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# How does a vending machine know what kind of a coin you put into it?

Ya0ya0

Lvl 1
2009-03-22 21:03:14

It has like a little slot where the money goes down, and the money pushes the slot back, it know how much money it is because it measures how far the little slot goes back. or some go by weight.

A bit of research turned up that most modern vending machines use a technology called "electronic signatures" to recognize coins, but that most vendors don't publish the specifics of their systems in order to limit "hacking" or "spoofing".

In addition to size and weight, coins are distinguished by their metal composition. When passed through a small electromagnetic field each coin creates ripples or other disturbances that can be detected and analyzed. Different combinations of size and metal cause distinct patterns when coins are passed through such a field. The distinct pattern of a particular coin is called its electronic signature. If a coin does not match a known set of signatures stored in the vending machine's computer, it's rejected.

One consideration in a coin's design is whether the signature is sufficiently distinct from other coins that are likely to be used. For example, that's the reason that special alloys were developed for the new euro coins introduced in the EU in 2002, and for the "golden" dollar coins used in the U.S.

Older machines did in fact sort coins by size and weight. In fact, the first known vending machine was a water-dispenser built by the ancient Greek mathematician and inventor Hero (or Heron) of Alexandria. It used the weight of the inserted coin to determine how much water to dispense into the purchaser's jug.

Machines built in the 19th century sometimes used size-sorters to filter coins; they would roll down a track with holes arranged in increasing order of size. The smallest-size coins would fall through the first hole, then next-smallest through the second hole, and so on. Anything that didn't match would be rejected. Others added a kind of ski-jump to help sort by weight - as a coin fell down the track it would have to clear gaps based on its forward momentum. A coin that was too heavy or light would fall or overshoot. Of course these methods relied on the purchaser using only specific

sizes of coins, and could easily be fooled.

20th-century machines used still other mechanical means, including magnets. At first they were used just to detect slugs and washers, but as the technology developed manufacturers realized that even though coins are generally non-magnetic, they would still disturb a magnetic field as they passed by. That technology was the direct precursor of modern signature-based coin acceptors.

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2009-03-22 21:03:14