If you have a command block with a comparator next to it with the command (ex. /testfor @a[r=3]. Out of the comparator you have a signal leading to a door, and a lever that separately leads to the command block. When you flip the lever within the radius the door opens, out of the radius the door still closes even though it shouldn't and the comparator doesn't get powered.
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Ok as far as I understand this is working correctly. When the Command Block receives a signal it checks for a player within radius given by the command. If so it tells the Comparator to output a signal. Now if you leave this radius and give the Command Block another signal it checks again, this time no player is within the radius - tells the Comparator - door closes.
Please provide some screenshots.