The self-contained ball-strike detector uses two transducers to detect the presence of an incoming pitch, and a series of transducers located on the upper surface of a home plate-shaped housing to determine whether the pitched ball is within the strike zone. Ultrasonic transducers are located near both...http://www.google.com/patents/US5401016?utm_source=gb-gplus-sharePatent US5401016 - Automatic baseball ball and strike indicator
