@Matteo_Ricchetti I am in the process of selecting a sensor for this purpose right now. I’d like to build and test your unit. Do you have links to the laser you used and a circuit diagram I could refer to?
Also, I’m not sure I explain your last post regarding using the “correct perforation.” Can you explain in a little more, please?
This is a great thread. 
Currently I’m talking with manufacturers of laser and fiber-optic sensors. We tried cheaper sensors but did not get good results. They are all suggesting fiber optic which costs about $300 per unit. I’m hoping @Matteo_Ricchetti’s design can help us save some money there.