How does coin recognition in vending machines work? — Joey, San Dimas, Calif.
Marshall Brain Answers:
For human beings, recognizing if a coin is real is pretty easy. We can look at the coin, weigh it in our hand, feel the temperature, listen to it hit the floor, etc. If someone handed you a coin replica made of plastic, you would immediately know it is fake even if it looked authentic to your eyes. If someone handed you a blank steel slug, it would obviously be “not a coin” because it wouldn’t look like a coin at all.
What we take for granted as humans is not so easy for a vending machine. A vending machine needs a relatively inexpensive way to detect if a coin is real.
If you look at How Gumball Machines Work you can understand how the simplest possible vending machine recognizes coins. It uses the simplest possible coin detector. All that it looks at is the diameter of the coin.
In something like a soda machine, there is usually a more sophisticated recognition system that usually depends on: A) diameter, and B) magnetic fields.
Checking the diameter is pretty easy, as demonstrated in this video:
The machine can also expose the coin to a magnetic field and see what kind of signal it gives off as the coin rolls by. A steel coin will have a different magnetic signature than a silver coin would. A plastic coin will not respond to a magnetic field at all. This page has a nice diagram.
Bonus: How coins are made: