Here`s the schematic of the device depicted in the photo. Each of the ten coil-assemblies has its own copy of the above circuit. The SS41 is a Microswitch Hall Effect magnetic field sensor IC that I found in Surplus catalog for $.50 each. Any similar hall IC should work, as long as it is fairly sensitive and is magnetically bipolar. All Electronics currently has one for $.50, HESW-5, but it is a surface-mount package with tiny, hard-to-solder leads. The professional supplier for hall-effect devices is Allegro Micro.