Abstract: A motion detector for use with trailers and the like. The motion detector includes a modular unit with its own power source, a motion detector, logic circuit and electro-audio warning device. The power supply is preferably a rechargeable battery. The motion detector may be an ultra-sonic motion detector which uses the Doppler effect to determine motion. The motion detector produces a signal which indicates the direction of travel and the speed of movement. The logic system discriminates between forward and reverse motion and issues a signal when the vehicle is moving in a reverse direction. The signal activates an electro-audio device which produces a pulse audio warning in response to the movement.