If I understand correctly, typically they use the sensor to synchronize the main axel of the projector with the stepper motor. Since projectors typically have the sprocket or claw synchronized with the main axel, that makes the turns synchronous with the advancing of the frame.
Another alternative, and a bit more precise, is the use of photointerrupters. That is the method I used on my first built to synchronize the main axle and the stepper position.
The schematic is for 5V output to Arduino, and it is quite simple. Resistor values may vary slightly for the sensor you choose, and 3.3V.
Actually, I got the photo interrupter from an out of service inkjet printer, the part number is below.
