I am building a box for a street performer that will have a slot to insert paper currency. When a bill i inserted, the box should recognize that something was inserted and "grab" the bill, and using a continuous rotation stepper, feed it into the box. Once the bill is inside the box, the motor should stop spinning and wait for the next bill to be inserted.
The problem though is how to detect that a piece of paper is being inserted. It has to be able to accept variable sized bills since I am in Europe. It doesn't have to analyze whatever is inserted, just know that something is being inserted.
What kind of switch(s) / sensors could work for this? How do vending machines themselves do it?