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CONVERT THE FISHER- PRICE BABY MONITOR
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CONVERT THE FISHER-PRICE BABY MONITOR
INTO A REPEATER FOR YOUR SCANNER OR SHORTWAVE RECEIVER
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-
citor.
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-
minals.
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
modification.
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
level.
If your neighbors have scanners, cordless phones, or
baby monitors, they can probably listen in to your
scanner, too!
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Bob Parnass, AJ9S - AT&T Bell Laboratories - att!ihlpm!parnass - (708)979-5414
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