Arduino Stack Exchange is a question and answer site for developers of open-source hardware and software that is compatible with Arduino. It only takes a minute to sign up.
Sign up to join this community
I have a 2 wire thermostat. R is +24VAC, W is Heater. I have no Ground wire (C), and I cannot open the thermostat.
When the heater goes on, the thermostat internal relay gets powered and connects R and W together.
How can I detect when the heater is on from those two wires?
With checking the temperature in the room. When the contact is closed store the current temperature. Check after a minute the temperature again. If it is higher, the heater is on.
R -----/\/\/\---|>|----- W... perhaps a high value resistor in series with an LED .... it is possibe that a small amount of current could pass without activating the heater ... the LED would actually be emitter part of an optocoupler ... when the heater is off, the LED is on ... the thermostat contacts short out the LED when they close, which causes the LED to turn off – jsotola Oct 28 '20 at 2:11Heater on → 0 (shorted)Heater off → 17MohmI hope I hooked it up correctly to Arduino to detect it. – zip85 Oct 28 '20 at 3:29