OK, here it is in all its glory, the pneumatic squirrel launcher.
It is balanced near the middle on the wooden support. Air is supplied via the hose to a port by the gage, visible on the right. Inside the barrel, all the way back, is a piece of ethrafoam with peanut butter smeared on it. The weight of a squirrel is enough to cause the whole thing to tilt slowly clockwise until the plenum hits a switch mounted on the right end of the base. This triggers the lawn sprinkler valve, the green object between the barrel and the air plenum, launching the little rodent far, far away.
Note to any budding mad scientists out there: A 24 VAC sprinkler valve will work just fine on 12 VDC. The cordless drill I use is a $10 (on sale) Drillmaster unit from Harbor Freight. It is 14 or 18 volts, depending on the vintage. If you run the valve at 18 VDC for extended periods of time, you will burn out the coil. Short time use is fine. The sprinkler valve is rated at 120 psi. The Spears company will tell you to never put compressed gases into its PVC tubing. Over time the oil that goes with the compressed gas will degrade the PVC, and it WILL fail. You've been warned.