Infrared sensors detect changes in the signal received from a light emitting diode due to the presence of smoke in the light path or the presence of an object in the light path.
Ionization chambersare devices that detect the leakage current between two plates that have a voltage between them. The leakage occurs when carbon particles from smoke are present and provide a conductive path between the plates.
Taguchi-type sensors are used for the detection of hydrocarbon gases, such as carbon monoxide and dioxide, methane, and propane. The Taguchi sensor has an element coated with an oxide of tin that combines with hydrocarbon to give a change in electrical resistance which can be detected. Periodically the element is heated and the chemical reaction is reversed, reducing the coating back to tin oxide. Likewise, the sensing process can be repeated. The tin oxide can be made sensitive to different hydrocarbons by using different oxides of tin, different deposition techniques, and so on.