Post by thread:looking for retriggerable 555 monostable circuit

> 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? ;-)