The electrical warning devices serve as early detection systems for dangerous gases. They are equipped with highly sensitive semiconductors that registers even low concentrations of gas such as propane, methane, or carbon monoxide in the air. If the composition of the ambient air changes, the resistance of the semiconductor also changes. Other gas detector sensors work with infrared technology or electrochemical or catalytic functions. Following a clear positive, the device activate a siren, and special design also cut of the supply of the detected gas. Stationary gas detectors tend to be used in boiler rooms and underground garages or in boats and motor homes.