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Fun Projects 20 Chapter

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                          (E/P)
                MrcRo RADIO
                     AM
        AM    willtune
Thismicro radio           stations thefrequency
                     strong      in               between
                                              range
530and1,600kHz.
Thecircuitusesonlyonetransistor the amplifier witha transformer
                              as            and
drives small
      a               As         is
            loudspeaker.thecircuit verypoor amplification,
                                           in           it needs
                 The
a longwireantenna. antenna  should from15to 50 feetlongforbest
                                   be
results. goodground
       A                    is
                   connection alsoimportant.
Of course, is a smallinexpensive
         this                  transistorradio               vol-
                                              withlowlistening
ume,particularly a weakstation. the station too weak,replace
              on              lf           is                the
SPKR a low-impedance
      by               earphone.
Thepower      is                                 drain very
        supply formed twoor fourAA cellsandcurrent
                    by                                is
   extending life. circuit
low,       their The           only
                         drains fewmicroamperes.
L1 is a tapped        connected variable
              loopstick,       to       capacitor Theseitems
                                                CV.
canoftenbe foundin non-working        radios, wellas the speaker
                             transistor     as
andtransformer.
FigureI shows schematic
             the       diagram thisproject.
                               for              Dl
                                           Diode actsas a
                                T1
detector Q1 as an audioamplifier. is a common
        and                                           output
                                             transistor
transformer.
ln Figure2 we showthe components      All
                                layout. the components be
                                                    can
housed a small
       in      plastic A terminal is used chassis anexperi-
                     box.       strip     as     in
mentalversion.
T1 is a transistor       (1,000:8) the SPKR a miniature
                transformer     and        is          type.
                              you
But,if youhave unworking radio cangetthesepartswithout
              an        AM                           spend-
inganymoney.
To use,closeSl andtuneto the desiredstation adjusting Youcan
                                          by        CV.
adjust forthebestresults
     R1                                        gain.
                        according yourtransistor
                                 to


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        Figure I




        Figurc2


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lilmhG8ffimffifipror                                              l
                                                                  I
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                PartsList - MicroAM Radio
     Q1    - 8C548general-purpose transistor
                               NPN
     D1    - 1N34           germanium
                general-purpose     dlode
     L1              -
           - loopstick seetext

     CV    - 365pFvariable
                         capacitor
     T1    - Transistor       1,000:8
                     transformer
         -     x
     SPKR I ohms 2 in.miniature
                             speaker
     R1    - 2,200,000 114W,5%
                     ohm,    resistor
     Cl    - 0.1uFceramic metal capacitor
                        or    film
     Sl    - SPST     or
                 toggle slldeswitch
     81    - 3 or 6V - twoor fourAAcells




                        ldeasto Explore
Tolearnmore                         performance:
           about circuit to getbetter
               the     or
     o Replace witha Darlington
               Ql               transistor, asa BC517, find
                                         such         and
     a bettervaluefor R1.Values the rangebetween
                              in                         and
                                                 2,200,000
               ohms
     10,000,000 canbe used.
                                                rod
     o Replace by20turns No. wireona ferrite andtrytuning
               L1          of 28
              stations. a long
     short-wave       Use      wireasantenna. Position theL1tap
                                                     of
     mustbe carefully     to getbestselectivity sensibility.
                    studied                  and
     o Experiment a crystalearpiecetheplace T1 andSPKR.
               with              in         of          A
     10,000         should wiredin
          ohmresistor    be        parallel theearpiece.
                                          with
                                          Fnllcr 20 ffilrilmlh

     o Explain crystal
             how                                  operate.
                                          receivers
                     setsor direct-detection
     o Define term"selectivitt''.
            the
     o Whatis a galena-crystal?
      projects:
Science
     o This receiver be usedto demonstrate antique
                    can                        how          radiore-
     ceiversoperatedwhenradio firstinvented.canbepresented
                              was              lt                 as
     a "modern        of        or
               version" a crystal  galena receiverin a demonstration.
                  exhibit be madewiththisset,the free-power
     An interesting      can                                     AM
     receiver                     and
             (described thisbook), a crystal
                       in                      set.
     a The circuitcan alsobe usedas an RF "snitfe/',           radio
                                                       detecting
           produced experimental
     signals        by            circuits theAMorshort-wave
                                         in                    band.
     Replace witha 1,000 choke remove to
             Ll            uH        and         C1    geta sensitive
     RFsnitfer.




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