Hi @DPXTIF,
I have also modified a projector to digitize old films, in my case Super8.
Personally, I am of the opinion that using a projector it is not necessary to use any sensor to detect the correct positioning of the frame to be digitized.
When a film is being projected, no type of sensor is used. The projector’s own mechanism correctly places the picture to be projected and the image remains perfectly stable on the screen.
It is about taking advantage of this mechanism to do the same by digitizing frame by frame.
I don’t know the mechanism of the projector that you intend to modify, but there is usually a main axis that in each turn makes the film advance exactly one frame.
With a software controlled stepper motor we can at our convenience advance the film frame by frame with total precision without the need for sensors which, on the other hand, must not be easy to install and adjust, not to mention the additional complication of software.
Here are links to my device and the software I use:
Regards