This instructable shows you how to use NES controller in Atari 2600 or ZX Spectrum (with Kempston Interface) with the aid of an Arduino.
Step 1: Background talk
I have recently acquired a couple of Atari 2600s and a bunch of joysticks. Both consoles were working but all of the 5 joysticks were damaged beyond repair. Indeed, it is very easy to find It is easy to find an old Atari 2600 console working but it's hard to find an original joystick in useful condition.
On the other hand, I had a spare NES 'generic' controller bought in a flea market that was working perfectly.
I thought about using a microcontroller to convert the serialized signals in NES joystick to parallel signals required by Atari 2600, but then I wondered that if instead of a custom microcontroller board I could use an Arduino. So other people without many skills in electronics could assemble their own adaptors with easy.