isolation - Monitoring open/closed-circuit on a mains line

I am building a custom refrigerator control circuit. The refrigerator is solely powered from a battery source, using an inverter. For power savings, the inverter should be off when the motor is not running.

The thermostat closes when the temperature is too high. This would normally start the motor. But the inverter is off.

How can I know when the thermostat is closed/open?

There is no voltage in the circuit, so it would be trivial to provide 5V to test for the closed circuit, except that this wouldn't be isolated, which could be problematic when the inverter is on.

The circuit should:

• take advantage of the internal thermostat (cannot be seperated from motor/mains power)


• should be properly isolated (e.g. induce a 5VAC signal that can be measured via a zero cross circuit, but without risk of back-current)


• can be powered from a 5V/12V source

Currently, I have a seperate temperature sensor, and it's a simple solution, but I'd prefer to use the thermostat. Also, the thermostat cannot be seperated, because it's built-in.

^{simulate this circuit – Schematic created using CircuitLab}