The heart of this diy metal detector circuit is CS209A. The metal detector is build with one 100µH coil. CS209A has one oscillator wich forms a LC circuit, the inductance of the coil will change when it is near metal objects.
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.