Nothing fancy really - Take one discarded CO2 bottle, the one I have is about 18" long, 4" in diameter. It's an old sample bottle I think, pretty heavy duty, prolly good for 2000 PSI. I don't think it needs to be that tough but I used what I had access to. Remove the Valve exposing 3/4" NPT threads. Install a 3/4" T fitting. At this point I aquired a 120 Volt solenoid with the same size inlet and outlet that was rated for the pressure etc. and mounted into one outlet of the T fitting. Make a nice exhaust nossle, and mount it into the outlet of the solenoid, making sure it its far enough away from causing any damage. Mine is about a foot out, and a foot up. I tried to frabricate some sort of device to introduce a little O2 into the exhausting stream for a better effect, but it didn't seem to make much difference, so either skip it, or try experimenting yourself. Anyway, make a nice support so the tank/nozzle assembly will sit pretty on the ground, and mount some sort of clamp to hold a propane torch to the exhaust outlet so it won't fall or blow out the flame when in use. I welded a piece of metal with a hole in it that the torch nozzle fits in nicely.Reduce out of the remaining T outlet and run propane rated hose to the desired distance away, and install into a regulator, which will bolt right to the propane tank. I used an adjustable regulator to play with pressures.
To operate the solenoid, run a cord to a switch, and from the switch to the solenoid, keeping in mind how far you want to be away from device. Incidently - you could use a 12v solenoid and battery for the solenoid, which for me will be the next step.
When completed and leak checked, light the propane torch at the nozzle,open the tank which will fill the accumulator with desired pressure. Now stand back and hit the switch. This will dump the contents of the accumulator, ignite it on the way out and POOF nice little fireball. A momentary contact switch is most desireable, so that after the dump, the accumulator will start to fill again for multiple BLOW-OUTS.
I managed to hide in the bushes in the front yard and hid the device behind a large ephegy of Death with red glowing eyes. When people would stop to check out the eyes..... BOOM . The worst part was cleaning up the shit on the driveway. hope this helps IMOK
never look behind- you'll see them gainin' on ya