How do I create a pressure-pad button that will release when I step off of it?

I am looking to make a game where to open a door, you have to step on a button, and I figure that I would use a OnCollideEnter type statement to activate it. The problem is that I want it to stop and close the door if nothing is colliding with it. I am not sure how to make a javascript for that, and on top of it, it gets a little more complicated. I also want the button to only activate when I collide with it, or if a box collides with it (I don't want the button to activate if I shoot it). I can't just make the bullets not have a collider on them, as that is an integral part of my game.

If you have any ideas, or even a script, it would be appreciated.

function Update () 
{
    //Make the tag for the box and player as Opener.
    var objectOperateable = GameObject.FindGameObjectsWithTag ("Opener");
    OnTriggerEnter(objectOperateable);
    OnTriggerExit(objectOperateable);
}

function OnTriggerEnter (other : Collider) 
{
    //Do door open animation script.
}

function OnTriggerExit (other : Collider) 
{
    //Do door close animation script.
}

This might not work, I just did it fast and didn't test it... But should get you along the right track.

Read the parts:

http://unity3d.com/support/documentation/ScriptReference/Collider.html

    function OnControllerColliderHit (hit : ControllerColliderHit)
{
    if(hit.gameObject.tag == "PressurePad")
    {
        Animation.Play("Door_Open")
    }
    else
    {
    Animation.Play("Door_Colse")
    }
}

This only works if your "Player" has a Character Controller attatched. And the presure plate needs to be tagged "PressurePad" then it should work. And your animations be named the same as in the script. :)