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Building the Heating / Air Conditioning Thermostat – Part 2

After looking at the documentation on our White Rogers thermostat, it looks like we can simplify the operation as such:

Now this would be fine, if we were just interested in manual control, but we actually want this to be automatic, like the original, based on temperature, so we need to give the Arduino the ability to “switch” these circuits, which are nothing more than 24vac relay coils in the furnace and air conditioner. I’m not sure what the current is that those coils pull, but I believe that these SSR’s will handle it.

The next step is the DS18B20 Temperature chip, as described previously in this blog, and at

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11 years ago

Hi thanks for posting this guide. Its really helpful for air conditioning.

Northumberland county Furnace installation

11 years ago

Thanks for starting this project. I have been searching for an Arduino controlled thermostat for some time now and most of the projects are sadly outdated and use hardware that is no longer available.

I have a single Carrier compressor that feeds two sets of ducts, each of which is controlled by a separate thermostat. It is a poorly designed system that constantly freezes up as one can only have cooling active in one of the two areas.

My objective is to attempt to lockout one of the systems when the other one is online to help prevent freezing. At about 8:00 pm, I want to start shutting down the downstairs system and enable the upstairs system.

It appears that your design would allow me to do that with some custom code which I hope to be able to write.

11 years ago

Hey, I was wondering if you got that solid state relay working with the Arduino and control circuit? I am trying to figure out an appropriate relay for my home thermostat. Thanks!


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