I am building a heat exchange ventilator (AKA: "HRV"). The purpose is to exhaust stale air from a home and to admit fresh air, but to use the heat from the outgoing air and to warm up the incoming air, to save energy. The efficiency of most HRVs is pretty good, but the less they have to run, the less energy is lost.

I have been working on a project to use a CO2 sensor to control fan speed, so that the fan speed is proportional to parts per million (AKA: "ppm") of CO2. My project uses a Telaire 6004 CO2 sensor, which has both an analog output and a digital output. I used the analog output method, which is probably good enough, but would have preferred to use the digital method instead. I searched for an Arduino library for the Telaire 6004, and could never find one. I didn't have the technical documentation on the Telaire 6004 either.

I did finally find the technical document HERE. But having found it, I don't know what to do with it. I could use some help.

HERE is a link to my project, which used the analog line from the Telaire 6004, and also accurately reads and displays temperature and humidity.