The design is based on an opensource metal detector called Smart Hunter. This Very Low Frequency (VLF) metal detector uses transmitter and receiver coils in so-called Double-D geometry.

The metal detector circuit shown here must represent the limits of simplicity for a metal detector, yet the design works surprisingly well. It uses just one 40106 hex Schmitt inverter IC, a capacitor ...

If you think of a metal detector, you’re probably thinking of a fairly simple device with a big coil and a piercing whine coming from a tinny speaker. [mircemk] has built a more modern adaptation.