problem with the power arduino uno R3 + motorshield v3

Hi there,
I have a problem with the power.
I have 2 motor, the arduino uno R3 + motorshield v3
When i'm in USB, no need of external power, everything works.
If i put USB + shield external power ,everything works too (with more power)
But as soon as i unplug USB, no matter what, nothing works.
So i need your help to solve this problem!!

Droug:
But as soon as i unplug USB, no matter what, nothing works.

And with what are you powering the project then?
Diagrams are really, really helpful (verbal descriptions are... not.)

I've had similar problems, but FYI for future posts, "nothing works" doesn't really say much about what's wrong. Try to be a bit more specific.

Basically, when you gather power through the usb, you're ignoring the V_in / barrel jack on the Arduino. So, if you have power when you use the usb, but nothing when you use external power, that means that there's something wrong with how your providing power to the board. You may not have enough current for your project from the power supply, or you may be short circuiting the voltage in. Check the external power with a multimeter to see if you're voltage is what it should be, and not shorting somewhere.