ROGFalcom:
I've got a little problem, when I connect my UNO to PC, the "L" LED doesn't turn ON, it is OFF. When I disconnect GND wire it turns ON, it never did before. Can someone explain me that ?
Is it an "UNO R3"?
With UNO R3 and pin-13 left INPUT and "not connected", the pin state is "floating" and the L-LED is either off, on, or even may blink from time to time. This is caused due to the R3 design and the LED driven by an OpAmp.
In older board designs, like first Uno, Uno R2 and Duemilanove, the LED should be off when pin-13 is left floating as INPUT.
In newer R3 design, the state of the L-LED is not determined as long as pin-13 is INPUT and not connected to anything.