A ball-in-cage switch is a mechanical alarm switch that works on an ingenious principle. It has a single relatively unstable “open” position, in which a metal ball rests in a small divot in an insulating base, inside a cage made from two U-shaped metal staples that overlap one another without contacting. The ball is too big to pass between the “bars” of the cage, but small enough that it can rest in the divot without contacting them.
When the switch is jostled, the ball rolls free from its divot and comes to rest against two bars of the cage, closing the circuit. The sensitivity of the switch can be adjusted by increasing or decreasing the size of the divot which holds the ball in the “open” position (a smaller divot makes a more sensitive switch) or by adjusting the stability of the insulating base (a base that is easier to tip over makes a more sensitive switch).
The ball-in-cage switch has a bit of a nasty reputation as a trigger for bombs and booby traps, but it has plenty of “nice guy” applications as a simple alarm, event sensor, or photo trigger. It is reliable, versatile, and easy to reset. This design uses a 1/2″ PVC pipe cap, a short section of wooden dowel cut from a broom handle, a couple pieces of brass rod, some wire, and a brass ball.