At 12:44 PM 3/30/2006, Rob Robson wrote: >Looks ideal! The absence of base resistor is a little surprising, but I'll >try it. Thanks! Don't be surprised at the lack of base resistor - its not needed. The PNP is configured as an emitter follower. That said: there is a potential problem with the circuit. If the input remains LOW, the timer will never time out. It will time out only if the input remains HI for longer than the RC time constant. If you need a true missing pulse detector, you'll need to differentiate the pulse fed to the base of the PNP transistor. That's the extra RC network Rob Rolf mentioned. Adding that network will allow the monostable to time out and signal the loss of input pulses regardless of whether the input is stuck HI or LO. If you need to detect only the condition of the input stuck HI (the condition of input stuck low is a "don't care"), the single added PNP transistor is all that is needed. dwayne -- Dwayne Reid Trinity Electronics Systems Ltd Edmonton, AB, CANADA (780) 489-3199 voice (780) 487-6397 fax Celebrating 22 years of Engineering Innovation (1984 - 2006) .-. .-. .-. .-. .-. .-. .-. .-. .-. .- `-' `-' `-' `-' `-' `-' `-' `-' `-' Do NOT send unsolicited commercial email to this email address. This message neither grants consent to receive unsolicited commercial email nor is intended to solicit commercial email. -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist