I've built a watchdog circuit for my Raspberry Pi using a NE555. In the astable configuration I'm counting ~5 minutes and then I kill the switch unless there is a signal from GPIO that discharges the C of the 555. I have a mosfet RFP70N06 with the gate directly connected to the OUT of the 555, and when C is 2/3 full out goes to GND, the mosfet is open and the power of the RPi is gone.
My question is about the ground. The RPi does not initially share the supply ground as the rest of the circuit, since the GND of the supply of the RPi is on the Drain of the MOSFET. But then I'm using a GPIO pin, and I also used one of the GND pins on the header.
Since the RFP70N06 has an Rds=0.014 ohm I have my RPi with a 0.014 resistor on its ground, but from the GPIO header I have GND directly to the GND of the supply.
Does this makes sense? Should I be using an opto-coupler for the GPIO? I don't want a track on the PCB of the RPi being used as fuse.