There are 2 parts to what you might think of as the photocell: A photo emitter (probably a near-infrared LED) and a photocell. The emitter is on all the time when power is applied. The photocell receives light from the emitter and goes on and off, controlled by the shutter wheel action. The photocell's pulses (caused by the light interruption of the shutter wheel) controls a transistor switch that then switches the current to the coil. The photocell by itself cannot switch the coil current; the coil current is waaay to large for the photocell to handle directly. All of this is inside the distributor module in the Unilite design.
