Vending Machines – Dale’s Daily Data

Ever wonder how a vending machine knows that the coins you put in it are the correct value and not just slugs?  Vending machines have pretty sophisticated coin-testing devices that not only measure the diameter and thickness of your coin, but also check its weight, alloy composition and magnetic properties. 

When you put a coin into a machine it enters a slot that accepts only coins that allow it to roll down a chute to two hook-shaped balance arms.  A coin of the correct weight presses one of the balance arms and moves a counter-weight on the other arm allowing the coin to pass through.  As that's happening the diameter of the coin is being checked…if it’s too small, it slips into the coin return.  If it’s too large, it gets stuck.  When you press the coin return button, a side flap opens and the coin falls into the coin return. 

If the coin passes the weight and size tests it then continues thru a magnetic field.  If it's not the right alloy, the magnet will catch it.  Again you have to use the coin return button to sweep the coin away from the magnet and into the coin return.  All of these tests take place in just a split second.  If the coin has the correct dimensions, weight and composition it passes thru and you get your vending machine item.

SOURCE: howstuffworks.com