CentralAlert Door Alert with Doorknob Hanger
CA-DX Door Knock sensor is mounted on a plastic doorknob hanger for portability. The hanger is placed on the doorknob on the inside of a door where it will pick up the vibration of someone knocking. A signal is sent to the CentralAlert receiver to alert the room's occupant. Ideal for hotels or dormitories where permanent installation isn't appropriate. The CA-DX has a sensitivity adjustment to avoid false alarms. Operates on 2 AA alkaline batteries (included). Battery life up to 10 months. One year warranty.