CO₂ buildup in a living area can be a serious problem, and a sensor that fails quietly cannot be relied upon. [William Collis] addressed this important issue by creating CO₂ monitors built to fail visibly.

[William] has a nice home setup that can monitor CO₂ levels in two places, but noticed an important issue: the system always displayed the last known-good sensor reading. An unchanging, healthy-looking number is good! Unless it isn’t, because a flat unchanging number is also exactly what a dead sensor looks like.

The solution was for [William] to change how the sensors are treated so a faulty sensor isn’t hidden behind apparently-good values, and the sensor itself flashes a visible fault LED in the case of a problem.

Each monitor is a Sensirion SCD30 CO₂ sensor, paired with a Seeed XIAO ESP32-C3 running ESPHome to communicate with Home Assistant, an open-source home automation framework. There was no need to rewrite the ESPHome firmware; all his modifications fit within the context of a configuration file and certainly seem to be improvements over the original.

Sensors that can be relied on are important, but if the system itself needs some ruggedizing maybe consider a homebrewed UPS for Home Assistant.