A Smoke sensor or Smoke detector is a device that can detect smoke, which may be an indicator of a fire. There are two basic systems: Simple, standalone sensors usually make a sound or flash a light when they detect smoke. More sophisticated sensors usually send a signal to an fire alarm panel, or system. Most smoke detectors use either an optical sensor, or they use a physical process called ionization. Many simple smoke sensors use batteries. Very often dead batteries are not replaced; when this happens, the smoke sensors stop working. But, the detector may "chirp" when the battery is low to try and stop this from happening. There are also systems which are directly connected to electrical power; these may use batteries just as a backup.
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 either, or both, methods. Sensitive alarms can be used to detect, and thus deter, smoking in areas where it is banned. Smoke detectors in large commercial, industrial, and residential buildings are usually powered by a central fire alarm system, which is powered by the building power with a battery backup. Domestic smoke detectors range from individual battery-powered units, to several interlinked mains-powered units with battery backup; with these interlinked units, if any unit detects smoke, all trigger even if household power has gone out.