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We had a self-made CO₂ sensor running in our living room for quite a while and it actually helps to determine when to open a window, especially in winter… The commercially available sensors are either super expensive or stand alone without a way to send a notification. So of course I made my own!
There are affordable sensors available that can be connected to a microcontroller, in my case an ESP8266, which makes it more versatile. The prototype that was running for over a year worked perfectly but didnt exactly look nice so I turned it into a proper device with everything on a single board – the ESP, the sensor, 3 LEDs as a “traffic light” to show the current air quality and a 4-digit display to show the current value.
The sensor I use is the MH-Z19C by Winsen which is available for less than 25€. The display is a standard 4-digit 0.56″ 7-Segment display driven by a TM1637 which is cheap and widely supported by libraries for about any platform – you also get this configuration as a module like the one in the prototype pic at your favorite cheap electronics dealer…
Everything is controlled by an ESP8266 on an ESP-12F module which is my go-to module for anything WiFi that doesnt need more I/O. Since we are using Home Assistant for a while now I chose to use ESPHome to create the software for the module. But it will also run with a custom version of Tasmota or any other platform that supports the parts used.
I also added a CH340C USB to Serial IC to make flashing the ESP easier but in hindsight that was not really necessary since ESPHome supports OTA by default after the initial install.
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The finished version looked a lot better than the prototype and is now sitting on a shelf in the livingroom. The sensor is not soldered in but uses a friction fit to stay in place since it is a quite expensive component. The board is a 5×5 cm 4-layer pcb which can be ordered for cheap, for example from JLCPCB (not a sponsor) and is easy enough to solder yourself.
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