Smoke detectors are devices that sense smoke as an indicator of a fire.
They can work as separate local alarms, giving an audible signal from the detector, or as part of an alarm system, in order to detect local fires and to generate an alarm in the complete building which as well can be transmitted to the responsible fire brigade.

The way of detection is either photoelectric (optical detection) or by ionization.
More and more countries have introduced regulations, to use smoke detectors in every household.