How does a dollar bill changer know whether your dollar note is real or fake?

If you’ve ever tried to cheat a dollar bill changer by inserting a piece of notepaper, newspaper, or whatever’s on hand when your money runs low, you’ll know the machine is no fool.

More than the correct size note is necessary to set off the circuits that send four quarters into a change receptacle. A common design, the one dollar bill validator made by Micro Magnetic Industries, Inc., performs no fewer than five authenticity tests on your dollar bill, so you might as well leave the Monopoly money at home.

This machine consists of a solid state system of control and sensing circuits and a motorized dual belt drive system that transports the dollar bill. The first test, a gross density check, occurs as the forward part of the bill enters the machine: a light shines on the bill, and a photocell measures the amount of light passing through it. If the bill is too thick or too thin, it can’t enter the validator.

Inside the machine a second, more elaborate optical test is performed, using a moire grid pattern, a stable light source, and a silicon photovoltaic cell. The moire grid pattern contains lines designed to correspond to the very fine lines that run diagonally through the words “ONE DOLLAR” on the back of a dollar bill.

As this section of the bill passes in front of the grid, a light shines through the grid, and the lining up of the lines (or the disparity between them) creates an optical wave pattern. A photovoltaic cell, in turn, transforms this pattern into frequency information. The optics circuits consist of an electronic frequence filter and an integrator. If the bill is real, a single frequency appears at the output of the filter, to be integrated and converted into “time presence information.”

Depending on the presence and duration of the signal from the optics scanning unit, the integrator accepts or rejects your bill. This interferometric scheme allows the machine not only to weed out play or foreign money, but to distinguish between a one and a five, ten, or twenty dollar bill; different printing patterns produce different optical “signals.”

Meanwhile, the bill is also being scanned by a magnetic pickup transducer. Magnetic characteristics, resulting from magnetic ink used in the printing of all valid bills, are transformed into an electronic signal. Again, depending on the presence and duration of the signal, your bill may be judged an acceptable candidate or a reject.

The bill must then travel on to interrupt a light path between another optic pickup cell and a light source. This triggers the validator to evaluate the results of the previous tests, the density, optic, and magnetic circuits. If any of those circuits is in the reject mode, the machine rejects the bill.

If the bill passes, it activates a circuit that measures its length and a circuit that “remembers” that the previous tests produced acceptable results. At last, a “credit” signal is activated to give you your change in return.

Once inside the machine, electromechanical interlocks prevent the retrieval of the bill by devious means.