Originally Posted by
rollagain
Light source on one side of the wheel, photocell on the other side; the detector circuit would count the interruptions of the beam from each individual spoke. Compared to the magnetic-pulse method, this scheme would increase the precision of the reading by a factor equal to the number of spokes in the wheel.
You can have that much easier with existing technology. Mount a magnet on each (/every other) spoke and divide the wheel circumference in your bike computer by the number of magnets installed.