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Soon our group will get a coffee machine. However, we will have to put it in a place that's accessible for other people, since it's a kitchen in Uni. Now we don't want other people to use it for many reasons - they might not clean it after using it, using machine also costs(and we don't want to offer coffee from our wallet), people might use our coffee beans, etc.
How can we protect this device from being used/accessed by other people, without much hassle?
There is a type of plug/socket combination known as a Walsall Plug, which looks similar to a standard 13A UK plug, but the pins go the other way [90° from standard]...
Left, Standard right Walsall...
You replace the regular plug on the coffee machine with a Walsall, then make a short extension cable from 'standard plug [any nationality] to a Walsall socket' - then only the holder of the extension 'key' could make coffee.
If you made inline mains extenders plug to socket, for either all group members, or just one as a 'loaner' to be returned to the 'holder of the key' then no-one else could even plug it in.
The Walsall would function on any international standard mains system, with or without earth [ground] as your 'lock & key' contains both elements.
The same system could be made from any non-standard connector, from as esoteric as the Mains Power XLR [Power Canon]...
...to "any plug/socket not readily available in your territory"
You're not really trying to keep out the 'serious hacker' who would investigate how to get his own 'key' made, just casual "ooohhhhh, free coffee" wannabes ;-)
Option 1: Put a sign on the machine
In a Uni environment, I think that putting a sticker on the coffee machine telling that it is for your group only would be sufficient.
If someone else uses the machine:
Either he/she does not do it regularly, in which case, you probably won't care.
Or it is regular, and the person will be caught one day in the action. When that happens, make a fuss, for example by sending an email to the lab mailing list.
Option 2: Card reader on the machine
If you happen to be in a science/engineering department, I am sure you could find a student with enough motivation to interface the coffee machine with a card reader. Many university access cards are pretty standard, so that you can easily buy a cheap USB reader to plug in a cheap computer (e.g. Raspberry Pi) and control a relay connected to the coffee machine.
This would typically cost about 100 USD and an afternoon of assembly. The advantage of the card reader option compared to other answers is that it is still very easy to make a coffee, and the coffee machine can stay pre-heated (if the relay just shuts down the buttons on the machine). The system can also be used to calculate a "coffee bill" for the users depending on how much they consume.
Option 3: Make the machine look dirty
This would work pretty well if the machine has a clear and visible water container. In that case, paint the container with transparent green paint so that it looks like the tank is full of algae.
Put a sign on it and say "Coffee available to all. 400PLN / cup" (about $10 / EUR). If you say it's "Not for you", people will be compelled to defy you. If you tell them they can have it but must pay a ridiculous price for it, they will not want it.
