On Mon, 6 Jul 2009 03:05:21 -0400, "Buerste" wrote:

Imagine a simple fractional hp DC gearmotor with a torque limiter or torque
limiting clutch that powers a small machine at 60 or so RPM. Lets say the
machine jams and the torque limiting device kicks out. How can I cheaply,
easily sense this condition and turn off the motor?

This is a personal project and when I get it running it will amaze and
astound!

Several have suggested sensing drive and driven parts but left the
signal processing details as an exercise.

I'm in dialup-land just now so it'd be tedious to download a
datasheet, but I'm pretty sure the CD4046 CMOS phaselocked loop would
work well here. It would produce an output signal if the frequency of
(digital) inputs differ, as they would here. I'd suggest something
like Allegro hall-effect geartooth sensors with integral magnets
because they're so easy to apply. They could easily sense screwheads
on the drive and driven parts. The CD4046 costs less than a buck and
will accept supply voltages from 3 to 18 volts DC.

You'll find a datasheet at www.ti.com Search for CD4046B.

Ping me if you're interested in this approach. I'll be back in town
tomorrow.