How does a dollar bill changer work?

A dollar bill changer doesn't just have to sense a bill's denomination -- it also needs to be able to tell whether bills are fake. Join Marshall Brain as he breaks down the science behind bill changers in this podcast from HowStuffWorks.com.

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Episode Transcript

Speaker 1 (00:00):
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

(00:21):
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

(00:44):
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

(01:05):
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

(01:25):
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

(01:47):
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

(02:08):
How Stuff Works dot com