LED 1 will light and the buzzer turns on when the coil is changing inductance. The setup is easy, VR1 is adjusted ( away from any metal objects ) so that LED 1 will light and the buzzor sounds on, and then VR1 will be trimmed until led and buzzer are off.
The heart of this diy metal detector circuit is CS209A. The metal detector it is build with one coil of 100uH. CS209A has one oscillator wich forms an LC circuit, the inductance of the coil will change when it is near metal objects.