I have a Playstation 2 controller and I'd like to try to connect it to my PS3. So, I saw on the web that it exists converters to do that. But, I'd like to know if it's possible to use arduino to do that convert. I think it's possible but I don't know how to do that. Can someone help me?
You do know there is an adapter that is made just for that. It converts the PS2 signals into USB for the PS3, and it costs about $10, less than that of an Arduino.
That should be enough to get you started, though you'll have to adapt it to the atmega. It's not as simple as connecting a few wires together. The ps2 controller uses a proprietary protocol that you'll need to decode in the arduino and convert to whatever the ps3 expects. This is one of those times where you really should figure out what your time is worth. Then look at the premade adaptors and store-bought ps3 controllers again.
I don't think it is possible, not straight through. Yes, you might get the arduino to understand the PS2 controller. But you can't get arduino to register as a USB gamepad (right?) with the desktop/laptop computer.
A workaround for this that might make is usable is to write some kind of "software device", a driver that acts as a gamepad but listens on a COM-port, where your Arduino in turns sends data in your own format that you need to decode in the software device on the main computer and translate to how gamepads outputs information.
So it is hard and I don't think it worth the effort. Althrough I haven't done any big Arduino project (I'm onto my first) I like to think that it is more fun to use Arduino in project where there doesn't exist any of-the-shelf hardware that is just to plug in (like the PS2->PC gamepad converter).