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,
designated '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
portable 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 capacitor.
Parts needed for the modification:
- 1/8" miniature phone jack
- 0.1 microfarad capacitor with rating of 25 volts or higher.
- insulated hookup wire
- heat shrink tubing or electrical tape
To modify the Fisher-Price transmitter:
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
Bob Parnass, AJ9S - AT&T Bell Laboratories - att!ihlpm!parnass - (708)979-5414
I've tried this on my Radio Shack baby monitor. I added a SPDT switch
so I can still use it as a monitor if needed. It works perfectly.
I wish I had come up with the idea myself.
Dave Keyser, San Diego