raspberry pi thermostat hookups

Posted on Feb 6, 2014

How to make the hardware connections between the Raspberry Pi and the house wiring. Check out this post if you want a PCB that does the connections for you. In my house (and the vast majority of others) the thermostat wiring runs at 24V AC. There`s a live wire (or two, depending on the setup) coming into the thermostat, and several others leaving to control

raspberry pi thermostat hookups
Click here to download the full size of the above Circuit.

the different components of the HVAC system. The thermostat`s job is to close the circuit between the live wire and the appropriate control wire based on the temperature. A relay is an easy way to do it, but I find relay`s cumbersome with their (usually) breadboard unfriendly pin layout and larger-than-can-be-supplied-by-gpio switching current requirements. I have found, in my opinion, a much better way of switching AC current: the TRIAC. If you`ve ever used a MOSFET for DC current switching, a TRIAC is similar in application (but not physics), only for AC current. There are some important differences, however. Probably the biggest difference is that once a TRIAC is activated, it will not deactivate until the current through it drops below some small threshold current, and I`m not talking about the gate current. Because of this the TRIAC is pretty useless for DC since once there`s current flowing it will never drop (unless you have another switch somewhere, which kinda defeats the purpose, or the battery dies. ). In AC, the voltage (and hence current) will go to zero twice every cycle, so shutting the thing off is not an issue. An important side effect of this is that it`s impossible to switch a TRIAC faster than the frequency of the AC line it`s connected to, so you can forget about using it for PWM. But you CAN control when in the cycle it turns on to control the average current flow available to whatever it`s connected...

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