The Beginning

<sometime in 2016>

I decided that I needed a new thermostat for the estate. The requirements were relatively simple: programmable schedule, automatic switch over between cooling/heating based on setpoint and space temperature, humidity control, touch screen control, and last, but not least the thermostat has to be standalone. It must not have any reliance on the manufacturer’s computing infrastructure for continued operation.

To make a long story short, the thermostats that fit the criteria were expensive ($250 and up) and were all ‘wifi enabled’. Theoretically I could live with a wifi enabled thermostat by simply not setting up the wifi, but the concern was how would the thermostat behave with the wifi not configured. Would it constantly nag me to configure it, have various network related delays, and etc. Price-wise though, for $200+ dollars one can get quite a bit of hardware. In fact, almost enough hardware to get a custom thermostat and a good start on a custom home automation system.

Initially I tossed around the idea of creating a thermostat with a touch screen and a single board computer. But that ate up the budget very quickly, I had trouble finding the right packaging (I wanted something that would not look out of place on a hallway wall), and the IO options really suck until you get into the ‘industrial control’ single board computers at which point the price gets real steep real fast. Then I had an epiphany – why do I even need a touchscreen on a thermostat? A properly designed thermostat should not need constant and ongoing interaction – it should just work.

Removing the display and deciding to forgo a proper single board computer freed up the budget significantly – enough so that I can get a very decent start on the custom home automation system I’ve been tossing around in my head. More about that later.

Broad design specs for the main controller:

• Passive cooling is a must.
• Completely open architecture.
• Native digital and analog IO.
• Wired Ethernet.
• Reasonable price point.

General system design specs:

• 24VAC control voltage
• Preferably current-loop based sensors (ICTD temperature probes and 4-20ma sensors)

Given the specs above, this is what I’ve decided on so far:

The main processor

The BeagleBone Black – checks off all the boxes. Thermostat duties will be it’s first job. With time I plan on expanding its role and running the whole home automation system on it.

http://beagleboard.org/black

Wall mounted temp and relative humidity probe

Another item that checks all the boxes – it is available with 4-20 ma output for both space temperature and relative humidity. Additionally, it has a local display and does not look out of place mounted on a wall.

https://www.dwyer-inst.com/Product/AirQuality/Humidity-TemperatureTransmitters/SeriesRHP-E-N

Money spent so far

Purchased and on the bench. The next update will be about this little bad boy.