Snd-500 dc smoke alarms provide battery powered smoke detection and alarm functions within a single unit. Advanced electronics in conjunction with a photoelectric smoke sensing chamber provide early detection of fire And high immunity against unwanted alarms. Snd-500 dc smoke alarms are battery powered, do not require a mains supply, and are suitable for general Residential applications. Photoelectric smoke sensor Integrated indicator provides visual indication of an alarm condition Test button verifies battery and alarm operation Loud 85 db sounder to alert occupants Single dc 9 v user-replaceable battery provides up to 18 months operation under normal conditions 30 day low battery warning signal Interconnection function available to connect up to 38 units together Battery missing indicator