Ok, so the $30 may be a bit misleading, but that was my approximate cost this year for this prop because I already had everything else.
Relay - $9
1x10x4 - $8
Audio Connectors - $12
3 year old daughter spotting a blower vac tube in someones bulk trash and exclaiming, "Cannon!" - PRICELESS
Other supplies already on hand:
2" PVC pipe
Black plastic takeout Chinese containers ("business end" of the cannon)
Red and Amber LEDs, resistor, various wires, shrink wrap, etc.
Lite F/X Constant Fogger
Portable CD Player
Powered Computer Speakers
Lightning F/X box
My problem was how to sychronize the fog, lights, and sound without an expensive prop timer. I wanted to use stuff I had already. If I could get the FX box to trigger a relay during the cannon blast sound on a looping CD, I could get it to light, fog, and BOOM all at once.
The cannon sound was too variable for the FX box and relay, so using Audacity I recorded that sound to the left channel to go to the speakers, and on the right channel I recorded a steady 7500Hz tone that would convert to enough voltage out of the FX box to trip the relay. The DPDT relay is wired to the fog button on the fogger remote, as well as the LED leads to the 9V battery, so when it is activated, it sets off both the fog and the LEDs in the barrel.
I kept the inside of the PVC pipe white to reflect the LED light for a brighter flash.
I'm pretty pleased with how it turned out. It will be pointing out of the side of our pirate ship.