Detection method photoelectric up to 24 units interconnection capability, Power source : 120 VAC, 9 V, audible alert 85 dB @ 10 ft., horn, visual alert red LED, diameter 5-3/4 in., height 1-7/8 in., width -, Digital readout : No, Battery backup : Yes. Features : Interconnectable, green power LED, red alarm status LED, quick connect power harness, end of life alarm, voice warning, Hush button : Yes, Locking pin : Yes, operating temp. : 40 degrees to 100 degrees F