Fire Sentry Battery Operated Full Size Smoke Alarm, Battery Operated Smoke Alarm Utilizing Ionization Sensing Technology, 4″ Unit Is Smaller In Diameter Giving It Discreet Design With The Same Quality You Would Expect In A Kidde Manufactured Product, Powered By A 9V Battery, Providing Continuous Protection, Even During Power Outages, Easy Install Unit Includes A Test Button To Verify The Detection Electronics Are Working As Intended.