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How Bill Changers Work

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You desperately want that last Baby Ruth in the vending machine, but all you have is a $20. Enter the bill changer. A bill changer is a vending machine that accepts dollar bills and dispenses coins. With a bill changer, you can convert your large bills to smaller ones, or to coins. This is especially handy when you come across vending machines that don't offer change. Some bill changers subtract a small amount as a surcharge for the transaction, but other machines make no charges at all. Machines that are free of charge are usually found in locations where the change can be used on the spot at another vending machine. Bill changers are commonly found in laundromats, video arcades, shopping malls, and next to other vending machines. Creating a good bill changer is a difficult task, not only does it have to be able to read both old raggedy dollar bills and crisp new ones, it also has to be reasonably good at distinguishing between real and counterfeit bills. Some bill changers also have to be able to sense different denominations of bills.

There are several different techniques that have been used over the years in the development of bill changers. Early bill changers were programmed to read the magnetic ink used in U.S. bills. Using magnetic heads, like the ones in a tape recorder, they picked up signals from the bills. Certain areas of the bill, such as behind the president's face, generate signals at a specific frequency. Detecting the frequency validates the bill and determines what denomination it is. Another method used is determining the bills conductivity. A genuine dollar bill has a certain conductivity and checking the resistance of the bill can determine whether or not it is genuine. Checking the fluorescent properties is another technique that has been used. Inks and papers have different fluorescent properties, so checking these is another way to tell real bills from counterfeit ones. The last technique is to check the optical properties of the bill, or of a strip of the bill. This technique is preformed with a photocell or a camera and works by comparing the bill to valid patterns in stored memory. Different patterns from different bills can determine the denomination. Good bill changers use a variety of these techniques to help outwit those who might want to cheat the bill changer.

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