orwhen resistance       themisvery
                                              between
high (morethan1,000,000                                  is
                        ohms) LEDis otf.When resistance low
                              the             the
(thetransistor in a conduction
            is                    the
                             state) LEDis on.
Thecircuit powered a 6- or 9-volt
          is       from               (AA    or      Current
                                 supply cells battery).
drainis lessthan10 mAwiththe LEDon.R2value depends the power
                                                 on
supplyvoltage. a 9V supply recommend or 1,000
              For          we          820      ohmsfor R2.
          diagram theContinuity
A schematic      of             Tester given Figure1.Twotran-
                                     is     in
     are                  pair.
sistor wiredas a Darlington Theconfiguration        the
                                              reduces current
flowbetweenprobes microampere
                to            values.
Components     placement a terminal
                        on         strip,usedas chassis, shownin
                                                       is
                           to      positions thetransistors LED.
Figure2. lt is veryimportant observe       of             and
House device a smallplastic anduseprobes alligator to
     the    in            box          or        clips
connect components
      the       under test.

                PartsList - ContinuityTester
      Ql, 2 - 8C558general-purpose silicon
                                PNP      transistors
      LED - Common green yellow
                 red,   or     LED
      R1   - 100,000         resistor
                   ohm,114W,5%
      R2   - 470ohm,114W,5%      -
                          resistor seetext
      S1   - SPST             switch
                 slideor toggle
      81   - 6Vor 9V (four cellsor battery)
                         AA
      PPl,2 - Redandblack
                        probes


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Figure I




Figure2


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          performance to learnmore
Togetbetter        or                 circuit devices:
                                  about     and
     o Invert power     poles LEDposition useNPNtransis-
                   supply     and          and
         Will
     tors. thecircuit                  version? transistor
                     operate theoriginal
                            as                A         such
                 can       in
     asthe8C548 beused thisexperiment.You alsoreplace
                                            can           the
                          x
     LEDandR1witha 6-volt 50 mAsmall incandescentlamp.
     o Determine highest
               the     resistance canbe connected
                                that            between
     probes causes LEDto illuminate.
          that     the
      projects:
Science
     o Conductability
                   of chemical        can        or
                            substiances bedetected demon-
     strated thiscircuit. twobare
           with         Use       wires probes.
                                       as
     o By replacing LEDwitha 0-200 microammeter canuse
                    the                uA               you
     thiscircuit a liedetector.
               as             Small         in
                                   changes skinresistance becan
             in
     detected thisway.  Skinresistance
                                     alterswithstress bythe straln
                                                     or
     caused an interrogation
            by               session.
     o Experiments plantscan be conducted
                    with                       involving
                                                       alteration in
     the resistance
                  between  electrodesplaced theirleafs. An interest-
                                          on
     ing studyaboutthis subject foundin the book The Secret of
                               is                            Life
            by
      Plants PeterTompkins Christopher
                            and           Bird.Theauthors associate
              phenomena plants a rudimentary
     electrical          in       to            nervous system.




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