OK, now that we know it's for vending machines, you absolutely must have a switch connected to a Pi GPIO pin that would signal if the machine is being tipped. Supposedly, more people have been killed by being crushed by vending machines they managed to rock over on themselves than lightning strikes, shark bites, and a bunch of other rare events, combined. If tipping is detected, then an audio file is played through the Pi's stereo output port that has someone yelling, "HEY, STOP THAT! ARE YOU TRYING TO GET US BOTH KILLED?"
In fact, I can see where a whole lotta fun could be had with a Pi-powered vending machine, especially if it were networked. The Japanese pioneered vending machines where you could pay for your goods using a cell phone, and I assume near-field communications (NFC) equipped vending machines have been fielded at least for trials (the major impediments being the limited number of NFC-equipped phones, standards, etc.). With a display and some buttons, you could put in games where a customer could win an extra product (mob boss and kneecap enforcer not included
). All sorts of proximity-based services to make it easy to find the machines using mobile devices, even advertising their presence to anyone within walking distance who opts in to be notified. Social media check-in functions could be provided (the machine would make its presence known to nearby devices and services already logged into on the devices would do the secure check-in.
In any case, if you weren't already planning on it, I would prototype the machine, put it in one or more high-traffic locations where it would be easy to service when, not if, problems occur, and let people beat on it for a month or three. A camera with a view of the display/user-input devices showing what people were doing, synched with activity logs, would make it possible to discover unintended user interactions, as well as allow you to interact with users in real time (you need to warn them they're being watched, of course). Voice input would require adding a USB audio digitizing device.
The best things in life aren't things ... but, a Pi comes pretty darned close!
"Education is not the filling of a pail, but the lighting of a fire." -- W.B. Yeats
In theory, theory & practice are the same - in practice, they aren't!!!