> I'm thinking of trying something much simpler to start.. Just an LC
> oscillator with a little coil for the L placed next to an arm on the 
> shaft
> of the motor. Then see if I can count the number of oscillations per 
> unit
> time and detect changes as the motor is turned. The nice thing about 
> an osc
> like that is if I drive it hard enough, the signal at the input pin 
> is
> basically digital.

If you are after revolution counting then a magnet and inductor will 
give you an essentially sinusoidal output. If you feed the inductor to 
a differential amplifier (as simple as a single opamp and not much 
else) then you will get a high amplitude square wave. Fails at low 
enough speeds when signal level gets too low wrt amplifier offset 
error, but is liable to be usable in practical situations. More turns, 
stronger magnet, smaller gap all allow lower speeds.


        RM 

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