Redstone to detect "falling" (self closing trapdoor)

Is there a way to get a redstone signal when a player falls through a hole (right after he falls, not when he lands)?

Background: I created a bubble elevator and want a trapdoor for the other way around. When you want to go down, you open the trapdoor and fall, but the trapdoor would have to be closed again immediately after. I know I could make this work with tripwire, but I lack the space for that at the moment. If there is a better solution, I would be very glad for help.

This is how I solved it (for me personally, question is still not answered with this solution). Instead of hitting the trapdoor directly, I hit the button. Doesn't make much difference and I somehow like this approach even more.

For other people looking into this issue, I won't accept my own answer. If you come up with a more accurate approach to the question, feel free to post. I will give you credit for it.