A while back my brother hit me up to build him some sort of animated coffin. After some misfires involving pneumatic pistons and overbuilt coffins, I settled on a solution using a coffin made of relatively lightweight fence pickets and using a windshield-wiper motor to move the lid of the coffin.
The lid is moved via a snail cam, which has a radius that spirals out, lifting the lid and then dropping it.
Here's a video of it in operation:
The trigger mechanism is a motion-sensitive porch light hanging in the tree. I severed the light sockets and replaced them with plug sockets we could use for effects.
I was just trying to get something working, so this was somewhat rushed. That's why the motor mount is so jury-rigged. I also should have painted the inside black to hide the mechanism better (we ended up just turning it away). Maybe next year, along with a few more coffins.
Here it is in operation after darl:
Behind it is a skeleton, Wally, that was originally going to pop out of the other coffin, but we made that thing out of 3/4" OSB and it is just too damn heavy. The windshield-wiper motor was originally for it, driving a piston to open the lid, but it was just too heavy and broke the mechanism. Where-as the pneumatics were too vigorous for the picket coffin.
Wally is one of those floppy skeletons that I reinforced with dowels. I gave him guts using spray insulation that I spray-painted to be gory. I'm tempted to go big and cover my brother's driveway in entrails next year.