I didn't know sticky tape usage is a blessable act. TIL!
- know the total length of the tape, and when it's changed, and therefore predict the current radius of the roll
- a small sprung variable resistor pressing against the under side of the role
But that could be over-engineering something that almost more perfect if it's a little imperfect.
cellophane tape does emit xrays when unrolled, so correlate the strength of the radiation with the angular velocity of the roll to more precisely calculate how much tape was removed. I suspect a second photodiode may respond to the radiation.