Serene Innovations CentralAlert Wireless Door-Bell Knock Sensor,Dimensions: 2.5" x 3.8" x 1",Each,CA-DXPrice: USD39.95 Serene Innovations CentralAlert Wireless Door-Bell Knock Sensor detects vibration and sends a wireless signal to trigger the CA-360 to alert when someone rings your door bell. Install device on existing door chime or intercom door announcer for vibration detection or, you can attach the CA-DX to the inside surface of your door to detect the knocking on your door.Other products from H e a l t h P r o d u c t s F o r Y o u |