Abstract: A system for activating an operating element with respect to a moving substrate comprises a tracking line formed on the substrate, the tracking line having an activation mark formed therein which reflects light of an intensity which is distinguishable from that reflected from the remainder of the tracking line. A photosensor is mounted in relation to the tracking line to sense the intensity of light reflected from the tracking line when the substrate is moving. The photosensor generates an output signal indicative of the passage of the activation mark with respect to the photosensor. The output signal is utilized to activate the operating element.