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Convert the Fisher-Price Baby Monitor into a 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, 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
  • 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 connected, 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 conductors 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 terminals.

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 microphone 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!

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

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