> Optical is never a good idea IMHO for something like this
> because it will get dirty (just one puddle would be enough)
> and stop working.

I agree. The optos from mice are pretty good - I use them in
IR links (heck, I'll try and recycle anything) - but dirt will be a
problem. For movement detection a magnet and coil is better,
as Olin suggests. This is what my cycle computer uses. It's also
a very old method for pulse driving in timepieces. A magnet on
the pendulum passes by a coil which activates a power driver
which in turn sends a repulsing current to the coil, thus driving
the pendulum back towards the coil on the other side of its
swing. The major advantage of this pulse-processing method is
very low quiescent current, unlike Hall Effect or opto. For a
PIC application this would mean no power required for the
sensor and mostly only sleep current for the PIC

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