Anthony is firing my Tennis Ball Cannon for me for an experiment in the Armory back yard, as Mike Scott watches. The object was to try to get a picture of a tennis ball exiting the cannon.
The small white box stuck to the bottom front of the cannon (with Velcro) is a component from an ancient (1970s) alarm system. On the front of it is a normally-closed pushbutton switch poking through a metal plate. While a magnet is attached to the plate the pushbutton is pressed, opening the circuit. In an alarm system, the magnet would be attached to a tripwire, door, etc. I've screwed a bolt into a magnet and mounted it on this plate such that the bolt is in the path of the tennis ball. The bolt is attached to the box with a piece of elastic shock cord to restrain the bolt/magnet part. The cord dangling down from the box connects to the switch and is used to trigger a camera, which I'm holding and aiming.
Unsurprisingly, the camera mirror/shutter were nowhere near fast enough to catch the tennis ball, so all we get is this picture of the bolt/magnet flying away. The camera I was using didn't have mirror lock-up capability. I never got around to trying the same experiment at night with a strobe setup, which would certainly have been fast enough.
The camera trigger is described here.