Thermostat Three

Posted 2012.05.09 21.27 in Computers/Internet/Technology by Stephanie

This might just be the fastest project I’ve ever done. Saturday morning I started the hardware build, by Saturday evening I had also begun the software. By Sunday afternoon I was halfway through. Sunday evening saw it 90% completed. Monday was finishing touches and adding some extras just because. Tuesday I finished it. This afternoon I installed it.

Some of the things that aren’t obvious in a still photo: The the block above the screen has two RGB LEDs behind it. These aren’t programmable, but one cycles through the colours slowly and the other does so quickly. Together they provide a sort of swirly multi-colour effect that I think is reminiscent of ST:ToS effects.

The red circle ‘red alert light’ is wired to the XBee’s RSSI so when the XBee receives a wireless command, the red light comes on for a few seconds.

The white gridded rectangle is the DHT22 sensor (temperature and humidity). I felt it would ‘blend in’ enough that it should be mounted right up front for all to see. The little black hole to the right of the DHT22 is for the light sensor.

Why is there a light sensor? Why not? Also: because I had an extra one laying around.

The screen display is mostly self-evident. Time, day, date. Heat/Cool. Run/Hold/Override. Target temp (small) and actual temp (large). Fan status (on/auto) and humidity.

The last line is EV (exposure value) and free memory. That’s 657 bytes. Not kB or MB, just bytes. It probably won’t ever change but it’s there just because there was space for it.

The following images have some more build details / information:

The sketch code, and a text file with lots of my design and build notes, can be downloaded here:


  1. […] Adafruit forum regular, Stephanie, has another amazing project to share with us.  She ripped most of the guts out of a toy Star Trek wall communicator and turned the enclosure into a custom home thermostat, the Thermostat Three. The red circle ‘red alert light’ is wired to the XBee’s RSSI so when the XBee receives a wireless command, the red light comes on for a few seconds. […]

  2. Thi says:

    Hi Stephanie,
    What a great project you’ve done.

    I want to build same thing you got, could you provide schematic or some sort of wire run list?


  3. Nagaki says:


    Congratulation on Eagle thing!
    It’s not that bad , is it?

    I have read your ISB6 project also, what a great schematic you’ve produced (for us!).

    I have a small favor to ask: can you do the same for the ThermostatV3? Or you can throw at me your napkin “note” of your schematic, I will finish it in Eagle and send it back to you.


  4. Stephanie says:

    I’m working on a schematic to this now too. It might take a bit longer as I’ve already kinda forgotten some of the details. I literally threw the thermostat together over a weekend, installed it, then stopped thinking about it. So the details are not as fresh in my mind.


  5. Stephanie says:

    Just a quick note – I have finished and posted the Thermostat V3 schematic.


  6. Mike Kelly says:


    I’m looking into a project similar to this (though maybe less ambitious).

    It looks like you’re providing power to this from a wall wart or something, but you seem to have the black “common” wire in your wall, which I think is supposed to allow you to draw current from the red wire to power your thermostat. Have you looked into doing “something like that”? I assume you’d at least have to rectify it to DC and then step the voltage down, but I’m not very expert in this so I don’t know exactly how doable that’d be, or if you’d be able to draw as much current as your system would require?

    1. Stephanie says:

      Hi Mike, thanks for the comments. You are correct, I’m powering this with a 5v regulated wall wart supply.

      The black wire I have in my wall is actually the ‘green’ HVAC line. I know there is a way some systems draw power from the HVAC lines but I haven’t found any simple definitive details and I don’t want to mess up anything expensive (i.e. furnace or A/C).

      My house is really old and I’m not 100% comfortable with the wiring as it is, so I’d rather not risk making things worse there.

      For now, I’m ok with the wall wart, the only thing I’d like to change is to run the power cable up inside the wall rather than outside.


  7. chris says:

    The aesthetic appeal kicks ass! does your website; very cool.

    I’ve been trolling for thermostat schematics and ideas for weeks, but most are so boring. Thanks for the inspiration (and technicals).

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