Check the listing for details. Serene Innovations CentralAlert CA-DX Doorbell / Door Knock Sensor. Color: White, Condition: New. Listed at 45.95 USD. The wireless Doorbell/Door Knock 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). To see how to register (learn") the CA-DX doorbell/door knock sensor with the CA-360 clock/receiver, see the video below: Features:Detects vibrations of existing doorbell, door chimes, door intercom announcer or knocking and sends a signal to the CA360 unitEasy to install with adjustable sensitivity to avoid false alarms2.5mm jack for optional connection to existing doorbell/door chime (cord not included)2.4GHz operating frequencyUp to 200 ft operating rangeUses 2 AA alkaline batteries (not included)Dimensions: 2.5" x 3.8" x 1" What else do I need to make this work?In order to be notified, you will need to purchase a compatible receiver (see Accessories box).