Trebol-a:
Augh...thank you for your note!
I allways thought that Arduino required 5V, like USB connections. I will test with 9v.
The Arduino does require 5V, but when it gets it from USB, it goes straight to the VCC pin on the Atmel chip. When you power it externally, through the power plug, it goes to a regulator circuit, which needs a couple of volts over the regulated voltage to operate properly.