I had thought I could just connect the 24vac input to a relay input, and then setup a pin as an input on the arduino and be able to know that the relay had closed. But now I'm reading that relays need a minimum current or some such, and I don't really understand how I could meet that since I'll have no load.
I know just barely enough to even be dangerous. Am I even making sense?
Best Answer
No relay is needed. You could rectify the 24V into DC, smooth it with a cap, then drive the input of an optoisolator with a proper current limit resistor. When AC is applied the opto's open-collector output could pull a uC input with internal pullup resistor low to signal that AC is present. Example part: https://www.onsemi.com/pub/Collateral/HCPL2631M-D.pdf