Welcome to Brainstuff from house stuff works dot com where smart happens. Hi, I'm Marshall Brain with today's question, how does a dollar bill changer work? Creating a good bill changer is a tough problem. The device has to work with all sorts of bills, from crisp new ones to ragged old ones, and it has to be reasonably good at telling real bills from fakes. The changer also has to be able to sense the denomination of the bill. In order to accomplish all this, dollar bill changers use
a variety of technologies. Here are several of the techniques that have been tried. Many of the early bill changers took advantage of the fact that US bills are printed with magnetic inc They could therefore use magnetic heads like the ones you'd find at a cassette tape recorder to pick up signals from the bills. Areas of the bill, like the background behind the President's base, would generate signals at a specific frequency, and the detection of this frequency
would validate the bill. Different bills generate different frequencies, so the frequency can also help to determine what the denomination is. Genuine dollar bills have a certain conductivity, and checking for the resistance of the bill can authenticate it. Inks and papers have different fluorescent properties, and these properties can help tell real from counterfeit bills. The optical properties of a bill or a strip of a bill can be sensed with a photo cell or a camera and compared to
valid patterns stored in memory. Different patterns from different bills can determine the domination. A good bill changer will use several of these techniques. Because people seem to have a very strong desire to outwit bill changers. Bill changers have to handle such things as people inserting just half of a bill, people inserting bills with strings attached so they can jerk them back out, people who try unplugging the
machine with the bill half inserted, and so on. For more on this and thousands of other topics, is it How Stuff Works dot com