Atari 5200 controllers use 15 pin connectors compared to 9 pin connectors of most of their other consoles.  

The schematic below shows the controller has two potentiometers that control the x and y coordinates and couple of switched for the two fire buttons, and then a 4×4 keypad matrix. 

So that's total of 12 input lines needed for each joystick. 8 lines for 4×4 matrix, 2 for the stick position, and 2 for the fire buttons. To handle 2 joysticks, that means we need at least 24 input ports.

I decided to use a Pro Mirco micro-controller along with a 16 port PCF8575 I/O expander.  Created a schematic below.

I wired up everything and after a few hours of coding I was able to get it to work!

I had 6 ports to spare, so I added common buttons on the unit dor things like Start, Select, etc.. I then designed the PC Board, and cleaned up the code to get it to professional quality, designed and printed a case and success!

Here is a quick video showing how it works!

Youtube video walkthrough

Let me know what you think, and if you decide to get one, you can find them using any of the following links: