Jim wrote:
>
> I had an idea that pulses caused by the brushes on the commutator could be
> conditioned to digital levels and used as an encoder. Connecting a scope
> to a small dc motor while running shows a pulse of 20mv whose frequency is
> in proportion to the speed. It seems like capacitive coupling and amplifier
> to the motor and maybe using a schmitt trigger at the amp out would be a working
> alternative to opto sensors.


Hi Jim, here's one i've been using for years;
http://www.romanblack.com/encoder.htm

It needs some motor load (ie brush current) to
work and its not great for wide speed control.
It is ok for a simple tacho or for relatively
constant loads like fans, spindles etc. :o)
-Roman

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