I have this ODD problem.
If i plug a 7.4 volt Battery for supplying power to arduino all work flawlessly, If i plug the usb of my pc arduino blink the red led 7 times and dont run the sketch properly. But the USB Transfer with pc works fine.
I tryed to change usb cable, change pc, and to plug power directly from the usb jack, i'v plugghed the red and black usb wire directly to a 7.4 volt battery, and as well it works.
I mesured the voltage out in the two cases:
5.4v on vin 3.4v on 3.3v pin 5.4v on 5v pin
PC USB 3.4v on vin 3.4v on 3.3v pin 3.4v on 5v pin
I tryed to plug both USB cable and a 3.7v battery, but the result is the same. Red blinking led and not enough voltage output
So, is really odd...any suggestions?