A compact DIY CO₂ monitor built around a Sensirion SCD41 and Seeed Studio XIAO ESP32-C3.

Three LEDs provide an immediate indication of the current CO₂ level, while measurements can also be sent over Wi-Fi to a simple PHP server for CSV logging and graphs. The battery-powered version spends most of its time in Deep Sleep to reduce power consumption.

I also made a standalone firmware version that creates its own Wi-Fi access point and hosts a local webpage with current values and 24-hour graphs, without requiring an internet connection or external server.

The electronics are housed in a handmade plywood enclosure with ventilation holes around the CO₂ sensor.