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Andrew James - Its really simple - Get a USB Joypad - make sure the joypad works with your pc and that all buttons function - make sure it works and that buttons can be mapped - while your wheel/controllers for driving are plugged in and operational - no use going to all that trouble if it won't/doesn't work in the first place.
Next make a note of what buttons press which numbered buttons in Game Controllers on your PC - this helps because you can wire your box so that the numbers follow on from each other - ie, On my box the Red top left button is Button 1, The large Start button is 2, Green 3, blue 4, The first toggle switch pulled left is 5 right is 6 and so on. the bottom 4 buttons(red) on my box are Up, down, left right - which means they act as multiple press buttons, I understand you can switch this repeat rate off in windows somewhere - but I use them for seat adjustment - use them as they are for say brake bias or lights and it will cause the button to be pressed multiple times meaning you can't definitely switch lights on or off (if that makes sense)
Next pull the controller apart - what your looking for is circuit boards that are made of metal so you can solder to them - apparently some controllers don't have boards you can solder to (so I have been told).
Next you need to solder a wire to each side of the contact - try one at first - then if your confident - and making sure NO CONTACTS ARE BRIDGED you can plug the controller into your pc - touching both ends of the wires you soldered to the board should have the effect of pressing the button that corresponds to what you just soldered to. Thats just a test to make sure you haven't already fried your board.
If you logically follow the tracks on the board you will see that some will share the same 'earth' tracks meaning you can solder on wire and then connect off that to the 'earth' on your switches (saving fiddly soldering)
Don't use too high a powered solder iron or you can melt the tracks or even the chip as heat transfers through the board into the components.
Remember each button on the controller, as far as windows is concerned is simply a button - that applies even to the start,and select buttons if you controller has these - so can be attached to a button on your button box.
Ideally you should avoid programmable pads or pads with turbo modes etc because if you somehow end up programming a button it may cause major headaches getting it unprogrammed once you no longer have the casing that the board came in!
Buttons should be PUSH TO MAKE type and NOT- ON/OFF switches, as you want contact to only be made (the two wires joined together) when you are actually pushing the button - That is important PUSH TO MAKE - (push to make contact)
Thats a brief how to I hope it helps you or others as many of us have old controllers lying around maybe unused or even broken and its great to get use from something that may well otherwise end up in landfill.