Coin recognition

A very important and interesting part of a vending machine is the part where you insert money into the machine. How does the machine recognize real money. Certain machines accept credit cards, bills and coins. But most of them only accept coins. Now let's take a look how this problem is solved by several mechanisms.

http://blogs.howstuffworks.com/2010/03/31/32069/

http://web.mit.edu/2.972/www/reports/coin_tester/coin_tester.htm

http://www.ehow.com/how-does_4572360_vending-machines-read-money.html