That, is the essence of complex mechanics. You can easily build a pressure plate, but building it to do something complex is another issue entirely! You'll need some form of fluid, not necessarily a lot of it, set up for something like...
WWWWW
~FPFW
WWWWW
~=Water source
W=Wall
F=Floodgate
The outside gate, adjacent to the water, should be linked up to your desired mechanism, in this case the pressure plate in your hallway. The plate in the diagram should be set to activate when water is on it, and THIS mechanism should be linked to your bridge. What this means is that when someone steps on the plate in the hall, it'll open the floodgate, letting water pour in to wash over the plate. When the creature steps off the plate, the gate will close, but there should be 1/7 water or more on the plate already, to keep it active and keep the bridge down.
Then, when you want to turn off the bridge, you activate the back floodgate (the one away from the water source) and either let it drain out into an open area so the water evaporates, or link to a lever on /R so that the water gets repeatedly atom-smashed.
This has the small chance that a creature won't step on it long enough, and you'll get 1/7 water on the trigger plate, which would evaporate and cause the bridge to retract prematurely. You can solve this by making it vertical, so that water falls down to 7/7 very quickly, although it's a very slightly more complex setup.