link it to a hatch just above the landing zone and to an inverter[see wiki]
Um...I'm looking through the wiki, and I don't know what you're referring to. I don't see anything called an "inverter". There's a couple uses of the word in an article on building logic circuits, but it doesn't say what it is or how to build it, and it's described with sentences like "transmission gate logic can be accomplished by simply having an infinite water source in place of all +Vs, and infinite drainage for all grounds," which means absolutely nothing to me. Can you give me a more specific pointer?
an inverter is an electrical component that "inverts" a signal (ie changes 1 to 0, and 0 to 1). in dwarven terms, you want it to convert "triggered" to "not triggered".
for the sake of argument, suppose you have a device, Y, which you wish to always be turned on except for when a pressure plate, A, is triggered.
to accomplish this, you need a second pressure plate, B, which has something on top of it (water up to a certain depth is a reasonably easy solution... although also a potential source of FUN if you're not careful). B is the pressure plate which you actually connect to device Y. pressure plate A is, in fact, connected to a pair of doors, floodgates, hatches, or some mixture of those things. pressure plate B is in a small reservoir with both a drainage and filling end, blocked off at the drainage end by a floodgate, and always open at filling end. the filling end needs to have some sort of drainage area which water will flow to first instead of reaching the reservoir if that drainage are was open (this could be the same as the drainage area for the reservoir or not, it doesn't really matter, so long as it has infinite draining capacity). pressure plate A controls the floodgate/door/hatch which would allow water to drain away from the reservoir, as well as the floodgate/door/hatch which will drain water away before it can reach the reservoir with pressure plate B in it. as a result, stepping on pressure plate A will release the water from your reservoir containing pressure plate B, and will also prevent more water from reaching pressure plate B. the size of the reservoir can also be used to create a delay; if, for example, you have a pressure plate B set to function at 4/7 water or more, the reservoir must first drain to 4/7 before B deactivates (and the larger the reservoir is, the longer that takes). you could also use the diagonal trick (ie depressurize the water) to slow down the process further, if you wanted.
and that is how you make an inverter.