“I have recently been spending a lot of time indoors working with human breathing physiology. Prior Instructables have used O2 and CO2 sensors and I had tried switching to a new one from Sensirion SCD 41. It is very tiny and uses a new type of sensing for the level of CO2 …acoustic something or other. The previous version: SCD 30 works really fast and has a wide detection range…it is based on laser CO2 light absorption. When experimenting with the small model I found its speed too slow for my use and the range limiting so I was looking for something else to build with them. I have put together a short instructable on how to built a elegant very tiny CO2 sampler for home or school use. It uses the raw SCD 41 that is usually surface mounted or you can obtain it already mounted on a trial board but it is so much bulkier and not as much fun to put together. The other requirement was that it be unobtrusive and have a tiny buzzer to alert the occupants that the boring stream of data had gone over some limit—much like a fire alarm. Naturally it would have to have an App to graph your data stream and keep it locally on your phone.
There are only 3 components to this project. Total cost about $50.
1. 3v buzzer CMI-1295IC-0385T
2. SCD 41
3. TTGO T-Display ESP32 CP2104 WiFi bluetooth Module 1.14 Inch LCD Development Board
The SCD41 also comes as a development board from a variety of sources—some have STEMMA QT connectors for easy hookup. These boards would make the whole unit considerably bigger but the all the rest of the components would be the same.”