This smoke alarm is designed for living areas and hallways. Photoelectric smoke alarms have a sensing chamber which uses a beam of light and a light sensor. When smoke enters the chamber it changes the amount of light that reaches the sensor and the alarm sounds when the smoke reaches a certain level. They are best at detecting slow burning fires which burn for hours before bursting into flame. The alarm is always on, with no battery replacement required for the 10-year life span of the alarm.