Stephen Cass built a lockbox that opens only when the right gesture is traced with a magnet, and wrote it up for IEEE Spectrum. The sensor is SparkFun’s $100 magnetic imaging tile, a small-run board based on work by Peter Jansen. It carries an 8-by-8 grid of Hall-effect sensors that read both magnitude and polarity, and talks over SPI fast enough to track a magnet moved by hand.

A Teensy 4.1 runs the lock. Cass let Claude write two support libraries, one to record a gesture to nonvolatile storage and one to compare a new gesture against it, then wrote the top-level code himself. A servo rotates a beam into a notched piece glued inside the lid of a wooden craft box.

The sensing area is only 3 centimeters square, which made it hard to repeat a gesture from outside the box. Marking the spot would give away the hidden lock, so Cass traced a Celtic knotwork band across the front and uses it as a camouflaged reference grid.…