ArdPRS is an AVR/Arduino APRS transmitter/softmodem. APRS is a commonly-used protocol for transmitting telemetry; ArdPRS is intended for use in tracking balloons as they go to the edge of space (and sometimes myself, as I wander around town).
ArdPRS is implemented in straight AVR C, but it actually compiles via Arduino environment sketches. It’s designed for the Volksduino, so it will work on just about any ATMega328-based Arduino clone.
ArdPRS only uses one pin for output, with two resistors and one capacitor. That’s it. (And maybe another pin with a transistor to press the key on your HT, if you don’t want to use VOX). The actual modulation is done via a 1-bit DAC with a low-pass filter. One of the example sketches uses the serial lines to talk with a GPS unit; another just transmits any string it receives from a computer attached via an FTDI cable (KISS mode is possible, but not yet implemented).
If you’re a ham radio operator who’s looking for a simple APRS transmitter that you can hack, a Volksduino, ArdPRS, and a cheap HT will do the trick! We’d love to hear any stories you have if you use this; email us with pictures of any hack that uses this!