After I got my Baofeng GT3-TP I wanted it to do APRS. After looking for a while, and joining information together I came up with a solution. I designed and built a quick and cheap interface to exchange sound between a computer and a Baofeng radio (or any radio with the same Kenwood type plug with the same pinout), and also the usual PTT signal over RTS/DTR serial. This is what I came up with:
First the circuit.
The circuit is really simple:
Nothing fancy in there. A bus for the audio, a connector for the PTT and a bus for the radio connector. The 2 capacitors decouple the DC component on the audio lines. The 4N33 IC is an optocoupler to do the PTTing.
Then, the boards.
For the board making I opted by OSH Park. They have a nice and cheap service (that is particularly cheap for small boards). The board was routed like this:
I’m not the best with soldering, so after a bit of work, this was what I ended up with:
I interfaced it with the computer like this:
And it works nice. I figured out not connecting GND on the serial connector avoids group loops. The sound get noisier with it connected.
On the sotware side I recommend soundmodem for Windows. Soundmodem is a tool designed tro provide a “soundcard TNC” solution. It emulates AGWPE’s TCP API, but I prefer soundmodem. I use UISS for the “front end” or my own python scripts.
The design is far from perfect. Despite the fact I use an optocoupler for the PTT don’t be fooled. GND lines aren’t isolated, as no transformers were used. I used an optocoupler only because they are easy to use, but a simple transistor could be used. Also, as a friend noticed, non-polarized caps should have been used (and even the ones used might be the other way around, but I never had any problem). Also, no attenuator for the audio lines was used. That means you should be careful with the audio levers, or you can easily overdrive both the computer or the radio audio input. This solution is stable at 1200 baud rates. 2400 is also fairly stable. Over that, the radio can’t code/decode such fast rates. Finally, this solution isn’t exactly mobile friendly, but a version of the design for use with something like aprsdroid will be coming.
This is what is looked like in the end: