Like many people, I have a CO2 sensor connected to HA. When it reached 1000pm, my desk light would flash, and I open a window. But I wanted something more, even more visual (and also play a bit with addressable LED lights).
So, I build a CO2 level display.
On the inside, it is a WS2812B 12-LED ring light connected to an ESP8266. It is housed in a 3D-printed case with a top that clips on very snug. I also inserted a piece of paper to diffuse the LED-light a bit. The 3D STL and Fusion360 can be found here.
The ESP8266 runs on a ESPHome config that separates each LED into its own lamp in HA. There I created a scene for each state (all green, one red, two red and so on). Then I created automation for CO2 levels from 400ppm to 1000ppm in 50ppm increments and connected them to the corresponding scene.
It may be inelegant, but it is the only way I know how to do it (More on that in this thread, maybe someone has a better idea).