I just finished the first tutorial(from the Nerdkit ebook) where the USB is connected to the circuit. I was playing around measuring the voltages and saw that there is a voltage of 2.31V on one rail while having taken away the battery. I put a led light between the negative and positive of the rail. I am aware that we can have voltage without current but I don't understand how the led can light up if there isn't current. The only source of energy is the USB that is connected but then again the red cable is just hanging there thus there can't be a current. How can the led light up then? :S

Hi pithikos,

The power in this case is actually coming from the signal lines of the usb serial cable. The signal line can sometimes remain high and provide enough power to keep the chip on and a small voltage across the power rails. If you disconnect the usb programming cable your led should turn right off.

Humberto