The sprinkler control valve is the heart of the cannon - it lets you release the air from the air chamber into the barrel, pushing the potato out.

You CAN control these electrically (they operate on 27 volts) but if you get the kind with a flip-up Manual Release Valve, you can use that as the trigger - just flip the lever and fire away.

BEWARE: These manual release valves come in 2 flavors: Flip (good) and twist (bad.) You want the flip version.