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I never understood mechanically how a vending machine prevents people from accessing goods through the dispensing panel. From what I understand it's not a locked door that unlocks only upon payment, dispenses goods and then re-locks. In my experience it seems to be a non-automated mechanical system powered by the weight of the dispensed goods when they fall into it.

I remember as a young child I would out of curiosity try to stick my hand into the panel and see if I can reach up but was unable to do so as my hand would be blocked by what felt like a literal wall. So I know there is some mechanism in place to ensure goods can only flow out and nothing can come in, but I don't understood mechanically how such a thing works.

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    $\begingroup$ Anecdote: Some of the vending machines have large items like sandwiches and drinks, which can get stuck in the "fall through", as they get wedged against the glass. Most of these machines are also programmed to return the money in the event the item did not fall into the "output bin" down below. Now, if you knew which items are likely to get stuck and they are in the lower levels of the machine, you could get an item stuck, get your money returned and order another item, delivery of which will get the stuck one unstuck and get away with say...five sandwiches and cola for a price of a cola $\endgroup$
    – mishan
    Jul 11 at 1:01
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    $\begingroup$ Anecdote: When I was in university many many years ago there was a small Kit Kat only vending machine. With some manual dexterity it was possible to empty the machine of stock through the outlet without payment - obviously a poorly designed machine. There was also a pinball machine that would register a payment if thumped in the right place ;) $\endgroup$ Jul 11 at 11:58
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    $\begingroup$ @mishan: From the moment I read your comment, I knew you program software (or engineer of some kind). $\endgroup$ Jul 11 at 18:12
  • $\begingroup$ With the numerous styles and designs of these machines maybe you should ask about a specific one which you encounter. Provide a picture, maybe? $\endgroup$
    – MonkeyZeus
    Jul 12 at 13:33
  • $\begingroup$ Strictly speaking, they are designed to minimize undetected theft. (easy to smash the glass front and take it all). The various one-way hatches and so on keep all but the most flexible folk from extracting unpaid items $\endgroup$ Jul 13 at 11:19
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The vending machines have different product delivery box implementations.

One of the most common ones -I've encountered- is the following:

enter image description here

Basically you have a box which has an opening, as shown in the picture. The opening is big enough for the products to fall in.

When the box pivots, then the closed section of the curved face "closes" the access from the product storage area.

In most cases, there is physically no way to access the products inside the storage area.

enter image description here

Figure: Closed position the product can pass

enter image description here

Figure: Open position the product can be picked.

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    $\begingroup$ This is essentially the same as bear-proof rubbish bins and many donation bins. There are vulnerabilities involving long flexible strips, though extra baffles at the edges and close tolerances can help to some extent; even the most basic version of this approach deters opportunists. You can often see how it works if you look closely, and can occasionally see it fail e.g. if buying more than one item and they land in a stack that blocks the hopper $\endgroup$
    – Chris H
    Jul 12 at 9:17
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I think the locking mechanism is more or less like this:

enter image description here

Note, the actual setup could be different but the concept of locking through a barrier that prevents the slide plate been pushed up remains the same.

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