A 4-step Arduino based sequencer for the Gakken SX-150 synthesizer (and probably any other ribbon-controlled mini synth, for that matter).
This project was detailed on my blog (http://houseofjeff.com/2013/09/12/hacking-the-sx-150-building-an-arduino-based-sequencer/) but the final product is presented here.
The hardware uses a digital potentiometer & transistor to play individual notes, and the software abstracts this away to "play note". Then the hardware uses a set of momentary buttons to select which of the 4 steps to change and a rotary encoder to choose the note.
It's a simple hack, but a good example of repurposing noise-making equipment for new uses!
If you use this code or the schematics, drop me a line and show me what you built.