Device Machine Bridge Control

I made a bridge for a map that I'm making (not telling the name because of leakers =D). I want it to have a control that makes it extend , but I have no idea how to make a control for it. I tried to find the answer by looking at various machines from the Halo campaign levels, but that didn't help me at all. Could you tell me how to make a control that makes the bridge extend and stay like that?

[GNS]Joker

If this is for multiplayer, you might want to get the synchronization script.

You have to mess around with power groups and position groups, and use

(device_set_power <device> <boolean>)
(device_set_position <device> <boolean>)

The problem is that I've never got it to work.