Universal Security Instruments USI-1209 120-Volt AC/DC Wired-In Ionization Smoke and Fire Alarm AC/DC Ion Smoke Detector 120V 1PK Alarm and Power Leds. 85Db Alarm 5-5/8" Base Interconnect up to 18 units, 12 smoke alarms maximum; when one alarm goes off all units sound their alarm The pack includes a separate large size 5-5/8-Inch mounting plate, which attaches to any standard rectangular or octagon junction box and covers drywall cutouts This 120-Volt AC powered alarm also includes a 9-Volt super heavy duty battery backup which provides power in the event AC power is lost The chamber and cover design protect the alarm from bugs and contaminants The alarm sounds with its loud 85 decibel horn and the red LED indicator flashes. A green LED light indicates AC power