Universal Security Instruments USI-7795 120-Volt AC/DC Wired-In Combination Smoke and Carbon Monoxide Alarm *2-Pack* Interconnect up to 18 units, 12 smoke alarms maximum; when one alarm goes off all units sound alarm Convenient silence button with automatic reset allows you to temporarily override an alarm Alarm and power test buttons; includes a battery to power the alarm in the event of an power failure The alarm sounds with loud horn and red LED indicator flashes; green LED light indicates AC power The quick-draw pull-out drawer makes battery replacement easy