By the "light," I assume you mean the light on your Pip-Boy 3000a. Real simple, it's the same button you use to bring up the Pip-Boy 3000a wrist device, instead of just pressing it though, just hold down the button till the light comes on with a ominious sounding hum, then release the button, to turn it off, do the same.
Source(s):
http://www.fallout.com

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