First Alert 9120B Smoke Detector, Hardwired Alarm with Battery Backup, 1-Pack - Amazon.com
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Smoke Detector, Hardwired Interconnected Smoke Detectors, Smoke Alarm with Replaceable 9V Battery, Interconnects Up to 12 Fire Alarms Smoke Detectors, Photoelectric Fire Alarm with Test/Silence Button - Amazon.com
X-Sense Hardwired Smoke Detector, Hardwired Interconnected Smoke Alarm with Battery Backup, Interconnects Up to 18 AC-Powered Alarms, XP04-S, 1-Pack - Amazon.com