Making slot machines isn't too difficult.  They have a much easier life than most of the things I've made because they only work when a coin is inserted - unlike clocks and interactive museum exhibits. They are worth the effort because they can be brilliantly satisfying to make. I've split it into 3 sections


Coin mechanisms
 The coin mechanism checks the coin is genuine, and then switches a momentary electrical contact to start the machine running. If you're not too worried by people putting coins of the wrong value in - and washers etc, its simple to make a coin mechanism yourself. If you want to be sure the correct coin is inserted, buy a commercial coin mechanism. The coin acceptor only makes a momentary contact, so you will also need a circuit to connect it to - see control



The simplest is a horizontal slot with a gentle slope behind. The slope has a bit of wire running down each side. A coin simply makes an electrical contact between the two wires as it slides down the slope.

Another way to make a coin mechanism is to buy a low torque microswitch.

The weight of a coin is enough to trip the switch as it falls. You can make the coin slot and slide out of metal, plywood or even cardboard.    
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if you know a bit about electronics, its even simpler to use a slotted opto switch or inductive sensor, as adjusting the microswitch can be fiddly.

There 2 basic types, mechanical and electronic.

In the mechanical ones the coins roll down a slope. Coins of the wrong size and metal fall off and come back out the reject slot below. Genuine coins travel along the slope and off the end, tripping a microswitch as they fall.
The advantages are: they’re simple, can be used outdoors. Disadvantages are they jam more easily than the electronic ones. The Coin Controls S1 (pictured) is available from Eurocoin for about £45.

Inside the electronic coin mechanisms, the coin rolls past two coils which sense the eddy currents generated by the moving coin. A microprocessor then compares the eddy currents with those of genuine coins, stored in its memory. An electromagnet then deflects fake coins out the reject slot. Real coins produce an output on a serial or parallel port (the parallel port is simplest to use, one pin usually changes state for each coin denomination).

The advantages are that coins jam less often and the electronics can recognise every different coin (some even have logic to add the coins until the required value has been inserted). Commercial arcades prefer them as they take more money because people can use the machine with whatever change they have in their pocket. The disadvantages are that they were more expensive – almost £100 - and can be less straightforward to connect to the rest of the machine.

The simplest one to use I've found is the Multi Coin Acceptor CH926 from Tiwan which you can buy on ebay for about £25.

You can also buy a timer/relay board for it which gives an adjustable timed output when the right amount of cash has been inserted.  There's even an add on LED display to show how much has been put in.