This instrument is designed for precision parking.If you find it difficult to judge how far the car is from the inner wall when you garage your car,the instrument puts an end to all those trial-error bumps and knocks.It uses ultrasonic technology and work automatically .it is powered by a 9v battery(6f22) or an external power source which provides dc voltage of 12v. it consists of a display unit, a sensor unit and a cable. Low battery indication is also provided.