In Xylophone Player, Becky Lippmann built an ingenious LEGO robot that could play a toy xylophone. The robot moved back and forth along a track, using a reflectance sensor to keep track of which key it was over. A spring-loaded arm held a mallet which could strike the keys with just the right touch.
People could get the robot to play by moving along a huge paper keyboard taped to the floor. An ultrasonic position sensor detected the person's position, then relayed the information to the robot via an infra-red signal. Knowing where the person was standing, the robot then played the corresponding note !