Universal Security Instruments SS-785 120-Volt AC 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 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 The alarm sounds with its loud 85 decibel horn and the red LED indicator flashes. A green LED light indicates AC power The chamber and cover design protect the alarm from bugs and contaminants Ideal for replacement of most existing smoke alarms