Gaurenteed State

I need to ensure that a pin never stays high for a period of time longer than 1 second, regardless of what the system is doing. The pin is PWM controlling a heating element, which if left on can overheat and burn down the machine (1300 degrees max temp).

I know about monostables where they produce a pulse of a specific width, however I want a system where it will pass the PWM pulse widths but limit those widths to less than n milliseconds. Does anyone know of such a thing?

Maybe you could charge a capacitor via a resistor and feed the voltage level into a Schmitt trigger NOT gate. If you AND the result with the PWM. This should automatically turn off the output if the charge of the cap reaches above the threshold voltage of the NOT gate.

external watchdog like MAX6369 or MAX697 ?

Atmel used to make fail safe relay drivers but they are all end of line for the ones that Mouser stock.

I though about using a capacitor, since PWM would be like an AC voltage and capacitors let AC pass but block DC.

PTC is interesting, do they normally have a fast response time?

I think I’ll also have to invest in an external watchdog as well. I can’t afford to have the system running unstably when one or more of the uCs go missing.

The watchdog IC controlling the power to the relays would be a good idea. As long as the watchdog is active, the power is always on.

This is how we do it with a machine control system. We have solenoid drivers to operate a hydraulic machine. If the software fails to trigger the watchdog, the solenoids all switch off.