One of the exciting things in amateur radio at the moment is the unbelievably low price of the Chinese handheld radios. One of the very cheapest is the BF-888S. It’s a 16 channel 70cm unit putting out about 2.5W. I got a pair for $42 from a Chinese ebay seller.

These radios are so cheap, they make you want to invent projects to use them. One project I’ve had in mind for a while is to use them for ARDF/fox hunting (you hide a transmitter, and people drive/wander around with receivers trying to find it) with kids armed with little Yagis.

All this would need would be a little programmable logic board plugged into the speaker/mic plugs so the transmitter could be keyed up and play the CW tones required to output my call sign.

Some variant of Aduino is probably the obvious choice to run this sort of application on for me. Although it could be done on something even simpler, I also have in mind to build an Arduino TNC on a very similar circuit, and that will need a bit of grunt.

I worried a lot about the TX/RX interface, and looked at many circuits on the internet that used 600 ohm transformers on both to isolate the radio from the electronics, one circuit went as far as using opto-isolators. In a conversation with VK6MT when I was looking for advice about this, he suggested “start with the simplest thing that works”. Sounds like good advice.

As mentioned, in parallel with this project, I’ve been thinking about a low cost TNC based on Arduino. This would have exactly the same type of audio/radio interfacing issues. Looking for circuits for this led me fairly directly to Mobilinkd who sell a bluetooth TNC based on an AVR chip for interfacing to APRSdroid. They are good sharers, and they provide a hex file to make your own KISS TNC and have a blog post to explain it.

The circuit they show for this is pretty simple (I’m not assuming it’s the same as in their bluetooth verison, but maybe). It has a voltage divider on the sound output of the Arduino to bring it down to 0-500mV and a capacitor to block any DC before it is fed into the mic input on the radio.

Their input from the radio audio (which I don’t currently need) also uses a capacitor, then adds a DC offset to the signal to use more of the range of the Arduino ADC. A straightforward NPN transistor is used to switch the PTT to ground.

This diagram of their circuit is from M0PZT

I plan to use the input circuit eventually, partly for the aforementioned TNC but partly because it opens up the opportunity of remote control of the fox.

I built it up today on a prototyping shield to test the circuit – pretty much a 20 minute job and had the satisfaction of it working first shot.

As you can see it’s currently based on an Arduino Uno, It makes sense to shrink it down to a Nano or Mini Pro eventually. I’ll build it properly onto a blank UNO shield for the time being, and order some PCB breadboards for a mini version. According to Miklor, there is some voltage available in the speaker/mic plug. If there’s enough to run a 3V mini-pro, I could end up with a matchbox sized unit that just plugs straight on the side of the radio.