A detection device detects smoke as an indicator of a fire. As part of a fire alarm system, commercial detection devices send a signal to a fire alarm control panel. Household smoke detectors emit audible or visual alarm from the detector itself or multiple detectors. Detection devices usually housed in a plastic enclosure shaped like a disk. About 150 millimetres (6 in) in diameter and 25 millimetres (1 in) thick, but shape and size vary.
In large commercial and industrial buildings, smoke detectors typically linked to a central fire alarm system. Domestic smoke detectors range from single battery-powered units to multiple interconnected units with backup batteries.