The Gentex 7139 for ceiling mount is a photoelectric 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 hard-wired so you would need an electrician to hook it up.
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 with 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. The 7139 series provides the temporal 3 (T3) evacuation tone as a standard feature. When testing it may take up to 16 seconds longer to go in or out of alarm.