Sunday, July 1, 2012

Building the Heating / Air Conditioning Thermostat - Part 3

We continue with the project by building the temperature sensor module. We are using our favorite, the Dallas DS18B20 digital sensor. It uses one pin to communicate with the Arduino, along with +5vdc and Gnd. We add a 4.7k resistor between pins 2 & 3. Because we are using a single sensor, we do not need to know the address of the DS18B20 ahead of time. If you want to add a second sensor, to display outdoor temperature see our other tutorial. We are using the DS18B20 code from and the LCD code from

Don't forget the updated One Wire and Dallas Libraries from

Pin 1 on the left is Gnd, Pin 2 in the middle is +5vdc, and pin 3 on the right is data, according to the GVS standard we follow.

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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