Instructions
Use the left analog stick to navigate your way through a darkened area until you reach an area where you no longer need the Pipboy 3000A's light to be able to see your surroundings.
Locate the button on your controller mapped to the Pipboy 3000A. On the PlayStation 3, this will be the "Circle" button.
Press the "Circle" button and hold it down for about three seconds. This will cause the Pipboy's light to switch off.