I’ve been looking for an excuse to build one of these clever tamper-detecting switches for a long time. When a mystery critter recently started raiding the fresh catnip I grow outside as a treat for my indoors-only cat, I saw my opportunity. Sure, I could’ve bought or borrowed a commercial trail camera, or hacked a PIR motion detector circuit, or set up an IR webcam with software-level motion detection, but I’ve long been charmed by the simple low-tech design of the ball-in-cage switch. Here’s how I did it.