It’s something [mircemk] demonstrates, with an Arduino-powered magnetic field strength meter that uses a UGN 3503U Hall effect device. The circuit is extremely simple, comprising the sensor ...

Getting a magnetic field to balance on another magnetic ... he got off of Taobao (the Chinese version of eBay), a hall effect sensor, and a handful of current regulation ICs.

Diodes’ AH4930Q sensor detects magnetic fields along the X, Y, and Z axes for contactless rotary motion and proximity sensing.