On the second episode of Make: television I showed how to build an air cannon (Burrito Blaster) using PVC pipe and a sprinkler valve. The valve is electronically triggered, using three 9V batteries to activate a solenoid. But what’s actually going on inside to allow a tiny solenoid to hold back and then release a huge amount of air pressure? Well, the solenoid doesn’t do it by itself; it’s actually a “pilot” that opens a tiny hole to relieve pressure from a larger diaphragm that’s doing all the real work. These diagrams and descriptions from The Spudgun Technology Center do a great job of explaining the clever trickery of the sprinkler valve.