How do they recognize a coin? In the US, the most widely used coin is probably the quarter dollar. We recently came out with a comemorative coin for each of the 50 states, so there will be 50+ varieties in circulation. Do the vending machines check coins solely by weight, or do they recognize the picture? If solely by weight, it would seem that counterfeit slugs would be more of a problem.
It’s partly by weigh, partly by diameter and partly by metallic composition. Weight and diameter are fairly trivial to determine, but composition relies on the same electromagnetic effects as a metal detector uses. One of the technical challenges for developing the golden Sacagawea dollar coin was that it had to work in existing vending machines that were designed to accept S.B. Anthony dollars. Getting the metallic composition just right was the hardest part. It’s very difficult to fool a modern vending machine coin acceptor.
I don’t have a cite at the moment (I’m sure someone will be along to provide one) but different machines work in different ways. They generally use a combination of size filters, weight filters and the electrical resistance of the coins. I don’t think any of them use photo-imagery to recognise coins.
I was surprised to find that How Stuff Works didn’t have a Vending Machines article!
God damn, beaten to it again. That’ll teach me to do ‘real work’ mid-post 
So is what makes it so difficult the part about getting the metalic composition/electrical resistance just right? I always assumed it was hard to fool them but I wasn’t sure why.
I’m not aware of any that use electrical resistance as a test, since coins in circulation tend to accumulate oxides, dirt and whatnot that would throw off such a check. But yes, basically it’s the electromagnetic signature that makes a coin so difficult to fake–at least to the extent that it would be uneconomical to do so.
Actually, the tricky thing about the Sacky is that Congress, in the enabling legislation, specified that the new coin must be gold-colored. There are very few alloys that are gold-colored, and most of them have very, very different magnetic properties from the components of the Susie (I think it’s actually magnetic permeability that’s used in vending machines, but I’m going by memory from a long-ago article). Add to this the fact that various vending machines rely on different parameters in measuring this quality, so the Sacky had to have the same properties throughout, not just as a whole.
Actually, the electromagnetic properties you mention are primarily the conductivity. It’s just that it’s internal, not on the surface. The way it works is that when you subject a conductor to a varying magnetic field, a current will be induced in the conductor. That current will itself generate a magnetic field, which your metal detector can detect. The magnetic field you get out will be determined by the magnetic field you put in (which is produced by the machine, and hence common for all coins), the geometry of the conductor (which will be the same for two coins (or slugs) of the same shape and size), and the conductivity of the conductor. Incidentally, this means that contrary to popular belief, aluminum (a fairly good conductor, as metals go) will be more easily detected by a metal detector than iron (which is ferromagnetic, but not as good a conductor).
Another test you could use fairly easily would be the moment of inertia of the coin (measured by how quickly it rolls down a slope), but I suspect that most coins and slugs have a fairly uniform density, which would make this test redundant with tests for diameter and mass.
They very first vending machines were found in Egyptian temples, for dispensing a kind of “holy water” equivalent.
Drop coins through the slot onto a balance arm, until the right weight of coins were achieved.
The balance arm then lifts a plug out, causing water to flow into a cup.
When the balance arm end with the coins on tilts far enough, the coins fall off, making the coin end lighter. Arm goes down, plugs hole, water flow stops.
Weight, diameter, and thickness, with a magnetic trap for slugs.
I have a coin mechanism that I bought from American Science and Surplus some years ago. That’s all it uses.
However, high-speed ones use electromagnetic signatures.
Yeah, but could you get Doritos from it?
In my 7 years of arcade experience, epepke nailed it in one. If it fits the size and shape, and is pretty much non magnetic, it will take it. Generally speaking its not worth the effort to make perfect copies of coins for weight and size, and not worth the potential savings to use any kind of trick magnetic permeability except in higher end units that will commonly deal with things like sackies and silver dollars.
Doritos are for yo-yos, & people who turn to the Encyclopedia Brittanica rather than the Celestial Master Cecil Adams. We will forget you said it.