No need for analog.
Tie the cathode of the photo diode to the PIC pin with a 10M ohm pull up. The anode of the photo diode to zero(ground). When no light is detected the PIC pin will go LOW.
Used an IR diode for a dedicated light source.
You will have to shield from ambient light to keep from having false readings.
This is very similar to how a mouse works. Very fast and reliable.
I have used this setup to detect parts moving in excess of 6 inches per second.
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