They are devices can detect smoke and carbon monoxide, 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. Since smoke alarms work to constantly monitor the air 24 hours a day, they may begin to malfunction over time from exposure to accumulated dust, insects, airborne contaminants, and corrosion. Most smoke detectors will have an expiration or replace-by date stamped/printed on them; however, if your smoke alarm does not have a date on it, it is important to replace it once every 10 years. At the end of 10 years, the smoke alarm will have gone through millions of monitoring cycles and the components may have become less reliable.