Abstract: A crib mobile responsive to body motion of an infant. The mobile system is based on use of a passive infrared sensor and associated electrical switching of the type employed for security systems that turn on floodlights in response to detection of the presence of an intruder. An A.C. motor, drive shaft, and support rod for the mobile are mounted on an arm that extends from a housing connectable to a ceiling, and the sensor is mounted on another arm. Electrical connection may be made to a conventional A.C. ceiling outlet. The sensor provides a signal to activate the motor when it detects motion above a predetermined level, and it includes a time delay feature that prevents turning the motor off for a period after such motion is no longer detected. Sensitivity of the sensor and the duration of the time delay are adjustable. The mobile structure includes horizontally extending arms for supporting toys or decorative objects for being rotated in a circular path above a crib.