Interfacing with home heating system

So after buying a second ThermoDisc in as many years I've decided to replace it with something solid state.

Some background
A thermodisc functions as a time-delayed relay. When your thermostat calls for heat it bridges two wires together completing a circuit. When that circuit completes, a heating element in the thermodisc heats up, causing a little disk (the disc part of thermodisc) to change shape which causes the disk to push a little pin closing a contact which then causes the circulation fan to turn on. When the thermostat is done, the circuit opens, and the thermodisc cools down, and disengages the fan.

While this works, its wildly inefficient. The relay engages 20-30 seconds after the burners fire, and disengages 20-30 seconds after the burners turn off. My goal is to use either an IR temperature sensor, or a high-temp thermocouple to read the temperature of the heated surface and use that to turn on/off the fan.

None of that should be to difficult (I hope). My question is this though, what is the best way of interfacing to the existing system to get notified when the t-stat calls for heat. The wires put out 24v AC when the t-stat calls for heat. My first thought was to wire that into a relay that would then pull a pin high. But I would like to eliminate as many mechanical pieces as possible.

Does anyone else have another idea?

-dan

Hi Dan, You could use an opto-coupler. Take the 24V AC put it through a resistor and then into the LED of the opto-coupler. Connect the transistor of the opto-coupler between a Netduino pin and ground. Configure the Netduino pin as an input with internal pullup resistor. When the t-stat calls for heat the LED will pulse the transistor ON, and the input will pulse low. There is a slight problem - the input will be pulsing at the same frequency as the AC. This might cause issues in the software. If it does, you could use a bridge rectifier and capacitor to turn the AC to DC before it goes into the LED. No moving parts - except the electrons and photons! Hope this helps - Paul

Hi Dan,

You could use an opto-coupler. Take the 24V AC put it through a resistor and then into the LED of the opto-coupler. Connect the transistor of the opto-coupler between a Netduino pin and ground. Configure the Netduino pin as an input with internal pullup resistor.

When the t-stat calls for heat the LED will pulse the transistor ON, and the input will pulse low.

There is a slight problem - the input will be pulsing at the same frequency as the AC. This might cause issues in the software.
If it does, you could use a bridge rectifier and capacitor to turn the AC to DC before it goes into the LED.

No moving parts - except the electrons and photons!

Hope this helps - Paul

Thank you Paul! I hadn't even considered using an optoisolator!

-dan