Or if the OP wants a no brainer hardware solution, 74HC221 retriggerable monostable. If he want's to do it with 555, couple the short negative going pulse into the timing cap using a diode so that the charge is pulled off for every negative pulse and so the cap never reaches 2/3 Vcc to flop the flip flop and activate 555 reset. Use the negative pulse to trigger the 555 as well (sets the flip, clearing reset and driving output high). If the input signal can't handle the cap loading, they he'll have to use the NPN to discharge the timing cap, and add another series cap and R & Diode to ground to differentiate the incoming pulse and turn the NPN on for long enough to reset the cap. Robert Bob Axtell wrote: > I'd do it with a PIC10F20X. It would be 1/10 size, cost 1/5 as much, and > would be more reliable than a 555. Take about 5 minutes to write this code. > > --Bob > > David VanHorn wrote: > > >>You're looking for a retriggerable one shot, AKA "Missing pulse detector" >>I've seen 555 ckts to do this, check an older National semi data book >> >> >> >>On 3/30/06, Rob Robson wrote: >> >> >> >>>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? >>> >>>Many thanks, >>>RR >>> >>> >>>-- >>>http://www.piclist.com PIC/SX FAQ & list archive >>>View/change your membership options at >>>http://mailman.mit.edu/mailman/listinfo/piclist >>> >>> >>> > > > -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist