Universal Security Instruments USI-5204 120-Volt AC/DC Wired-In Ionization Smoke and Fire Alarm Interconnect up to 18 units, 12 smoke alarms maximum, when one alarm goes off all units sound their alarm Silence feature, a convenient silence button with automatic reset, allows temporarily 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 warning, alarm and power test buttons, includes 9-volt super heavy duty battery Full-size plastic dust cover protects the smoke alarm from spray applications, dust, and other contaminants