A smoke detector is an electronic fire-protection device that automatically senses the presence of smoke, as a key indication of fire, and sounds a warning to building occupants.
Smoke detectors are housed in plastic enclosures, typically shaped like a disk about 150 millimetres (6 in) in diameter and 25 millimetres (1 in) thick, but shape and size vary. Smoke can be detected either optically (photoelectric) or by physical process (ionization). Detectors may use one of the two, or both sensing methods. Sensitive alarms can be used to detect, and deter, people smoking in areas where smoking is banned. Smoke detectors in large commercial and industrial buildings are usually connected to a central fire alarm system.