Missing Pulse Detector (Watchdog Timer) Circuits Using LM339/393, CD4093, or 555 2017-04-15

Missing pulse detector circuits commonly use a circuit with a 555 and a transistor to perform this function.
As an alternative, here's a similar circuit using only a single active device package, the LM339/393 comparator.
The total number of parts is about the same but with one less active device.
The circuit will time out when the pulse stops, independent of the pulse DC voltage at that point.

Below is the LTspice simulation of the circuit.

**broken link removed**

Repeating pulses...

Click to expand...