The wireless Door Knock/Doorbell and Door Chime Sensor is part of the CentralAlert Notification System by Serene Innovations. Simply install the CA-DX sensor on the existing door chime or intercom door announcer for vibration detection. When someone rings your doorbell, the CA-DX sensor unit detects the vibration and sends a wireless signal to trigger the CA-360 clock/receiver unit. (An electrical signal from your doorbell activates the CA-DX). Alternatively, the CA-DX sensor can be attached to the inside surface of your door to detect door knocks. The CA-DX has a 2.5mm jack for optional direct connection to an existing doorbell/door chime (no cord included). The Door Knock Sensor with Door Hanger is part of the CentralAlert Notification System by Serene Innovations. It sends to the CentralAlert receiver to notify you when someone is knocking on the door. The CA-DX Door Knock Sensor is ideal for bedrooms or hotel rooms and is safe and secure since there is no part visible outside the door. Features: 2.4GHz operating frequency Uses 2 AA alkaline batteries (not included) Dimensions: 2.5" x 3.8" x 1" Click for user manual (Doorbell & Door Chime) Click for user manual (Hanger) Options available: Doorbell & Door Chime ($49.95) (pictured above) Detects vibrations of existing doorbell, door chimes, door intercom announcer or knocking and sends a signal to the CA360 unit Easy to install with adjustable sensitivity to avoid false alarms 2.5mm jack for direct connection to existing doorbell/door chime (cord not included) Up to 200 ft operating range