Monitoring my home's air quality (CO2, PM2.5, Temp/Humidity) with AirGradient's DIY sensor. I've made three of these so far (ordered a 10 pack of the boards and have been sharing them with friends). It's even handier using ESPHome to integrate with Home Assistant. It's fascinating to me how the measurements, especially for CO2 change over time and whether I'm in the room or not. For instance, I have one in the bedroom so I can see what sleeping is like. It's also possible to tell how sealed my house is, the CO2 drops every time we get a wind-storm without opening any windows...