> > * What kind of coil would need to be used? Thickness? Resistance?
> 
> As many turns around a magentic core as you can stand.  Several
> layers of thin magnet wire around a steel bolt should to it

Analogue clock coil is a lot of turns. DC resistance ~ 250 ohms. Also
on the test bench is the step-up transformer from a disposable camera
(collateral damage from the flash tube project). Secondary DC resistance
is ~ 320 ohms. Both produce a 2V pulse with a strong HDD magnet
moving past

The only downside to a coil is that it would not register a turn at low
speed as a bicycle has no detent points

cf

http://www.piclist.com/techref/io/sensor/pos/enc/stepper_as_encoder.htm


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