This metal detector it's based on the CS209A made by Cherry Semiconductor.The CS209A its a bipolar monolithic integrated circuit for use in metal detection/proximity sensing applications.The CS209A metal detector IC contains two on-chip current regulators, oscillator and low-level feedback circuitry, peak detection/demodulation circuit, a comparator and two complementary output stages.The oscillator, along with an external LC network, provides controlled oscillations where amplitude is highly dependent on the Q of the LC tank. The peak demodulator senses the negative portion of the oscillator envelop and provides a demodulated waveform as input to the comparator. The comparator sets the states of the complementary outputs by comparing the input from the demodulator to an internal reference.
The detector, is a single 100uH coil. The IC has an integral oscillator the choke forms part of an external LC circuit, it's inductance being changed by the proximity of metal objects. It is the change in oscillation that is amplified and demodulated. Led 1 will light and the buzzer will sound when the inductance it's changed. Set up is easy : R5 is adjusted with the LC away from any metal source so that the LED lights and buzzer sounds. The control is backed off so that the LED goes out and buzzer stops. When the choke comes into contact with any metal object that alters its inductance, LED 1 and the buzzer will activate.