I'm trying to design an Arduino-based central heating controller. I've worked out (I think) turning things on and off, but I'd like to be able to detect whether there is voltage across the terminals leading to the thermostat on the hot water tank. Has anyone got any advice for that? It's literally a 240v thermostat switch being supplied power. When cold enough, there'll be current and I want to detect that current to make decisions in software about whether to turn the hot water on or not.