Output serial commands from the audio port! You can control any serial device (Arduino, BASIC Stamp, etc.) from your android phone without rooting or bluetooth. This app can be called as an intent or integrated with other apps. Order a cable for this for only $15, or build your own - email [email protected]
for source code and schematics.
Added support for differential-stereo output so that no fidgeting with a trim pot is necessary anymore