Hi Bruno,
Tell me pls; is it not a simple digital (I mean with full supply amplitude as outp. pulse) reflexions sensor with some chopped excitation?
Are you sure, thatit changes the frequency or duty cycle depend of distance to your papers?
I think it is a some square wave supplyed LED for synchron detection & environmental light independent working and it sens only the presence of papers_reflected light or nothing!? Can you agree pls?
I.e. Hamamatshu, Panasonic & Rohm has very good products for such sensor solutions...
How far is the sensor from the sensed papers & is it a specially build component or a usual some modul?
K.