I'm looking to implement a watchdog circuit. Tried the standard "missing pulse detector" schematic such as one from http://www.falstad.com/circuit/e-555missing.html - and it works well when the input stuck to high.

However if input stuck to low (for example my application crashes at the exact moment when I'm resetting the watchdog pin), this circuit fails to detect missing pulse. This could be seen in the same schematic with logic replaced by AND.

Is there a reasonably simple schematic which can truly detect a missing pulse, no matter whether input got stuck on high or low? I know there's always MAX6814 or TPL5010 would work, but their price tag isn't attractive (10x-3x of attiny which can do the same).

What I want is basically:

INPUT: 010101010101     OUTPUT: 1111111111 
INPUT: 010100000000     OUTPUT: 1111110000 
INPUT: 010111111111     OUTPUT: 1111110000
  • \$\begingroup\$ The easy answer is to just take two copies of that circuit and invert the input to one of them, then and them together. This is not an efficient way of doing it, though. \$\endgroup\$
    – Hearth
    Commented May 27, 2019 at 0:44
  • \$\begingroup\$ you are asking for a missing transition detector, not missing pulse detector \$\endgroup\$
    – jsotola
    Commented May 27, 2019 at 2:02
  • \$\begingroup\$ Is this a standard term? Google search for "missing transition detector" doesn't find much relevant content. \$\endgroup\$
    – George Y.
    Commented May 27, 2019 at 2:08

1 Answer 1


You simply need to AC-couple the pulse, which turns the circuit into a falling-edge detector, rather than a low-state detector.

Here's the genreal idea, but component values may require tweaking for reliable operation.


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ This works, thanks! \$\endgroup\$
    – George Y.
    Commented May 27, 2019 at 17:24

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.