How do vending machines work?  Specifically, I'm looking for a list of
the major components, a brief description of how they work (especially
the currency recognizer), and a layman's explanation of how the
components work together.  The target audience for this answer is
junior high school students, so the answer need not be excessively
technical.  The machines in question are typical soda or snack
machines commonly found in the United States.

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Request for Question Clarification by pafalafa-ga on 03 May 2004 07:50 PDT

Oddly enough, there doesn't appear to be a readily-available
description of how a vending machine works.  Instead, there are
descriptions (at all levels of details) of individual components --
how coin recognition works, how bill recognition works, etc.

You might want to reword your question a bit to zero in on a single
aspect of vending machines, to increase the likelihood of getting an
answer.

Cheers.

pafalafa-ga

I don't have a complete answer, but this article (which had a very
promising title) might help you with the math aspect of things:
http://www.math.swt.edu/~carol/research/vendingmachine/vend4.pdf

This might be useful:

http://www.eurocup-vending.info/articles/showproduct.asp?pid=150

The Eurocup link above describes the currency recognition in rather
simplistic terms.

In modern coin acceptors, a magnetic field is generated by sensors and
the coin passes through this field. The presence of the metal coin
causes changes in the magnetic field which are detected by the sensors
and converted into several readings (perhaps 10 each time a coin goes
through).

The accuracy of the coin acceptor then depends on how these readings
are used. In simpler coin acceptors there may be lower and upper
limits, for each reading, for each type of coin.

Take a particular type of coin - let's call it coin A. For coin A, the
acceptor will have upper and lower limits for all of the (say 10)
readings. When a coin comes in, the 10 readings are found. If each
reading falls within the allowed range for coin A, the coin will be
accepted as coin A. The process carries on for each different type of
coin the acceptor is programmed to recognise. If the coin does not
"look like" any of the coins it is programmed to recognise, the coin
is rejected.

More sophisticated coin acceptors use advanced statistical techniques
to define "what a coin is", based on the readings. These statistical
methods can more accurately "describe" what a coin actually is.

The acceptors are programmed by passing lots of coins through lots of
coin acceptors, averaging out all the readings and examining the data
to determine the best lower and upper limits for each coin. The limits
are set to accept as many real coins as possible, but accept as few
fake coins as possible.

Thanks for the links.  I'd actually seen these before, searching for
this answer myself.  As pafalafa-ga points out, there doesn't appear
to be much info available for a vending machine, as a complete system.
The link from mvguy-ga is cool, but it's more of an academic
discussion about "vending machine as state machine".

As far as pafalafa-ga's request for refinement, I don't see any way to
reword the question in a way that preserves its intent, as a detailed
description of one component doesn't really meet my purposes.

I Googled this topic pretty extensively myself, and came up empty, so
I don't think there's much in the way of "low hanging fruit."  I'm
hoping that somewhere, out there, a vending machine specialist is
looking for an easy $5.  :)