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.
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