Grass block trapdoor on one side then on the same side of the trapdoor leave one air block and then place a repeater under the air block. when the trapdoor is in the open state and a redstone signal is sent into or take away from the repeater the trapdoor will close. If a redstone signal is sent to or taken away from the repeater the trapdoor will remain closed.
Also happens when a repeater is placed right underneath, or one block over, an open trapdoor it will also close.