Wouldn't a simple LED/photodetector pair attached to a simple bracket that would straddle the crank gear teeth generate a very useful, and high-resolution, train of pulses? I constructed a wooden ramp with sides containing two such pair to conduct basic acceleration experiments in a high-school physics lab. It was easy to write programs to perform the detection and timing counts. In fact, those pulses alone would drive a speaker/headphones in an interesting sounding manner much like that aforementioned trading card in the spokes arrangement. You could power the LED by battery or the bike's generator, and options for a circuit to process the pulse stream could be both simple and richly functional. Frank