This tiny SCD41 CO2 Sensor breakout board is pretty much as small as it could possibly be. The SCD41 is a relatively new sensor from Sensiron that has both a high accuracy as well as a large detection range from 0 to 40000ppm of CO2! Inside this tiny sensor is some extremely cool technology called Photoacoustic NDIR sensing.
Like most CO2 sensors, it exploits the fact that CO2 strongly absorbs very particular frequencies of infrared light. The difference is that instead of measuring the absorbed light directly, the SCD41 measures the absorption acoustically. This works because CO2 absorbs infrared light periodically, creating molecular vibration which leads to a pressure wave inside the sensor unit. A small microphone picks up the amplitude of this pressure wave, and this is used to calculate the percentage of CO2 present!
Why go to all this trouble when you can simply measure the light directly? Well, the photoacoustic method allows for much smaller sensors, which is one of the biggest forces driving innovation in sensors these days. Manufacturers want to make their sensors as small and accurate as possible so they can fit inside tiny, battery-operated devices like smartphones, smartwatches, and just about anything in the IoT space. Another advantage is that there is no sensitive optical path that needs to stay in alignment, meaning the sensors are typically more robust to sudden acceleration or high G impacts.
The sensor also includes an accurate temperature and humidity sensor, which can be used in combination with the CO2 reading to give an accurate reading of the air quality inside a particular space. As was widely discussed during the pandemic, stale air that is high in CO2 impairs humans’ cognitive abilities, making us feel tired and unmotivated. CO2 can even be dangerous at high enough levels, so it’s important to keep an eye on the air quality in your home, office, and hackerspace!
If you’re interested in reading more about Photoacoustic NDIR CO2 sensing, Sensiron has a short info note that summarizes the topic and includes references to more advanced papers.
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