Ok, so here it is in all its pride and glory. Of course you have to have some sort of air tank to build this air cannon. The good news is, you can get them pretty cheap if you are on the look out. Try garage-sales or ebay and you could possibly pick one up for $5-10. I will not add any measurements because different tanks could be different sizes, but I basically measured how long and high I would want the steel pipes to be and went from there. Of course I put teflon tape on all threaded ends so air would not leak out.
A. NPT Check Valve. It basically lets air in but not out. Makes it portable.
B. NPT Air Pressure Regulator and Gauge. It helps maintain and regulate air flow.
C. Electric 12v Solenoid Valve. When powered, it allows air to pass through.
D. Galvanized Threaded Steel. You will need a Tee, Bushing, or anything to properly assemble this build.
E. Simple Prop Timer.
This is the simple prop timer that I have had for a while from Fright Props. While you can choose a different method of powering the solenoid valve, this is a very simple and effective way of doing so and you can set a timer on how long you want the solenod valve powered.
The settings on this timer are: Trigger, Negative (-), Positive (+), NO (Normally Open), C (Common Lead), NC (Normally Closed)
Fright props has a "how to" on wiring this but all I did was connect the trigger to the Trigger and Positive, Connect the 12v Power Supply to the Negative and Positive, connect the Solenoid Valve to the Negative and the C, and then cut a separate wire and run the ends to the Positive and the NO.
All in all I built this prop for $35 not including the simple prop timer that I already had. If you find the right prices you could possibly build it cheaper, but for $35 I still think it is the best scare for your buck! If you have any questions, email me at email@example.com