My desktop computer is an always on machine. It's role is to automate my home. Rarely will the machine genuinely lock up or the automation software made via FLOWSTONE would stop working. I always wanted a watchdog timer to automatically reboot the machine should those conditions occur.
I found a low cost 5 volt delay timer on Amazon that has a trigger input. Once this trigger goes to above 3.3 volt, the timer relay is energized and counts down to a preset value. And the end of the timer counter, the relay de-energizes.
Step 1: Wire the Relays.
Two relays were needed to create a short duration
pulse to reset the motherboard. Both relays had their power and trigger inputs wired in parallel.
Each relay has a potentiometer to adjust the time it's relay will denergize. Relay 2 has a slightly longer time expiration of 2 seconds.
The third pic gives the wiring of the relay contacts. The reset pin on a motherboard is active low (0v gives a hard reboot). I wired the output to the reset pin.
Step 2: Provide a Regular Pulse to Reset the Timers.
I set both timers to expire approximately 7 minutes after the trigger terminals are driven low. I dedicated a digital out channel on my labjack u3 and via the FLOWSTONE software to send a low pulse of 1second once every 10seconds.
Now if my pc locks up or the FLOWSTONE fails to function, in 7 minutes my machine will hard reboot.