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Simple sensitive metal detector

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Simple sensitive metal detector

... underground and in freshwater bodies of water, in the ceilings of buildings and in the thickness of concrete, it will be detected by an electronic device - a metal detector. This is how it works.
Fig.1 Structural scheme of the metal detector.

The reference generator EG (Fig. 1) produces a sinusoidal voltage of 50 kHz. Contour coil, which determines the generation frequency, is the sensor D of the device. The sinusoidal signal through the coupling capacitor Cp is fed to the quartz filter KF. If the frequency of the generator and its own resonant frequency KF coincide, the signal falls on the threshold device PU. It registers an alternating voltage at the input, extracts a constant component from it, and supplies it to an arrow indicator I.
Approaching a metal object causes a change in the frequency of the EG. Since it is now different from the resonant frequency KF, the voltage at the input of the PU decreases, and the arrow deflects 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, thanks to which the sensitivity of the circuit increases dramatically. Here is how it works.
Fig.2 The waveform at the input and output of the threshold device.

A sinusoidal signal arriving at the PU input is limited from below (Fig. 2), and voltage pulses appear on the indicator:
In = Io - Yip,
where Io is the input signal level at rest, Ip is the set threshold voltage.
The sensitivity of the device is expressed by the ratio:
s = DI / Ii = DI / (Io-Ii),
where DI is the change in the sinusoidal voltage during detuning of the EG, depending on the size of the object and the distance to it. In fact, s shows by what amount the indicator needle is deflected when the sensor-circuit mismatches. Therefore, selecting the value of Ip, it is possible to achieve the maximum deviation of the instrument arrow with an arbitrarily small change in Io. But in real devices one has 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 T1 transistor (Fig. 3). Contour coil L1 is a device sensor. Capacitors C3 - C6 are designed to tune the generator to a frequency of 50 kHz. Pic.3
Fig.3 Schematic diagram of the metal detector.

Through the separation capacitor C7 sinusoidal voltage from the generator is fed to a quartz filter. Capacity C7 is chosen small - 5 pF. Thus, the influence of subsequent cascades on the operation of the generator is practically excluded.
The threshold device is assembled on a T2 field-effect transistor. The voltage threshold Ip is set by the divider R5 - R7.
Capacitor C8 smoothes pulsations on the indicator IP1.
The filter R4, C1 provides the decoupling of alternating current between the threshold and master oscillators.

The device consists of two units: measuring (with sensor) and power supply. The first includes a circuit board, indicator, controls and adjustments. The sensor is a rigid ring frame made of Plexiglas, on which 65 turns of the PEL 0.2 passage are wound. The winding is encased in an aluminum foil screen and coated with epoxy. The sensor is connected to the measuring unit with a coaxial cable RK-75.
The power supply contains five silver-zinc batteries. The voltage of each element is 1.25 V, capacity 2A-h. Particular attention should be paid to the frame of the metal detector. It should be light weight, be stiff and elastic. Otherwise, even with light strokes, unavoidable when working with the device in the field, the generator frequency “goes away” - the metal detector becomes upset.
The base of the frame is an annular frame made of plexiglas or polystyrene d = 300 mm. The winding is screened with aluminum foil 0.05 mm thick. But it is impossible to interconnect the ends of the screen (a short-circuited turn is formed).
The terminals of the winding are connected to the cable RK-75 with a length of 0.3—1 m (with the braid of the cable also connect the screen of the coil). This place is poured with epoxy. Sensor connection to the electronics unit one-piece.
The metal detector has a high sensitivity. The indicator arrow deviates 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. So, for example, steel, aluminum and brass disks give equal distances of equal arrows at equal distances. They do not depend on whether a solid object or hollow.
During the work with the metal detector it is necessary to consider background hindrances. Sandy and peaty soils, black soil, wood, water do not give a background signal. Therefore, the device works well in fresh water, in wooden buildings and on non-stony soils. A strong background gives a brick (burnt clay has magnetic properties) and some minerals.
The readings are also affected by temperature changes. Therefore, it is better to place the frame in a case of a heat insulator, such as foam plastic.
To work under water, the metal detector must first be held 10–15 minutes in the water and then adjusted.
On the ground, searches should be carried out in cloudy weather or in the evening to avoid direct sunlight on the device.