This detector will help you find fairly large objects that consist of materials with a relatively high permeability. Also, it indicates whether the magnetic object inside the detection coil has good or bad conductive properties.
Examples of materials that couple good magnetic properties to a fairly high electrical isolation are ferrites pressed from metal oxides. The detector is not suitable for coin digging, for which it is not sensitive enough. The more fanciful stuff like bombs and treasures left by pirates are, however, reliably located.
The metal detector is powered symmetrically by two 9-V batteries, each of which is loaded with about 15 mA. The detection coil, L1, forms part of a sine wave oscillator built around transistor T1. Normally, the central frequency of the VCO (voltage-controlled oscillator) in the PLL ( phase-locked loop) contained in IC1 equals the oscillator frequency of T1. That changes when a metal object (Ferro-metallic or non-ferrometallic) enters the field Induced by L that happens, the sine-wave oscillator is detuned, and the voltage difference between pins 6 and 7 of IC1 indicates the difference between the sine oscillator, frequency, and the VCO frequency. This difference causes moving-coil meter M1 to deflect. The needle deflection itself is a measure of the frequency change, while the direction of the needle depends on the type of material detected by the coil. The meter used here is a center-zero ±50 μA type.
The coil consists of 40 turns of enameled copper wire on a plastic former with a diameter of about 10 cm (4 in). The inductance so obtained ensures that the sine wave oscillator works at a frequency which is roughly equal to that of the VCO in the PLL.
Use an oscilloscope to check that pin 2 of 1CI supplies a sine wave signal with a frequency of about 75 kHz. Next, adjust P1 such that the edges of the rectangular signal at pin 4 coincide with the positive peaks of the sine wave at pin 2. Next, null meter b turning potentiometer P2.
Since the null adjustment will drift a little as the battery voltage drops, it will be necessary to redo the balance adjustment every now and then during use.
Ri = 6.8 lid/ R2, R3 = 4.7 k.C2 R4 = 680 t2 R5, R6 = 5.6 Id/ P1 = 10 ki.(2 multiturn preset P2 = 470 S2 linear potentiometer
C1, C2 = 100 p.F 16 V radial C3 = 68 nF C4 = 15 nF C5:C8 = 10 nF C6:C7 = I nF
L1 = details in text
T1 = BC547B T2, T3 = BC557B