Thanks Both.
At present it is just a plain old piece of wood - so it needs to be lifted, held up for 20 minutes and then released by whatever I am adding.
Whether or not the solution involves trapping the wood in the 'up' position and then later releasing it again is open to deisgn.
As for dropping, I can assure you nocat's will be injured in the making of this device lol
The wood is actually dropping and landing on more wood, for what it is worth. It isn't actually cat flap! It's an old-fashioned timed game from the 1930's. In the olden days, you'd pull a big long lever (which lifts the wood) to release the balls for the game and I think traps the wood open. It also pulls a spring and traps it with a metal bar. Then a clockwork timer works away, slowly moving a pole until it releases the spring enough to un-trap the door, allowing the wooden bar to drop and thus ending the game.
The problem is that the timers are well past their best, don't work properly and are practically impossible to get spares for. Because of their age and speciality, a working second-hand timer fetches 300 GBP or more!
I have seen versions of the game that have been converted to run from a battery powered button (and from a mains powered button) but obviously I have no idea how to do it.