Apparatus is disclosed for obtaining torque and angular velocity of a load transmitting shaft of a prime mover to provide control functions that can be fed back to the operational controls of said prime mover thereby to monitor and control the operating characteristics thereof. The apparatus is described primarily in connection with the preferred embodiment thereof in which torque and rpm data from the drive shaft of an automobile are used to derive further data in the way of electric signals which in turn are fed back to control the automobile output and provide, among other things, pollution and/or efficiency control for the automobile. The torque values are obtained using a light modulation scheme whereby the modulated light provides electric signals from which the torque information is derived.