On Tue, 07 Jul 2009 18:50:18 -0800, "Paul Hovnanian P.E."
wrote:

Bill Noble wrote:

simple electronic way. Have a magnet on the rotating part. Place a reed
switch near enough that it closes every time the magnet comes by. Have a
timer you set to two seconds. Reset the timer when you presss "start", and
every time the reed switch is closed by the magnet. You can make this timer
from a 555 chip, or just buy a timer. If the timer expires, drop out a
relay to de-energize the motor.

That'll work until the output shaft stalls with the magnet over the reed
switch, holding the 555 timer in a reset state permanently.

Google "watchdog timer", or go to maxim-ic.com and search on that term.
There are one chip solutions that handle this (and other) gotchas.
Don't use a reed switch - use an inductive pickup - passing the
magnet GENERATES a pulse, instead of switching one.

Or just put a capacitor in series with the reed switch (assuming you
are using DC - which is pretty well a given connecting to a 555 timer)
that will limit the duration of the pulse.