By hinkmond on Feb 03, 2014
If you are this far along, it means you do have a "C" wire at your thermostat. Yay! That means you can connect the Bridge Rectifier GBU608 and the DC to DC step-down transformer from your thermostat wires to power your Raspberry Pi from the 24VAC of your thermostat wiring. We are using the C wire and either the (Rh or Rc wire) or the R wire to power the RPi, since one of the "R" wires is considered in AC terms "hot" and the "C" wire is considered "neutral".
NOTE: You need a "hot" wire and a "neutral" wire in AC household electrical circuits to complete or close an AC circuit. With most U.S. household thermostats, we are working with a lower 24VAC standard, and not your typical 110VAC found in U.S. homes. If your thermostat happens to be a higher voltage 110VAC thermostat, stop here and do not proceed. Your wiring for a 110VAC thermostat is not the same as the 24VAC we need for this project. If you are unsure whether your thermostat is 24VAC or 110VAC, ask an electrician friend or electrical contractor to check for you.
As a review, the Bridge Rectifier turns the 24VAC of your furnace relay from 24 volts of AC power to 33 volts DC power, and the step-down transformer turns the 33 volts DC down to 5 volts DC for the Raspberry Pi (and all its peripherals).
As with any $3.2 billion startup, you begin with a prototype, and that prototype is typically built using a breadboard, so that you can easily put it together and change it if necessary.
Once you have the screw set on your transformer, then your are ready to connect your Raspberry Pi in the next step... Fun!
Full series of steps: