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Here are some pictures of the pop up.
I made the entire thing from PVC pipe and fittings. 
The air is controled by an automatic sprinkler valve which is triggered by a control circuit, which you can find elsewhere on this site.
Last Halloween I just hooked this straight to the compressor, at 100 psi. It shot up fast and hit hard.
I am going to redesign this so that it uses less air, by incorporating some flow control valves.
One note about this, these PVC pneumatic cylinders are way cheap to make, but you will need to have a lathe or some other way to turn down the piston to the right size to go into the pipe. At 100 psi it doesn't need to fit real precise, I even took off extra to slow down this one, probably about 1/8 of an inch! So it was real loose and it still hit hard.