Figure 14 shows the circuit diagram of the slotted optical vane switch. For the Mark 2, I had used a slotted optical switch and narrow vane to detect 360º cam rotation. See Photograph 27. For the Mark 3 the slotted optical switch is used to control the pulses to the winding motor to keep a constant tension on the film as it exits the film gate. For this use the vane needs to be wider and robust enough on which to mount the tension spring. The fixed angular step per pulse of the take up reel winds more film per step as the reel fills up. If the vane width is too narrow it will fail to block the slot until sufficient film has advanced so that the spring can return the vane to unblock the slot and let the pulse stream resume. See Photograph 23.