i'd like to kno if it's possible to power an arduino UNO with a car battery (12V). I'd like to create a circuit with a 12v motor, but the arduino's output is about 5v, so I decided to use firstly a transistor, but a chose a relay whereas a transistor. Is it relevant ?
Your scaring me.
The dc jack on the UNO can accept up to +12vdc , That is the maximum dc voltage you can plug into that. The Minimum should be around 9V. (the specs say 7V but 9V adaptors are much more common). I don't know what you mean about the transistor so why don't you just draw a quick schematic and take a photo of it with your cell phone and post it so we can make sure you're thinking of doing anything stupid.