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Elevator Doors, By Jason Otto 01/03/2005
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Step One:

Set up a properly working elevator complete with all the switches that will control
it.
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Step Two:

In the object hierarchy create a new concrete room, call this room, TrigRooms.  To
this room add the script TrigRoomCreature.
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Step Three: 

Create a subroom from the Trigrooms type for each stop the elevator will have. 
Name these room Trig1, Trig2, Trig3, etc.
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Step Four:

Create a room at each level of the lift, Trig1 at level one, Trig2 at level 2, etc.
Fill in the other spaces of the lift with standard room brushes.  Link the 
roombrush at each level to a door or gate via controldevice.
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Step Five:

On the lift create a rat.  The rat is the creature that will set off the rooms, the
rat needs to be modified.  Change render>render type to not rendered, change voice
actor tags to blank so its quiet, change the class schema to nothing so it won't 
make noise if bumped.  Change the physics model radius to 0.1 units.
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The Effect:

The rat will now ride the elevator, as it passes through each trigger room the 
associated gate will open, as the rat leave the gate will close.  Because the rat
isn't visible it seems like the gates are reacting to the elevator.
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Application:

Any mission with elevators that would be enhanced by a working gate, its great 
almost anywhere and doesn't require a custom stim.  The system can also be used to
turn a light on when an elevator arrives and off when it leaves.
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Other Issues:

The speed of the gate reaction can be adjusted by changing the height of the 
trigger room appropriately.  Because AI will become efficient when the player is
far away its less resource intensive than a custom stim.  The effect is somewhat 
strange in a multiple elevator system when a gate opens on a floor as you pass
because the rat is moving through its trigger, the system is likely best for
elevators between only two floors.  This system also uses fewer objects than a 
stim because the buttons and revert traps needed for that set up are eliminated.