Abstract: A sensor having a shutter mounted upon a shaft for rotation within a housing in opposition to a light path between an LED and photodetector is described. A potion of the shaft extends outside the housing connected to an elongated arm. A runner secured to the elongated arm, engages a moving photosensitive surface and deviations of the edge position of the photosensitive surface rotate the shutter in relation to the light path between the LED and photodetector for tracking the edge position of the moving photosensitive surface by providing signals representing shutter position.