After reading about the impact of CO2 levels on cognition I created a simple prototype of a CO2 monitor and level indicator. It also reports the readings to a mobile phone using Bluetooth LE.
Posted a few years later hackster.io See the project on hackster.io
This prototype is still functioning very well, now many years old. There have been a few iterations since.
Here are the parts. Everything from Adafruit except from the K-30 CO2 sensor from http://CO2meter.com
The most basic enclosure possible. Not pretty but it get's the job done. The gap in the back is because the parts are actually too deep to fasten the screws on the back plate. In use at home I use a rubber band to keep it from falling off. It makes it look like it has a head-band. lol.
Once I got into Fusion360, this is what I designed. I think it looks like a dial-up modem from the 90's. Much better than the original enclosure though. You can buy my design here on Scuplteo https://www.sculpteo.com/en/printnew/co2btle_enclosure/vj8RfEkK