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. In many cases the changer also has to be able to sense the denomination of the bill. In order to accomplish the task, dollar bill changers use a variety of technologies. Several different patents listed below discuss many different techniques that people have employed. Some of these techniques have changed over the years as microprocessors have allowed more capability in the device.

Here are several of the techniques that have been tried:

  • Many of the early bill changers took advantage of the fact that U.S. bills are printed with magnetic ink. They could therefore use magnetic heads (like the ones in a cassette tape recorder) to pick up signals from the bills. Areas of the bill (like the background behind the president's face) would generate signals at a specific frequency and the detection of this frequency would validate the bill. Different bills generate different frequencies and the frequency can help determine what the denomination is.

    The backgrounds from behind the president's face of
    a $1, $5 and $10 bill. Note the different spacing
    used in each one. The spacing can help determine
    the denomination.

  • Genuine dollar bills have a certain conductivity, and checking for the resistance of a 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 photocell or a camera and compared to valid patterns stored in memory. Different patterns from different bills can determine the denomination.
A good bill changer will use several of these techniques because people seem to have a very strong desire to outwit bill changers. If you read these patents you will find that they handle things like people inserting just half of the 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, etc.

Here are three interesting patents: