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'looking for retriggerable 555 monostable circuit'
2006\03\30@171142 by michael brown

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> I'm looking for a 555-based monostable (one-shot) circuit whose output
will
> STAY high as long as short negative-going pulses are present at the
input.
> The textbook 555 monostable circuit doesn't do this.  I have a spare
NPN on
> the board to utilize if necessary.  Has anyone come across a 555
circuit
> that will do this?

Like Dave V says, you want a "missing pulse detector".  I use this
circuit right here for a telephone ring detector:
www.eed.usv.ro/misc/mirrors/cc/circuits/circ0081.htm
It gives me a nice continuous high pulse for each ring, instead of
having to deal with a bunch of pulses.  Just set the time delay to be
longer than the period of the incoming pulses.  Note that this circuit
uses a PNP to short the cap out.  No base resistor and no current limit
on the cap shorting.  I just love living on the edge, don't you?  ;-)

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