simple co2 sensor for indoor usage with led and buzzer signalling. based on esp32 and mh-z19 co2 sensor
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co2ampel

idea

higher concentrations of CO2 inside will make you sleepy and have an impact on your wellbeing. during the current pandemic higher CO2 concentrations inside are also a pretty good indication for higher levels of aerosol and as such a higher risk for infection. the 'CO2 Ampel' (CO2 traffic light) will give you a visual representation of the current situation and remind you to open windows to keep you happy and healthy

build

just solder everything according to the connection schema.

schema

ESP32 PIN peripherals note
VIN mhz-19 (VCC)
GND mhz-19 (GND)
GND KY-012 (GND) passive buzzer will do as well
D12 KY-012 (Signal/+)
3V3 SSD1306 (VCC)
3V3 WS2812 (VCC)
GND WS2812 (GND)
GND SSD1306 (GND)
D4 WS2812 (DI)
RX2 / D16 mhz-19 (TX)
TX2 / D17 mhz-19 (RX)
I2C SDA / D21 SSD1306 (SDA)
I2C SCL / D22 SSD1306 (SCL)

ws2812

if your ws2812 don't work on 3.3V power (most will do) you can try to use 5V instead. there are plenty different ws2812 builds so ymmv... if the leds don't work with 5V try to add a pullup resistor to your data line and a diode towards D4 to raise your high level while protecting your GPIO (https://forum.arduino.cc/index.php?topic=578735.msg3941756#msg3941756) looks like this:

either way it might be a good idea to have a capacitor between VCC and GND. my prototype worked without any of these, but from time to time some LEDs just randomly turn on...

readings for inside rooms

CO2 measure light buzzer meaning
400 ppm none no more or less the baseline of the sensor hardware and means 'fresh air quality'
400-800ppm none no safe zone and can be considered good air quality
800-1500ppm yellow no warning zone (consider to open windows) and is between medium and bad air quality
> 1500ppm red yes bad air quality, ppl in the room will be affected in a negative way

todo

  • cooldown time for buzzer (alarm only once every n minutes) -> lambda/interval/on_time?
  • add openings for mh-z19 to the case
  • adjust light tunnel to use frosted acrylic glass
  • add a silent switch for meeting rooms and such (no buzzer, but flash at a regular interval to attract attention)