Brand New Directed DEI 509U Ultrasonic Sensor Car Alarm Security New DEI XpressKit 509U Key Features Ultrasonic Sensor for Security Systems Detects air pressure changes in the cabin of the vehicle Adds extra protection when used in conjunction with other security systems Detect thieves entering vehicle without glass breakage Effective only when all entrances are secured and closed (doors windows and sunroof) NOT RECOMMENDED FOR CONVERTIBLE VEHICLES Dimensions: 4.5 L x 5.3 W x 2 H