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Fun Projects 21 Chapter

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                  TOUCHSWITCH(E)
Small          lamps motors becontrolled a simple
     appliances,    and      can             by      touchwith
thisexperimental     Loads ONduring timeadiusted Pl fora
              circuit.   are           the           by
fewseconds more
          to     than10minutes.           byusing NC(normally
                                conversely,      the
closed)       youcanalsoturnotf an appliance theadjusted
       contacts                            after        time.

Thesensor formed twosmallmetal
          is     by            plates, shouldn't placed
                                     and      be      far
             The    plates finger-touchedthesamemoment-
fromthecircuit. metal    are            at

Thecircuit be powered a 6- or 12-volt
          can         from            power      depending
                                           supply,         on
therelay     Do                  power
        used. notusetransformeless supplies  withthisproiect'
                                                           to
avoidshocks dangerous
            or               Transformerless supplies not
                       shorts.            power        are
isolated theAc-power andrepresent dangerous
       from           line           a         shock hazard.

K1is a relay with6- or 12-volt andis energized current to 50 mA'
                             coil            by      up
Withrelays this range can power touchswitch
            in           you         the          fromAA cells(6
volts) small
      or       Power suPPlies.

A schematic theTouch
             of                                        uses 555
                        Switch shown Figurcl.The circuit
                               is     tn                   a
lC timeras itsheart, timedelay determined adjusting
                    and           is        by        P1'

circuitmounting,using small
                     a     homemade            board, shown
                                   printed-circuit  is      in
Figure2.lf you:d to etchyourownboard thispartsplacement
                like                  use                 dia-
gram guide.
      as
ln Figure3we  show sensor canbemade twosmall
                   the     that          with           plates
                                                    metal
ona-plastic wood
           or                        shouldn't long. maximum
                       Wires thesensor
                   base.    to                be    A
lenght 10feetis recommendedavoid
       of                     to         operation.
                                   erratic

       of                                                and
                                                capacitors
                    components, astheelectrolytic
 Pos16n the polarized           such
       should observed.
 diodes,     be         Loadis connected terminals and B. Re-
                                       to        A
 member the relay
        that       actsas a switch.

 Don't    this                                    danger-
                 fromtransformerless supplies avoid
     power circuit                power     to
 ousshocks shorts.
         and
 A sensitivity                                  manner. a
            control be added thiscircuit a simple
                  can       to         in            use
 1,000,000 potentiometer thistask.
          ohm           for
              -            :       t        !



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        R3                         lr \ ^
                                      <B
        10K

                                      lfi
                                 a2
                                 8C548       B1
                                            a12V
 R1
100K                                  c2
                                   lOO!f
 Q1
8C548
                  (.) sEE TEXT




              Figure 1




              Figure2

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                    PartsList -Touch Switch                       I



      lC1   - 555 integrated      timer
                           circuit,

      Q1,2 - 8C548general-purpose transistors
                               NPN

      D1    - 1N914     diode
                  silicon

      Kl    - relay- see text

      Rl    - 100,000          resistor
                    ohm, 114W,5%

      R2    - 47,OOO         resistor
                   ohm,114W,5%

      R3    - 10,000         resistor
                   ohm,114W,5%

      R4    - 1,000ohm,1l4W,5olo
                              resistor

      P1    - 1,000,000         resistor
                      ohm,114W,5%

      C1    - 10 to 470 uF,12 WVDCelectrolytic       -
                                             capacitor see text

      C2    - 100 uF,16 WVDCelectrolytic
                                       capacitor

      S1    - SPSTtoggleor slideswitch

      81    - 6 or 12V- M cells,batteryor powersupply



                           ldeasto Explore
Togetmoreperformance to learnmore
                  or                 the
                                 about circuit:
                                                         tran-
      o Sensitivity beincreased replacing witha Darlington
                 can          by        Ql
           such a BC517.
      sistor,  as
      o Byconnectingreedswitch
                      a       betweenpoints andX youcanuse
                                          Y
                do
      thiscircuit       movement. a smallmagnet the piece
                   detect      Fix            on         to
      acton the reedswitch.

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                            '3

                           Figure 3


      and      applications:
Science uncommon
    . Thiscircuit be usedto close trap. touch
                can              a    The         will
                                             sensor de-
                    enters trap.
    tectwhentheanimal    the
    o Projects needa wayto detect
               that                        switching load,
                                  movements,       any
    canbe       with
          made thiscircuit.                 like
                                kind sensors, microswitches,
                          Several of
    pendulum  sensors anyothermomentary-type canbe used
                     or                   switch
    inthiscircuit.
    o Connect LDRbetween
               an           points andB.Alsoconnect 100,000
                                 A                    a
    ohmpotentiometer                       pole
                    between andthenegative ofthepower
                           X                               sup-
    ply.    have light-operated or remote
       You'll    a           relay                A
                                           control. flashof light
    produced a lantern evena cigarette
             by        or              lighter close relay,
                                             can       the
    powering load. circuit be used detect
            the     The     can        to      flashes lightin
                                                       of
    scienceexperiments. theLDRintoa telescope youcandetect
                      Fix                     and
    meteorfallings.
    a Experiments        thunderstorms nightcan be conducted
                 involving            at
                            The
    automatically thiscircuit. firstflashof lightning be used
              using                                 can
    to power theequipment.
           ON



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