The 7139 for wall mount is a single station smoke alarm designed to give reliable early warning of the presence of smoke where both audible and visual alarms are required. This model is AC powered so an electrician is not required for installation.
The smoke alarm operates on the light scattering principle, a superior method of detection in smoldering fires. It has a pulsing LED light source and a photodiode sensor in a fully screened sensing chamber. When smoke enters the chamber, the infrared beam is deflected onto the sensor. The LED rate increases to 8 times the normal pulse rate, and after the photodiode confirms the presence of smoke for 2 consecutive pulses, it will produce the signal necessary to trip the audible and strobe light alarms.