Replicating a vending machine paper bill insert slot

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?

If you built the box carefully, you could use a proximity sensor such as the following...

VCNL4010 Proximity/Light sensor

I've never used one but it looks like it would work for what you're trying to achieve.

I'm also not affiliated with Adafruit in any way, it was just the first proximity sensor that came up on Google.

EDIT:

You could also hook up a small piece of rough tube (maybe PVC covered with sandpaper?) to a motor that would then turn, pulling the money inside? Just an idea.

-Chase

The PVC idea is what i was thinking already as well.

That sensor looks cool, but im not sure it could handle the small distances that it would need to. It would at most be only 1cm away from the walls of the box so the paper insertion would have to have a quite noticeable change in signal which im not sure could be achieved. Worth a shot though if I am ordering something from adafruit anyway.

Any other ideas?

Taken directly from description of product:

our experimentation we found it worked best at distances of about 10-150mm.

10mm = 1cm

If I were to do the project I'd give the previously mentioned, or some other Photoelectric Sensor a shot.