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A sensor that detects both smoke and carbon dioxide has been developed a
Chicago-based company. At the heart of the device is a miniature microphone
sealed inside a transparent tube filled with CO₂. A pulsing infrared
source strapped to the side of the tube creates tiny pressure pulses in the gas,
which are picked up by the microphone (GB 2358245). The tube is housed in a
larger, perforated container with a second infrared source built into it.
Ambient air drifting into the container absorbs some of the infrared light from
the second source. The inventors at Pittway Corporation say that comparing
signals from the microphone when each bulb flashes will show how much
CO₂ and smoke is in the air.