This homemade metal
detector circuit will help you find objects composed of materials with
relatively high magnetic permeability. It is not suitable for buried
coins discovery that is not sensitive enough but you can detect pirates
The metal detector is powered by 2 × 9V batteries, each of it
charges with 15mA. L1 detector coil is part of the sinusoidal oscillator
built around transistor T1. Normally, the center frequency of the
voltage controlled oscillator (VCO) from the PLL
loop that is contained in IC1 is equal to the oscillation frequency of
T1. This changes when entering a metallic object (ferrous or nonferrous)
in the field induced by L1. S1 is a miniature 2-pole switch.
Meter needle deviation is a measure of frequency change, since the
direction of deviation depends on the type of material detected by the
The meter tool used for this homemade metal detector is zero as central, +-50µA.
Coil L1 consists of 40 turns of enamelled copper wire, wound on a
plastic template with a diameter of about 10 cm. Inductance thus
obtained ensure the functioning of the oscillator at a frequency
approximately equal to the VCO included in the PLL loop.
Use an oscilloscope to check that pin 2 of IC1 delivers sinusoidal
signal with frequency about 75 kHz. Adjust P1 so that fronts rectangular
signal from pin 4 to coincide with the peaks of the sinusoidal signal
from pin 2. Then, adjust P2 in order to obtain 0 on the meter. Since the
neutral zero setting “runs” with the battery’s decreasing voltage it
will be necessary to restore it (zero balancing) from time to time
during use of the metal detector.