Great project by Brian Chamberlain up on Hackster.io.
This project shows how to build a CO² for use in your home or office. With this device you can know at a glance if the air you are breathing is high in carbon dioxide. Using a simple color coded LED bar graph it displays the current CO² level. Each band is roughly 100 ppm of CO² increase. The range of the device is from 300 ppm (1 led lit) to 2,700 ppm (all 24 leds lit). For the precise reading use your mobile phone and connect to it over Bluetooth to monitor/track readings over time.