Background: I bought a soda from a vending machine today.
It cost a dollar, and I had a few singles.
The first $1 bill I tried looked fine. It was flat, fairly crisp, no missing pieces, no weird wrinkles or folds. And the machine wouldn’t take it —the bill acceptor would take it in, pause a second, then spit it back out. I tried 5 or 6 times with no success.
So I grabbed another $1 bill. It looked pretty similar —it was maybe a little more worn, but no significant damage. And the machine took it on the first try.
My question: What are the likely reasons a vending machine would reject a particular $1 bill that looked, to my eyes, fine? Is it likely to be counterfeit? That struck me as unlikely with a $1 bill. (If it was counterfiet, oops, I’ve spent it already.) I’d guess a problem with the machine if it hadn’t worked with the very next bill I tried.
(I don’t mean that people should try to guess why my particular bill was rejected. I’m just curious about why any given $1 bill would be rejected, if it wasn’t damaged.)
That’s probably not a GQ question with a GQ answer, but here come some speculations:
There was some accumulated gunk in the bill reader from all the prior bills. It had gotten just gunky enough that it rejected your first bill. A few tries later you’d dislodged enough gunk that the next bill would be accepted. By luck you changed bills right at that point.
There is a tolerance on how well-centered the printed image is on the piece of paper. It’s tight, but it isn’t zero. That particular bill was at the raggedy edge of the tolerance, and the reader is at the other raggedy edge of the tolerance.
Readers work by looking at just a few small spots on the bill. It’s not doing image recognition on the whole thing. You happen to have a smudge or a discoloration or something at the exact critical spot. But you don’t know exactly where to look or what to look for, so you missed it.
Note that all 3 of these reasons are separate but they could be additive. The reader develops a confidence score on the bill and rejects at some lower limit of confidence. Your bill might have gotten all its “defect points” from just one of these causes or a bit from each.
I used to maintain the pop machine at our clubhouse. The bill validator is interesting, as LSLGuy mentioned, it doesn’t scan the whole bill. There are two plastic lenses about as wide as the end of a pencil. That’s the only 2 strips that get scanned. They are replaceable as part of the validator overhaul kit. You also get new feed belts in the kit.
The validator could be set from a menu of options. Tightest scan was: 1 dollar bill only, portrait up, facing left. Then: portrait up, facing either way. Then: one dollar bill, any way. Then: singles or 5 dollar bills, etc.
I worked with someone who had previously worked at a company that made such devices. He said the software developers were required to work in teams to prevent some programmer from writing malicious code. He mentioned the possibility that a writer could write a section of code to look for 2 pencil dots in specific places. When detected, the machine would vend the soda, then eject the bill. Free Pepsi!
I don’t know if this actually happened or if it was something they were just paranoid about.
Did you try reversing the crisp dollar? The candy machine on our floor sometimes gets extremely picky, rejecting a significant percentage of the bills. Often turning the bill around is all that’s needed to get it to accept.
Odds of your bill being counterfeit are vanishingly small. Unless it looks like a photocopy, the cost of making counterfeit ones is too high to be profitable.
This has happened to me many times over the years - a crisp new bill being rejected. In almost every case I can remember crumpling the bill up and flattening it our again solved the problem. Always assumed that somehow the crisp bill wasn’t getting gripped tightly enough by the rollers and so was getting misaligned.
I’ve also noticed that if I have a very worn bill that is rejected, I can get the machine to accept it by playing tug-of-war with the machine - as the bill is being sucked in, hold it back slightly so that the rollers slip a bit before the bill is completely in. Again, I always assumed a roller grip problem (maybe grime of the rollers that I am scrubbing away by letting them spin against a stationary bill).
Conclusion: I spend far too much money in vending machines.
I’ve similarly wondered this in the past, because it seemed like perfectly crisp dollar notes that I just got from the bank seemed to fail regularly (at least in the mid-to-late 90s when I most often used vending machines), while those with normal wear but fully intact were accepted fine.
If I have a crisp bill that keeps getting rejected I usually fold the bill in half lengthwise so it has a nice crease right down the middle, unfold it and try again. I always assumed that the crease forces the bill against a top or bottom reader whereas a crisp bill may be narrow enough to just skirt by both.
Huh. That kinda surprised me. I haven’t exactly taken apart a large number of vending machines to see how they work, but I was under the impression that most of them used a magnetic head (money is printed with magnetic ink) instead of anything optical.
My grandmother used to build these machines. She’s been retired for almost two decades, but even at the time, her advice was always to crumple the bill up and try it again. According to her, smooth bills would, in fact, slip enough to make the captured images look enlongated to the sensor–and that enlongation was a very specific thing the machines were built to reject.
I vaguely remember her saying (or perhaps reading somewhere else?) that the most common counterfeits of the time were inkjet printer/color copier outputs (no risk of being caught at a vending machine, so the counterfeit didn’t have to pass human inspection), and that they tended to enlongate the copied images slightly, so the machines were particularly sensitive to this sort of “stretching” in their images.