The clever part of the design is the single rectangular coil made by etching one continuous track on the printed circuit board. you can see that the rectangular coil is tapped at one turn i.e. a single coil with a precise tapping at one turn. The coil starts at the top side of the double-sided printed circuit board and makes square loops towards the center, A via in the center of the printed circuit board takes the coil to the other side of the printed circuit board and the same set of tracks are on the bottom side.
the circuit includes three bipolar junction transistors, a few resistors, and capacitors. The Audio part in the module is a standard active piezo-buzzer, and there’s also a small trim pot to fine-tune the circuit.
the kit needs to be soldered and needs basic soldering skills.
Here is the basic schematic of the metal detector module.