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I'd like to build a set of miniature two-way (half-duplex) radios that operate on the FRS band. It seems that transceivers for this band would be widely available. However I've not been able to find any. Ideally they would be controlled digitally (I'm thinking to use a PIC controller) and have analog inputs and outputs for earbuds and a microphone. Where can I find a transceiver?

I was planning to use this transceiver from Sparkfun but it doesn't look like there will be an easy way to get an analog output and input.

I'd actually not mind building a basic transceiver from scratch but I can't find a good tutorial or set of schematics.

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    \$\begingroup\$ You are right that RFM12B modules will not work with an analog signal. \$\endgroup\$ – jpc Mar 22 '11 at 19:57
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Operation of FRS radios might be license-free, but in most countries you are not allowed to build your own equipment. It has to be certified, like anything that transmits RF. If you want to build your own equipment for two-way radio communication, you need to get an amateur radio license and use the amateur radio bands.

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  • \$\begingroup\$ I think that (at least where I live) you can build your own for the ISM band as long as it does not violate the terms of use of the band. Or as one of my professors put it: if you ever want to do something stupid then just be 130% sure you stay out of army and flight control bands or they will hunt you down fast. :) \$\endgroup\$ – jpc Mar 22 '11 at 21:36
  • \$\begingroup\$ @jpc What would the consequences of running voice communications over the ISM band be? \$\endgroup\$ – Andrew Buss Mar 23 '11 at 3:15
  • \$\begingroup\$ Also, are there any FCC-certified walkie-talkie transceivers (without enclosure)? I read that they will not certify anything until it has an antenna. I would not mind buying something with a hardwired antenna and putting it into an enclosure. The primary purpose of the project is to end up with a small, pocket-sized radio for use with a headset. All commercial walkie-talkies that I could find are relatively bulky and are built for use in the hand, with integrated speaker and microphone. \$\endgroup\$ – Andrew Buss Mar 23 '11 at 3:23
  • \$\begingroup\$ I am not qualified to offer you legal advice on this but I believe the ISM band is free to any use and limited only on the power and duty cycle of the signal (the exact numbers and measurement methods vary from country to country). \$\endgroup\$ – jpc Mar 23 '11 at 3:25
  • \$\begingroup\$ I believe you could rip the walkie-talkie apart and unless you mess things up a lot it will still transmit the same power on the same frequency so you should be OK. \$\endgroup\$ – jpc Mar 23 '11 at 3:28
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All of this assumes you are in the US.

My understanding of the associated FCC rules is: You cannot modify or create an FRS, GMRS or MURS radio without going through FCC certification (legally, Part 95 rules). You can create up to 10 ISM band transmitters (Part 15 rules) for research and development or experimentation as long as the transmitters meet the FCC technical requirements for part 15 ISM transmitters. This means you can use parts like the AT86RF212 (which is available in modules) and an audio codec like G.711, G.729 (now patent free) or SILK/OPUS to convert the analog audio to a digital stream. G.711 can be implemented on any microcontroller, G.729 or SILK generally require an ARM running at 72Mhz (or equivalent in other processors like the PIC32s).

Using a codec and a digital transmitter is a little more difficult but much more interesting from a project perspective. You can also transmit data including GPS, inertial sensor data, biometric data, etc. If you do go with the AT86RF212 or a module using that chip, you can expect most of a mile @ 100Kbps (line of sight) or several miles @ 40kbps or with an external power amp.

If you want to experiment, take a look at the FreakDuino 900Mhz boards. You will only be able to do G.711 with the processor on that board but you can get started really quickly. G.711 required 64Kbps, do you'll need to run at least 100kbps for your on-air data rate. The board also has an RF power amp, and the two boards I have did meet FCC rules for part 15.

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