A shot of my customized 11th Doctor Sonic Screwdriver. I removed the spring, altered the inner shaft a LOT (giggedy), carved a channel in the grip with my Dremel tool, and added a momentary contact switch, that isn't actually wired to anything.
It works just like the show prop now. Press the release button and give it a flick of the wrist to extend. Pressing the momentary button does nothing. SLIDING it, however, allows you to turn the screwdriver on... and LEAVE it on. When it's extended, holding the button up will activate the light and sound as well. No more opening the pommel cap or reaching into the cage to press the button.
Doctor Who @ teh BBC
Sonic Screwdriver @ Victor Pemberton