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.
What's that Lassie? You say that Stuart Wheaton fell down the old rec.crafts.metalworking mine and will die if we don't mount a rescue by Tue, 07 Jul 2009 19:04:38 -0400:
If you have a spring loaded ball type limiter, then you could mount a roller lever switch to trip when the ball/plate rides up.
so, just capacitively couple to the reset pin - one part, no problem.
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.
The old LM567 tone detector chip would work, too.
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