By Hinkmond-Oracle on Jan 27, 2014
So, let's get started in building your own $3.2 billion (with a "b") Nest Startup using a Raspberry Pi, some hobby electronic parts, and Java SE Embedded Technology. The journey of a $3.2 billion startup begins with a single step-down transformer... er, or something like that.
First, we'll need a crash course in home thermostat technology. Here in the U.S., if you first flip off the circuit breaker to your home heating furnace and A/C, then take off the thermostat panel in your house, you should see these standardized labeled wires (with various colors of wires that are not standardized):
Red - R - 24VAC or Red - Rh - 24VAC (dedicated to heat call) Red - Rc - 24VAC (dedicated to cooling call) Green - G - Fan on White - W - Heat call Yellow - Y - Cool call Blue or Black - C - Common
If you see a wire labeled "C" (Common) and the rest of the labeled wires (above), then you are OK. If you do not see the "C" label on a wire, you must ask an electrician friend or hire an electrical contractor to run the common "C" wire from a furnace relay to your thermostat. Otherwise, if you do not have the "C" wire at your thermostat, stop here since the "C" wire is needed to power the Raspberry Pi and especially for the Wi-Fi adapter to have enough power to allow your new SmartThermostat to be networked.
If after one way or another you do have a "C" wire at your thermostat, then you are ready for the first step which is to connect the Bridge Rectifier GBU608 and the DC to DC step-down transformer from your Raspberry Pi to your thermostat wires to power it from the 24VAC of your thermostat wiring (C and either Rh or Rc or R). 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).
Come back to the next blog post to see how that's done... It's a fun step since it's your first one!
Full series of steps: