Universal Security Instruments SS-795 120-Volt AC/DC Wired-In Ionization Smoke and Fire Alarm with Silence Feature Interconnect up to 18 units, 12 smoke alarms maximum, when one alarm goes off all units sound their alarm Convenient silence button with automatic reset, allows temporary alarm overriding Includes a separate large size 5-5/8-inch mounting plate, which attaches to any standard rectangular or octagon junction box Audible low battery indicator, alarm and power test buttons, includes 9-volt super heavy duty battery Ideal for replacement of most existing smoke alarms, fast and easy installation