> (LEDs and Photodiodes typically are only active at one frequency: such as 900nm or 700nm). That's not entirely correct. The spec sheet for the LED will give

> (LEDs and Photodiodes typically are only active at one frequency: such as 900nm or 700nm). That's not entirely correct. The spec sheet for the LED will give an intensity vs. wavelength chart, which depending on the LED can be 10s to 100s of nanometers wide. As long as the source LED has a sufficiently bright output in one of the CO₂ absorption bands, it doesn't have to be exactly tuned like a laser. NDIR is the technology used in most consumer-grade CO₂ sensors: https://en.wikipedia.org/wiki/Nondispersive_infrared_sensor https://en.wikipedia.org/wiki/Nondispersive_infrared_sensor