Detector circuit consists of oscillators, mixers, and frequency - voltage converters and other components. High sensitivity, visual display and so on. Can locate the metal, the maximum detection depth up to 2 meters.
IC1 (555) and L1, D1, RP1 probe oscillator components, L1 for the detection coil, mounted inside the probe handle. The oscillation frequency f1 = 0. 72R/L1, icons corresponding to the frequency parameters of 26KHz. Select the long 26KHz frequency electromagnetic wave to decrease the absorption of the soil. IC2 (555) and L2, D2, R1 etc. reference oscillator. VT1 two oscillations were added to the mixer, then the difference frequency signal into the IC3. IC3 for the LM2917, is a circuit with integrated charge pump and comparison, here used as a frequency voltage converter, the linear degree is generally less than 0.3%. It will convert the input voltage difference frequency in the range is 3V DC voltage shown in the table, the load can also be used to replace the audio circuit.
IC2 reference oscillator frequency 25KHz, L2 the inductance is 150mH, choose products inductance. L1 with a diameter of 0.49mm enameled wire coil around 30cm in diameter and 50 turns.