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TUCoPS :: Radio :: fpmod.txt

Turn a Fisher Price baby monitor into a scanner repeater


                        Bob Parnass, AJ9S

     The Fisher-Price baby monitor (model 157)  consists  of
     an AC operated, low power FM transmitter, and a battery
     operated receiver.  Both transmitter and  receiver  are
     equipped  with  2 crystal controlled channels in the 49
     MHz range.  The unit  modified  for  this  article  was
     equipped  with  49.845  and 49.875 MHz channels, desig-
     nated 'C' and 'D' respectively.

     This modification allows a hobbyist to connect the baby
     monitor  transmitter  to  the TAPE jack of a scanner or
     shortwave receiver and rebroadcast the transmissions in
     the  49  MHz  range.   One can roam around the house or
     yard with the portable baby monitor receiver or a port-
     able  scanner  tuned  in the 49 MHz range, listening to
     transmissions intercepted by a base receiver.

     No changes are needed in  the  baby  monitor  receiver,
     although  one  could  disconnect the red light emitting
     diode  (LED)  to  prolong   battery   life.    In   the
     transmitter,  we  will  be disconnecting the microphone
     and installing a 1/8" phone jack and DC blocking  capa-

     Parts needed for the modification:

        - 1/8" miniature phone jack

        - 0.1 microfarad capacitor with rating of  25  volts
          or higher.

        - insulated hookup wire

        - solder

        - heat shrink tubing or electrical tape

     To modify the Fisher-Price transmitter:

       1.  Make sure the transmitter  is  disconnected  from
           the AC line.

       2.  Turn the transmitter upside  down  and  use  your
           fingernail  or  a small screwdriver to remove the
           rubber feet, fastened with  rubber  cement.   You
           should  now see 4 Phillips screws holding the top
           and  bottom  of  the  plastic  cabinet  together.
           Remove the 4 screws and save them for reassembly.

       3.  The rubberized antenna and  microphone  are  con-
           nected, using crimp on connectors, to pins on the
           printed circuit board (PCB).  Using a needle nose
           pliers,  temporarily  disconnect  the  white wire
           that connects the antenna to the PCB.

       4.  Using a needle nose pliers, disconnect the 2 con-
           ductors  of  the  shielded cable that connect the
           microphone to the PCB.   Tape  the  ends  of  the
           shielded  wire  and  stuff  them  back  into  the
           cabinet top.  The microphone is  disconnected  to
           prevent   the  transmitter  from  repeating  room
           noises.  If you don't disable the microphone, you
           will be "bugging" your own house!

       5.  Remove the cabinet top and drill a 1/4"  hole  in
           the side of the cabinet top.

       6.  Mount a 1/8" miniature phone jack  in  the  hole.
           Don't  overtighten  the  nut  or else the plastic
           case might crack.

           Note: although most phone  jacks  have  3  solder
           terminals,  we will be using only 2 of the 3 ter-

       7.  Solder the end of a  piece  of  insulated  hookup
           wire to the sleeve terminal of the jack.

       8.  Solder the other end to the pin on the PCB  where
           the microphone shield had been connected.

       9.  The original circuit placed a DC  voltage  across
           the microphone.  We must add a blocking capacitor
           to  prevent  any  DC  from  flowing  between  the
           transmitter   and   the   scanner   or  shortwave
           receiver.  Solder one end  of  a  0.1  microfarad
           capacitor to the tip terminal of the jack.

      10.  Solder the other end of the capacitor to a  short
           length of insulated hookup wire.

      11.  Slip a length of  heat  shrink  tubing  over  the
           capacitor.  If  you  have  no tubing, you can use
           electrical tape instead.  Solder the other end of
           the  wire  to the pin on the PCB where the micro-
           phone center conductor had been connected.

      12.  Reconnect antenna to the PCB.

      13.  Using the 4 Phillips screws from an earlier step,
           reassemble the cabinet top and bottom pieces.

      14.  Replace the  rubber  feet.   That  completes  the

     Use a shielded patch cord to connect  the  Fisher-Price
     transmitter  to  the  TAPE  jack  of  your  scanner  or
     shortwave receiver.  If your receiver has no TAPE jack,
     try connecting the transmitter to the earphone jack.

     As a last resort, you could use  the  external  speaker
     jack.   If  audio  from  the speaker jack overloads the
     baby monitor transmitter, producing distortion, use  an
     attenuating  patch  cable  to  reduce  the audio signal

     If your neighbors have scanners,  cordless  phones,  or
     baby  monitors,  they  can  probably  listen in to your
     scanner, too!
Bob Parnass, AJ9S - AT&T Bell Laboratories - att!ihlpm!parnass - (708)979-5414

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