What you probably want is a rotary encoder (shaft encoder). These are
readily available devices, or you can make your own out of slotted disks
and photo-diode/transistor pairs. Either way, the support circuitry is
These pages might get you started:
If all you need is a count of revolutions, then a single slot would be
all you need. Another technique is to put a cam on the shaft and
position a roller microswitch to register the revolutions.
Hope that helps,
Have a look at the inards of an old mouse (the ones with a ball), I've used
one for a simple rotation counter.
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