I followed this tutorial to make a switch-activated sticky piston door...

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and I want to add a pressure plate to the other side of the door so that the door closes behind me, however when i put down a pressure plate it closes the door when I'm on it, but opens again when I'm off the pressure plate.

Any help will be much appreciated! Look at the Youtube link to see the circuit I built, it's the same to the one in the video

I'm a newbie if you haven't guessed