I'm trying to design a monitor circuit for my folks steam boiler furnace. For whatever reason, when the weather changes, the boiler demands water and then the internal valve gets stuck on... the boiler eventually overfills, filling the steam registers and it's a BIG MESS
so I'm going to throw a microcontroller and a liquid flow sensor at the problem.
The uC will monitor the thermostat circuit - when the thermo requests heat, the uC will switch on an external valve and monitor the water flow. When the boiler is operating normally, it doesn't need water but every few days, but the internal safety system needs to see water pressure, otherwise the boiler won't fire. The thermostat circuit is 24vac.
So what I need to do is detect when the thermostat circuit is closed, and pass that information to a uC, which will open a valve, monitoring the water flow, and shut down the valve if it flows for more than a few seconds.
How do I read 24vac with a 5vdc microcontroller?
I don't think there is much current involved in the thermostat circuit, as far as I can tell, a relay in the thermostat closes the circuit when it calls for heat, and that in turn completes a circuit for the gas valve ... a steam pressure sensor and a water pressure sensor are in series with that circuit and are normally closed except when their values are out of bounds (steam pressure too high, water pressure too low).. as far as I can tell, there are no "electronics" in the furnace, its from the mid 1900's.