KEY FEATURES OF METAL DETECTOR MODULE:
Built-in PCB trace detector coil
Buzzer detection alert
Sensitivity adjustment
3 to 5V operation
Virtually all metal detectors work on the same basic principal of using an oscillator that incorporates an inductor as part of an oscillation circuit.  That inductor is composed of wire windings in the sensor head (or in this case, it is PCB traces in a spiral pattern).the sensor-head of the circuit is a 50 turns (25T top + 25T bottom) ‘printed’ coil tapped at first turn
Any metal that comes into proximity with this inductor causes its electrical characteristics to change.  That change affects the oscillator frequency or amplitude and that is what alerts us via a buzzer or some other user interface as to the presence of a metallic object.