This instructable can be accessed at the author's website – http://www.neatinformation.com/howto/Pi%20GPIO.html. If you link to this instructable from another website, please include a link to the Neat Information website.
The project described in this article requires basic mechanical and electronics skills and is provided for information purposes. Do not attempt to replicate it or use it for any purpose unless you've got the proper knowledge and skills and are willing to accept the responsibility for your own actions.
One of the Raspberry special features is the GPIO (General Purpose Input / Output) a 26 pin connector to the outside world. This is where you connect external electronics devices to your Pi. It can be as simple as a LED, or as sophisticated as a factory automation system.
In theory you can connect electronics components directly to the GPIO pins, but in practice it’s best to have some kind of extension cable to an external circuit board. Commercial Raspberry Pi GPIO cables are available for $$$, but it’s easy to make your own for almost nothing.
Step 1: Find a junk computer
Find a junk computer that’s about six years old – or even older. You’re looking for a computer with a DB-25 parallel connector on the case which is not part of the motherboard (e.g. one where the connector is mounted in an expansion slot). In almost all cases the circuitry for the parallel port is on the motherboard and an adapter cable goes from the motherboard to the connector on the back of the case. The end which plugs into the motherboard is a 26 pin IDE connector, the connector you need for the Pi’s GPIO.
In almost all cases the ribbon cable which goes from that connector to the DB-25 connector on the back of the case only has 25 pins. You can use that cable as-is if you don’t need GPIO pin 26 (GPIO 7) or replace the cable with a 26 pin ribbon cable. If you’re using the original cable, slice off the DB-25 connector.
Step 2: Split cable and crimp on connectors
I haven’t been able to find a 26 pin crimpable DIP connector, so I used two 16 pin DIP connectors. Take the ribbon cable and carefully count 16 pins from the pin 1 side of the cable. Pin 1 should be marked with a different color (typically red). Use a sharp knife to split the ribbon cable for a couple of inches. You’ll have a 16 pin ribbon and 10 pin ribbon.
Crimp one 16 pin DIP connector on to the 16 pin section, and the other connector on the 10 pin section.
For more detail: A Really Inexpensive Raspberry Pi GPIO cable