I want to monitor the cooling fan. All I want to know is if it is on or off. I have a relay controlled via a thermocoupler that goes to ground at 195 degrees, triggering the 12V relay to turn the fan on. I am trying to figure out how to use that ground to trigger a digital input. I could run it off the 12V+ going to the fan, but using the ground seems safer (no level shifting)
I don't know how you want to use the ground to switch anything. Please provide a wiring diagram of the idea you had for this. I would use a voltage divider from the 12V of the fan going to a transistor that pulls a digital input to GND.