Coin detectors used in
modern soda machines use several methods to detect the type of coin
deposited. One of the most common methods involves a series of
optical sensors and a
coil that the coin passes through. As the coin passes through the coil, it changes the electrical
properties of the coil. This electrical change is used to determine if the coin is valid and if the coin is an acceptable type.
To set up a new coin machine, it needs to be placed in test mode. The technician then sets the type of coin to be "taught" (for example, US dimes). The technician then runs a certain amount of dimes through the system, and the averaged electrical properties of the coins are stored in the machine's memory. This process is used for all other denominations. Some more advanced systems allow you to just upload a data file instead of pumping coins through the slot.
The optical sensors track the coins through the coin mechanism. If a coin is detected as invalid, a set of flaps are used to direct the coin back to the customer via the coin return cup.