Kidde 21006371 p12040 Hardwire with Battery Backup Photoelectric Smoke Alarm LED - Red LED indicates that the smoke alarm is operating properly, HushTM, and alarm condition. Green LED indicates the presence of AC power Photoelectric Sensor  Advanced photoelectric sensor may detect visible particles (associated with smoldering fires) sooner than ionization alarms. Dust cover protects sensor during construction Battery Backup (9V Battery Included)  Provides continuous protection even during power outages. Battery Pull Tab eliminates battery installation time and keeps battery fresh Test Button  Tests units electronic circuitry, horn and battery function Quick Connect Power Harness  Makes installation fast and easy