Monday Feb 03, 2014

How to Build Your Own $3.2bln Nest Startup Using Java SE Embedded Tech (Part 3)

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.

  1. So find a hobby electronic breadboard, and add your GBU608 Bridge Rectifier. In this part of the project you will run the inner two wires to connect from the breadboard to your "C" and one of your "R" wires of your thermostat (refer to the pinout diagram above)
  2. Use a wire connected to the notched edge pin of your GBU608 which represents the positive terminal to solder to the positive "IN" pad on your DC to DC Voltage Step-Down Transformer, and do the same with your unnotched edge pin of your GBU608 which is the negative connector and solder that to the negative "IN" pad on your DC to DC Voltage Step-Down Transformer.
  3. Solder two separate wires to the positive and negative "OUT" pads of your DC to DC Voltage Step-Down transformer
  4. Connect the two inner pins (marked with the "~" symbol) of your GBU608 Bridge Rectifier to your thermostat wires as described in Step #1: one inner pin connects to the "C" wire of your thermostat and the other inner pin connects to one of the "R" wires (or better yet for workbench development purposes, connect them to a temporary 24VAC power supply like this one to represent the thermostat power as you develop on your bench first).
  5. Use a voltmeter to measure the OUT pads of your DC to DC Voltage Step-Down transformer and adjust the screw of the transformer until your voltmeter reads 5 volts.

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:
How to Build Your Own $3.2bln Nest Startup Using Java SE Embedded Tech (Part 1)
How to Build Your Own $3.2bln Nest Startup Using Java SE Embedded Tech (Part 2)
How to Build Your Own $3.2bln Nest Startup Using Java SE Embedded Tech (Part 3)
How to Build Your Own $3.2bln Nest Startup Using Java SE Embedded Tech (Part 4)
How to Build Your Own $3.2bln Nest Startup Using Java SE Embedded Tech (Part 5)
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Hinkmond Wong's blog on making Machine to Machine (M2M) and the Incredible Internet of Things (IoT) smarter with Java Embedded Technologies

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