Measuring a fraction of the size of a traditional smoke alarm, the First Alert Atom Smoke and Fire Alarm with 10-Year Battery delivers ultra-reliable smoke detection in a sleek, compact design. This smoke detector uses a photoelectric sensor to detect smoke from smoldering fires. A built-in 10-year lithium-ion battery provides continuous power, with no need to ever worry about changing batteries during the lifetime of the alarm. Much smaller than a traditional smoke detector, it features a modern, ultra-compact design that can be unobtrusively mounted on a wall or ceiling. When smoke is detected, the smoke alarm sounds a loud 85-decibel siren. The First Alert Atom Smoke and Fire Alarm features a built-in 10-year lithium battery that provides continuous power for the life of the detector. An end-of-life timer lets you know when the device should be replaced.