I want to make a raspberry pi arcade cabinet but I don't know how you would have it to only allow the user to place their game of choice when they have paid, and to let them play more than once if they pay more than necessary
I want to make a raspberry pi arcade cabinet but I don't know how you would have it to only allow the user to place their game of choice when they have paid, and to let them play more than once if they pay more than necessary
The requirements for identifying coins is a complex one, involving mechanics and elecro magnetics among others
Size of the coin (Thickness and diameter)
weight of the coin
Magnetic properties of the coin (Dont forget the double metal 1pound in the UK or the toonee in Canada) are complicated as is general shape
so before you can even get to the software there are a lot of hardware / mechanical issues to deal with
I would suggest getting an old vending or game machine coin slot and adapting that (Assuming it has a readable protocol or is purely mechanical with contact outputs denoting the identified coin) but this will limit your acceptable coins
you could go modern and issue an RFID tag to players where they can "Charge up" the tag and play within it. but this will require a link to the "Cash Register" so to speak
google "Coint Tester" for more info if needed
Peter
I can confirm what peteroakes says. I've laid my hands on a genuine mechanical coin validator, and that's a very complex apparatus. It even evolves validating drop down velocity of the coins.
The unit I reviewed didn't use any electrical properties to validate the coins. It was all based on size, weight, and how it behaved in the drop down path.
I've done a short blog on the subject of drink dispensers:
Repairing a Vendo V-80 cola dispencer of the 1950s
And here are some videos that I made on bypassing a defect coin detection system 
and on fixing a soft drink dispenser:
I used video recorder internals to detect that something went through the coin path.
I can't say that my story has a happy ending. I did this modification for a friend who had bought this cola dispenser for his partner. They separated not long after.
I have developed a complex prototype for the coin accepting and programming, interfacing micro controllers on vending machines (including games machines). There are two different approaches: a complex coin acceptor that includes a simple yet ancient electronic based on a sort of high voltage serial protocol that is the MDB protocol and a simple mechanism that just accept the coin, identify it and do something, like starting a timer, that is the most frequently options.
About the more complex option take a look to this prototype I have created in 2014 https://youtu.be/fp5EP46eRMs
About the simplest way, that it is what I suggest instead, there are two options, as far as what is my experience. I bought some of these coin acceptors from China for very few $ (between 15 and 25 depending on the model), and if you need I can give you the contact or import for you at not overcharge. The first version I tried was the most complete as shown in the image below:
It is simple to program and almost easy to use, but I have discarded it for custom applications: it is too much a "finished product". The better use for this device is in Hotels for pay-per-use washers with a relatively wide set of time programming options.
Then I bought the following, that is more interesting, can fit in any customisation and has the advantage of a more understandable electronics (the producer got me a good support).
This second one (see the image above) has the advantage that the coin size and programming is very easy and is done independently by the control logic;. On the top right of the mechanical coin acceptor you see a sort of plastic clip with a coin. That's all: you put the desired coin there and the machine accepts only these coins 
The logic instead has a 4 characters 7-segments display for the user sire and a small board withe a programmable set of buttons: can be programmed on time or on infinite time for state changes. So, I have already explored the possibility to connect a micro controller board in parallel to the three programming buttons, the output signals and the activation relay to hack and get any signal that is needed to reprogram automatically the board "on the run".
Enrico
