Wiring Schematic diagram

This metal detector schema consists of two oscillators, both working at about 465 kHz. One uses a transformer and the other uses an inductor which is the search coil LI. The oscillators are coupled through a capacitor 10 pF. The diagram DE metal detector is a BFO (beat frequency oscillator), a tone beat produced if the two oscillators are working together through the diode is detected and sent to the audio amplifier. 

The oscillator coil is tuned by a variable capacitor of 10-365 pf variable. The search coil is made of wire with 22 turns between 24 and 36 AWG enamel junction to the center. The wire should be wound on a form of about 6 "χ 6" and the phones must be high impedance.