January 22, 2014 weblog
Spark: Look Ma, an open source thermostat
The user can change the temperature by turning a ring; the temperature appears on an LED display.
How did they do it, in spending little time and cash to put it all together? They cut out wood and plastic and added Spark Core and control logic. That's the short story. For those who want to understand, though, what they actually had to do to make this work, the Spark team posted a generous discussion, step by step.
First, the hardware. "In our case, that means sensors for temperature and humidity, plus a motion sensor to figure out whether you're home, and relays to control the furnace and the fan. We also need a display so you can see the current temperature, and an enclosure to protect the messy bits." Then they said, they did some "breadboarding" to create an early prototype. They chose the Spark Core as the "connected brain," and showed the temperature on Adafruit LED matrices. The display interface was a I2C bus. The primary sensor was a Honeywell Humidicon temperature and humidity sensor.
Several LEDs could represent whether the heat and fan were on and they added a Panasonic PIR motion detector. They built an enclosure using acrylic and wood. They laser-cut acrylic disks: for a faceplate, for the wall-mounting plate, and for the spinning wooden ring to a potentiometer; they soldered the components.
The thermostat, they said, is complemented with a cloud-based web app that handles the complex logic of the thermostat. "By doing this in the cloud, we can iterate faster using high-level programming languages and frameworks like Ruby on Rails rather than low-level embedded C."
The creators at Spark are proud of their outcome—they did after all build a thermostat in one day and open sourced everything— but they have also made something else clear: "We're not claiming to have matched the Nest thermostat in a day; far from it. But remember—every polished product starts as a rough prototype."
Spark's demo shows the DIY spirit and it also proves people who love building things, especially, connected things, can just do it.
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