> > shaft, or a loud clap nearby, will cause a small AC voltage on the
> > windings
> 
> But surely those are small signals compared to the result of true rotation?

Yes, when the motor is turned by hand faster than say 4 or 5 steps per
second, and the voltage goes up and the waveform is more defined. Slower
than that though, vibrational noise can be a fair part of the output signal. I
think a LPF will help, if it can attenuate noise without affecting the signal
from a fast-ish turn (ie the higher output frequencies) . Although at that
speed the step signal is a few volts. Some simple circuitry is probably all
that's needed, my hope was that using a stepper as an encoder was on the
web somewhere as a completed project

Might have a look around the robotics forums

And don't call me Shirley




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