Smoke Detector, Hardwired Interconnected Smoke Detectors, Smoke Alarm with Replaceable 9V Battery, Interconnects Up to 12 Fire Alarms Smoke Detectors, Fire Alarm with Test/Silence Button 4 Packs About this item [Hardwired interconnection] Up to 12 smoke alarms can be interconnected to cover your entire home. When one detects smoke, they all make a sound to alert everyone in the house, creating an interconnected safety net for your home. [9V Battery backup] Hardwired smoke detectors are equipped with DC 9V batteries. Ensure that all units are always connected and continue to function properly, even in the event of a power failure. Low battery alarm reminds you to replace the battery as soon as possible. If an error occurs, the system automatically triggers the fault mode, prompting you to check the device. [Test Mute button and LED light] It is connected with other alarms. Just press the test/mute button on one of the units to test or mute the entire interconnected system. If the smoke alarm is in normal working condition, the green LED light is always on and the red LED light flashes every 32 seconds [Photoelectric smoke sensor] Based on advanced photoelectric technology, smoke alarms can be more sensitive, fast, accurate measurement of smoke levels. More than 85 decibels of alarm sound, you can more clearly hear the danger coming. [Simple installation] The installation of smoke alarm is very simple. The AC connector is connected only to the mains power and signal cables. (Please connect as instructed) Package Included: 4 packs smoke detectors, 1*dust cover, 1*hard wiring,1*instruction manual,1* 9V battery,1*screws set