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.