Baby Monitor RF Repeater June 1, 2009

After I restored my Crosley 516, I was faced with a new problem: what was I going to use it for? Short wave reception was non-existent with a short wire antenna, and there’s nothing that I particularly care to listen to on AM; what I really wanted to do was listen to some old time radio! I’d seen other projects that replaced the guts with Internet radios, but since the electronics worked just fine I really wanted a way to stream audio from my computer to the radio without any modifications to the original radio circuitry itself.

The obvious solution was to build a small AM transmitter and modulate the signal with the audio output from my computer. I built a very simple, low-cost, low-power AM transmitter:

Simple AM transmitter schematic

Simple AM transmitter schematic

However, I had some self-imposed restrictions to overcome:

  1. The receiver could not pick up the low-power transmitter unless the two were right next to each other.
  2. A larger antenna would improve reception, but I didn’t want to have a large piece of wire hanging off the back of the radio since I planned on placing it in the living room.
  3. Adding an amplifier to the transmitter would improve reception, but would make the circuit more complex; more importantly, I would have to be careful about the FCC’s part 15 rules.
  4. I didn’t want to put a computer in the living room – most of the ones that I have laying around are noisy, so it would be best to keep the audio source (computer) in the basement.

I needed a way of relaying (preferably legally, cheaply and simply) the audio from the computer in the basement up to the AM transmitter located just behind the receiver. After considering several possibilities, I finally settled on using a baby monitor set to do the relay. My wife picked up this set from Target, which sells for $20, claims a 600+ foot range and operates on 49MHz:

Saftey 1st Baby Monitors

Safety 1st Baby Monitors

They aren’t Hi-Fi by any means, but neither is the Crosley. Besides, $20 for a TX/RX pair is about as cheap as it gets. The transmitter was easily modified to accept a direct audio line in by replacing the electret microphone with a 20dB attenuator and 1/8″ stereo jack; I simply drilled out the mic hole to accommodate the line input jack:

Transmitter with 20dB attenuator

Relay transmitter with 20dB attenuator

Transmitter, mic replaced by line input

Relay transmitter exterior view

For the receiver, I just replaced the speaker with a 1/8″ line out jack:

Receiver with line out jack

Relay receiver with line out jack

Receiver exterior view

Relay receiver exterior view

Initially I tried placing the AM transmitter in the relay receiver enclosure and powering them both from the same 9v wall wart, but this resulted in nothing but heavy static feeding into the AM transmitter. Instead, I enclosed the AM transmitter in a separate case and powered it off an external battery; given that the AM transmitter draws a relatively low 50mA of current, this was an acceptable compromise:

AM transmitter in case

AM transmitter assembled in case

The primary audio source for the transmitter is my PC, but I also wanted to be able to re-transmit CD, cassette and FM radio programs as well. I’m using an old Pioneer SX-2300 stereo to manage the audio sources; the Pioneer, cassette deck and CD player were all pulled from the dumpster:

Transmitting station

Transmitting station

Here you can see the audio output of the baby monitor connected to the audio input of the AM transmitter, and the RF output of the AM transmitter connected to the antenna input of the Crosley. Radio, “the way it used to be” (more or less):

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