iOS Bluetooth Automotive Dashboard via Uno - 12V on/off signal

I have a project that I am working on. I want to build a small dashboard that displays several readings from my engine. This is in a classic Mustang, so there is no OBDII or such to use.

What I am monitoring is RPM (via this little module here), vacuum via a 0-5V analog sensor, and air/fuel via a PLX module that will either provide 0-5V analog or, if I can figure it out, a serial connection. I have that part figured out. I have yet to try to get the BLE to send it, but that’s not the issue I bring today.

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 know this should be probably the easiest of the 4 to do, but I am having a mental block. Any suggestions in either direction?

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.

The thermocouples grounds when the temp gets hot enough (its 205* on, 185* off). That grounds the relay, (other side of the relay switch is 12v+ from ignition switch). That then allows the relay to allow power to the fan. The obvious choice is to use the 12v from the relay to the fan. I still haven't though of a way to use the ground to trigger. Probable go the route you suggested.

Okay, so I was right... it was way too simple. I'm using a Bosch relay that is a double throw. So instead of monitoring fan current (lots of amps) I am going to measure the other pole to tell me when the fan is off. Voltage on that line means fan is off.

Doing it this way, I would assume I need a pull down resistor? Since high means off, then I need to pull down (make sure it's 0v) when it's on?

I am regulating voltage via a zener diode. Would capacitors do me any good? Should I bother?

Please post a wiring diagram.

Here it is. The 1k resistor is for current limitation, and the 10k ohm is the pull down.

Okay... another dumb idea. I have a thermocoupler. When the temperature gets to a set level, it goes to ground, which closes a relay that turns on the fan. What if I have a digital pin pulled up, and when that thermo coupler goes to ground, it pulls it down? So HIGH would be off, and LOW would be on? No level shifting. Probably an external resistor just as a safety check against over-drawing.

That should work right?

Here it is. The 1k resistor is for current limitation, and the 10k ohm is the pull down.

Where in that diagram is that relay and which one is the fan? I cannot find the thermocoupler and not Arduino in that picture. Is that a joke?

You know what, just forget it. I’ll work it out on my own.