At 06:51 PM 11/16/2007, you wrote:
>Thanks for the ideas.  I was hoping for a one part solution, but a
>typical watchdog chip and an SR flip flop seems like a winner.
>
>Have a good weekend all.
>
>-n

You could also use a charge pump (a couple of ceramic caps, a BAV99) and
a single gate ST inverter, plus a resistor. You have to toggle the
input often enough that the resistor doesn't bleed off the voltage to
the ST trigger point.

One smallest-possible-part-count circuit (1) is an ADM705, which has a
/WDO output and an extra comparator. I've used these parts in production
and found them very good. About half a dollar in volume. There's  also an
equivalent 3V part. They may also be offered by M*x*m, at least in theory.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com



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