I'm looking for an odor sensor--specifically, one that senses those bathroom odors that make people reach for the air conditioner.
I realize it's possible to hook up a methane detector near the, ahem, exhaust, but this will only detect some odors. The contents of the John tend to produce an unpleasant smell that is often more persistent than the winds passed in the process.
Of course, a better-safe-than-sorry approach would be to decide that if someone is sitting on the can for some time, then they're probably doing big business and the fan should be turned on regardless of any smell. This would eliminate the indirect message telling you how you're affecting the climate, but for noise reduction purposes it might be better to leave the fan off until there's an actual use for it. (Which, I suppose, is a way of saying that only certain matter should hit the fan.)
But, does any such odor sensor actually exist?
