First Alert SA521CN-3ST Hardwired one Link Wireless Smoke Alarm, 120 Vac, Photoelectric, 85 Db Help keep your family safe with this hardwired smoke alarm that wirelessly interconnects with other First Alertenabled alarms to create a home safety network Equipped with photoelectric smoke sensor optimized to detect larger smoke particles produced by smoldering fires; helps minimize the number of false alarms Wirelessly connect up to 18 First Alert enabled alarms for the maximum in whole-home protection Hardwired alarm includes 2 AA backup batteries for continued monitoring in the event of a power outage Includes test/silence button, low-battery warning, EZ access battery door