Product review (5 stars): Informative, unobtrusive sensor
I was worried about CO2 levels in my basement office. Turns out they levels are fine, but there is some fluctuation. 400ppm with the windows in the house open, up to a max of about 950ppm when the windows are closed and I'm running around.

In the dark, I noticed that it shines an internal light every once in a while. Don't know if that's for sensing CO2 or relative humidity. Doesn't really bother me. A very useful, unobtrusive device.