For CSC 475 (Music Information Retrieval) I built a clap sensor that our robot drummer could use for input so that it can do something useful without a MIDI source. This involved designing the sensor, purchasing components for it, assembling it, and writing some signal processing code to detect claps and output digital signals.
I built the sensor from scratch using the steps given here.
This video shows the drummer responding to claps:
This video shows the drummer listening to a beat and then drumming along with it: