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Simple sensitive metal detector

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Simple sensitive metal detector

... Under the ground and in freshwater reservoirs, in the floors of buildings and in the thickness of concrete, it will be detected by an electronic device - a metal detector. That's how it works.
Fig. 1
Fig.1 Structural diagram of the metal detector.

The EG reference generator (Figure 1) produces a sinusoidal voltage of 50 kHz. The loop coil, which determines the frequency of generation, is the sensor D of the device. The signal of the sinusoidal form through the separating capacitor Cp is fed to the quartz filter KF. If the oscillator frequency and the resonance frequency of the FF coincide, the signal falls on the threshold device PU. It registers an alternating voltage at the input, extracts a constant component from it and feeds it to the switch indicator I.
Approaching a metal object causes a change in the EG frequency. Since it is now different from the resonant frequency of the CF, the voltage at the input of the PU decreases, and the arrow deviates to the beginning of the scale by an angle proportional to the dimensions of the object and inversely proportional to the distance to it.
Our metal detector has a feature - a threshold device, due to which the sensitivity of the circuit rises sharply. Here's how it works.
Fig. 2
Fig. 2. Signal form at the input and output of the threshold device.

The sinusoidal signal arriving at the input of the PU is limited to the bottom (Figure 2), and voltage pulses appear on the indicator:
In = Io - Ip,
where Io is the level of the input signal at rest, Ip is the threshold voltage set.
The sensitivity of the device is expressed by the ratio:
s = DU / Ui = DU / (Io-Ii),
where DI is the change in the sinusoidal voltage when the EG is detuned, depending on the size of the object and the distance to it. In fact, s indicates the amount by which the arrow of the indicator deviates when the sensor of the contour is detuned. Consequently, choosing the value of En, one can achieve the maximum deviation of the arrow of the device for an arbitrarily small change of I0. But in real devices it is necessary to take into account the instability of the circuit elements and the frequency of the reference oscillator.

The reference generator is assembled according to the capacitive three-point scheme on the transistor T1 (Figure 3). The loop coil L1 is the sensor of the device. Condensers С3 - С6 are intended for tuning the generator to a frequency of 50 kHz. Fig. 3
Fig. 3 Schematic diagram of a metal detector.

Through the separating capacitor C7, the sinusoidal voltage from the generator goes to the quartz filter. The capacitance of C7 is chosen small - 5 pF. Thus, the effect of subsequent cascades on the operation of the generator is virtually impossible.
The threshold device is assembled on the field effect transistor T2. The threshold voltage U is given by the divisor R5 - R7.
The capacitor C8 smooths the pulsations on the indicator IP1.
The filter R4, C1 carries out an AC decoupling between the threshold and the master oscillators.

The device consists of two units: measuring (with sensor) and power. The first includes a circuit board, an indicator, controls and adjustments. The sensor is a rigid ring frame made of plexiglas, on which 65 turns of PEL 0.2 are wound. The winding is enclosed in a screen made of aluminum foil and filled with epoxy resin. The sensor is connected to the measuring unit by a coaxial cable RC-75.
The power supply contains five silver-zinc batteries. The voltage of each element is 1.25V, the capacity is 2Ah. Particular attention should be paid to the metal detector frame. It should have a small weight, be rigid and elastic. Otherwise, even with light bumps, inevitable when working with the device in the field, the frequency of the generator "goes away" - the metal detector is upset.
The base of the frame is a ring frame made of Plexiglas or polystyrene d = 300 mm. The winding is screened with an aluminum foil of 0.05 mm thickness. But the ends of the screen can not be connected together (a short-circuited coil is formed).
The winding leads are connected to the cable RK-75 with a length of 0.3-1 m (with the braid of the cable, the coil shield is also connected). This place is filled with epoxy resin. The sensor connection to the electronics module is all-in-one.
The metal detector has a high sensitivity. The arrow of the indicator is diverted by one division when the frame of the device approaches the disk d = 13 cm at a distance of 80 cm.
The device almost equally reacts to any metal. For example, steel, aluminum and brass disks give equal distances to the arrows at equal distances. They do not depend on whether a solid object or a hollow object.
When working with a metal detector, background noise must be taken into account. Sandy and peat soils, chernozem, wood, water do not give a background signal. Therefore, the device works well in fresh water bodies, in wooden buildings and on stony soils. Strong background gives a brick (calcined clay possesses magnetic properties) and some minerals.
The temperature is also affected by the instrument. Therefore, it is better to place the frame in a case from a heat insulator, for example a foam plastic.
To work underwater, the metal detector must first be held for 10-15 minutes in water and then adjusted.
On the ground, it is better to search in cloudy weather or in the evening, in order to avoid direct sunlight on the device.