EYE ON NPI - Sensirion STCC4 Miniature CO2 Sensor

Adafruit Industries

This week's EYE ON NPI will help you breathe easier, with the smallest CO2 sensor we've ever seen: it's the Sensirion STCC4 Miniature CO2 Sensor (https://www.digikey.com/en/product-highlight/s/sensirion/stcc4-miniature-co2-sensor) Sensirion has always been our top choice for air quality sensing, and now they've got the tiniest sensor yet with ambient-air CO2 measurements. We've covered many Sensirion CO2 sensors before, and made breakouts for the most popular like the SCD-30 (https://www.digikey.com/en/products/detail/sensirion-ag/SCD30/8445334) and SCD-40 (https://www.digikey.com/en/products/detail/sensirion-ag/SCD40-D-R2/13684003). Sensirion has also made fully-integrated sensors like the SEN-66 which have an SCD sensor inside (https://www.digikey.com/en/products/detail/sensirion-ag/SEN66-SIN-T/25700945). There's also older eCO2 sensors like the SGP30 (https://www.digikey.com/en/products/detail/sensirion-ag/SGP30-2-5K/7400966) which did 'effective' CO2 measurements by estimating based on organic gas concentrations. While CO2 measurements have always been important for keeping humans and animals happy (https://en.wikipedia.org/wiki/Carbon_dioxide#Human_physiology) - our bodies and brains don't like it when the CO2 concentration goes over ~2000 ppm - it was fairly uncommon to see CO2 monitors in homes or offices. That changed with Covid, because CO2 became a good stand-in for air circulation / clearance: outside air is around 400 ppm, so the closer the indoor air is to 400 ppm the better the circulation. For folks who need the most accurate CO2 sensing, we'd still point them to the SCD-30 NDIR as a gold-standard (https://www.digikey.com/en/products/detail/sensirion-ag/SCD30/8445334) but it has the side effect of requiring a lot of space and is not particularly low power. The SCD-40 improved on the size/power requirements, using acoustic sensing instead of infrared light. However, if you want something really small, for wearables or phones or portable sensing, we now have a new sensor! The Sensirion STCC4 Miniature CO2 Sensor (https://www.digikey.com/short/nn982w9w) is only 3mm x 4mm x 1.2mm and uses thermal conductivity of the ambient air to calculate CO2 concentration. This means it works only for 'natural ambient air' measurements that have a similar profile to outdoor/indoor air, it's not good for scientific measurement or extreme/outlier locations and situations. Like the SCD30 and '40 series, the STCC4 will auto-calibrate (https://www.digikey.com/short/nn982w9w) to account for drift. To do that, it must be exposed to outdoor air, with approximate 400 ppm CO2 concentration once a week. Once it has completed its initial startup calibration, it will give measurements with +-100ppm accuracy. Note that this is not as good as the SCD30's +-30ppm or the SCD40's +-50ppm as the tradeoff for the smaller size and price. It also works best with separate temperature + humidity calibration - they suggest the SHT4x series such as SHT40 (https://www.digikey.com/en/products/detail/sensirion-ag/SHT40-AD1B-R3/14322709) or SHT41 (https://www.digikey.com/en/products/detail/sensirion-ag/SHT41-AD1B-R3/15296592) which you can wire up to the peripheral I2C pins for automatic readings. We noted that although the specifications for the STCC4 imply you can use 5V power/logic, that doesn't apply to the SHT4x series so its better to just have everything run at 3.3V. Sensor readings happen over I2C, and if you've used other Sensirion products you're probably familiar with their 'Command / Response / CRC' style of messaging. Thankfully no clock stretching is used, although it will NAK if the message isn't handled during a read. Two I2C addresses are available thanks to an ADDR pin. And if you want to get started fast, there's a ready-written Arduino compatible library available on GitHub (https://github.com/Sensirion/arduino-i2c-stcc4) as well as Python and embedded C (https://github.com/Sensirion?q=stcc&type=all&language=&sort=). For fast plug-and-play integration, Sensirion has also released an eval board (https://www.digikey.com/short/qwn75j80) and we really like that they went with a simple low-cost Qwiic/Stemma QT design (https://learn.adafruit.com/introducing-adafruit-stemma-qt/what-is-stemma) with integrated SHT4x that you can use immediately with dev board that has a JST-SH compatible connector. If you want to integrate the smallest, lowest-cost CO2 sensor we've seen, from the experts at Sensirion, check out the Sensirion STCC4 Miniature CO2 Sensor (https://www.digikey.com/short/nn982w9w) - it's in stock right now for immediate shipment from DigiKey! Order the STCC4 sensor today and by tomorrow morning you'll be taking measurements of indoor CO2 with ready-to-go eval board and firmware example code.