The GE Smart Home Ultrasonic Sensor Parking Alert uses ultrasonic, range-finding technology this item will help consistently guide your car to a safe stopping distance in your garage. This GE Smart Home Ultrasonic Sensor Parking Alert is a simple sensor system that easily mounts to the wall with the included velcro strips or pan head screws. Once mounted to the wall this battery operated sensor will detect how far you are from the wall (you can select the desired stopping distance between 1 and 3 feet). The alert light changes from yellow (slow down) to red (stop). The GE Smart Home Ultrasonic Sensor Parking Alert is simple to use and much cheaper than repairing your walls or car. The ultrasonic frequency is 40 KHz and has a maximum detection range of 6 feet. The dimensions of the sensor are 6 inches x 3 inches x one and one-quarter inches. The alert is 3 inches in diameter and measures 1 inch thick. A low battery indicator is built in to let you know when to replace the batteries. This unit requires 4 AA batteries that are not included.