A device that allows you to search for metal objects located in a neutral environment, for example, the ground, due to their conductivity is called a metal detector (metal detector). This device allows you to find metal objects in various environments, including in the human body.

Largely due to the development of microelectronics, metal detectors, which are produced by many enterprises around the world, have high reliability and small overall weight characteristics.

Not so long ago, such devices could most often be seen with sappers, but now they are used by rescuers, treasure hunters, public utilities workers when searching for pipes, cables, etc. Moreover, many "treasure hunters" use metal detectors that they assemble with their own hands .

The design and principle of operation of the device

Metal detectors on the market operate on different principles. Many believe that they use the principle of pulsed echo or radar. Their difference from locators lies in the fact that the transmitted and received signals operate constantly and simultaneously, in addition, they operate at the same frequencies.

Devices operating on the principle of "reception-transmission" register the signal reflected (re-radiated) from a metal object. This signal appears due to the impact on a metal object of an alternating magnetic field, which is generated by the metal detector coils. That is, the design of devices of this type provides for the presence of two coils, the first is transmitting, the second is receiving.

Devices of this class have the following advantages:

• simplicity of design;
• great ability to detect metallic materials.

At the same time, metal detectors of this class have certain disadvantages:

• metal detectors can be sensitive to the composition of the soil in which they search for metal objects.
• technological difficulties in the production of the product.

In other words, devices of this type must be configured by hand before operation.

Other devices are sometimes referred to as a beat detector. This name comes from the distant past, more precisely from the time when superheterodyne receivers were widely used. Beating is a phenomenon that becomes noticeable when two signals with close frequencies and equal amplitudes are summed. The beating consists in pulsing the amplitude of the summed signal.

The pulse frequency of the signal is equal to the difference in the frequencies of the summed signals. By passing such a signal through a rectifier, it is also called a detector, the so-called difference frequency is isolated.

Such a scheme was used for a long time, but today, it is not used. They were replaced by synchronous detectors, but the term remained in use.

The beat metal detector works using the following principle - it registers the frequency difference from two generator coils. One frequency is stable, the second contains an inductor.

The device is set up by hand so that the generated frequencies match or at least are close. As soon as metal enters the coverage area, the set parameters change and the frequency changes. The frequency difference can be recorded in many ways, ranging from headphones to digital methods.

Devices of this class are characterized by a simple sensor design, low sensitivity to the mineral composition of the soil.

But besides this, during their operation it is necessary to take into account the fact that they have high energy consumption.

Typical design

The structure of the metal detector includes the following components:

1. The coil is a box-type design, it houses the receiver and transmitter of the signal. Most often, the coil has an elliptical shape and polymers are used for its manufacture. A wire is connected to it, connecting it to the control unit. This wire transmits the signal from the receiver to the control unit. The transmitter generates a signal when metal is detected, which is transmitted to the receiver. The coil is installed on the lower rod.
2. The metal part on which the coil is fixed and its angle of inclination is adjusted is called the lower rod. Thanks to this solution, a more thorough examination of the surface occurs. There are models in which the lower part can adjust the height of the metal detector and provides a telescopic connection with the rod, which is called the middle one.
3. The middle shaft is the node located between the lower and upper shafts. Fixing devices are fixed on it, allowing you to adjust the size of the device. on the market you can find models that consist of two rods.
4. The top bar is usually curved. It resembles the letter S. This form is considered optimal for fixing it on the hand. An armrest, a control unit and a handle are installed on it. The armrest and handle are made of polymeric materials.
5. The metal detector control unit is required to process the data received from the coil. After the signal is converted, it is sent to headphones or other means of indication. In addition, the control unit is designed to adjust the operating mode of the device. The wire from the coil is connected using a quick-release device.

All devices included in the metal detector are waterproof.

This is the relative simplicity of the design and allows you to make metal detectors with your own hands.

Varieties of metal detectors

The market offers a wide range of metal detectors used in many areas. Below is a list that shows some of the varieties of these devices:

Most modern metal detectors can find metal objects at a depth of up to 2.5 m, special deep products can detect a product at a depth of up to 6 meters.

Operating frequency

The second parameter is the frequency of operation. The thing is that low frequencies allow the metal detector to see to a fairly large depth, but they are not able to see small details. High frequencies allow you to notice small objects, but do not allow viewing the ground to a great depth.

The simplest (budget) models operate at one frequency, models that are classified as average price levels use 2 or more frequencies in operation. There are models that use 28 frequencies when searching.

Modern metal detectors are equipped with such a function as metal discrimination. It allows you to distinguish the type of material located at depth. At the same time, when ferrous metal is detected, one sound will sound in the searcher's headphones, and another when non-ferrous metal is detected.

Such devices are referred to as pulse-balanced. They use frequencies from 8 to 15 kHz in their work. Batteries of 9 - 12 V are used as a source.

Devices of this class are able to detect a gold object at a depth of several tens of centimeters, and ferrous metal products at a depth of about 1 meter or more.

But, of course, these parameters depend on the device model.

How to assemble a homemade metal detector with your own hands

There are many models of devices on the market for searching for metal in the ground, walls, etc. Despite its external complexity, making a metal detector with your own hands is not so difficult and almost anyone can do it. As noted above, any metal detector consists of the following key components - a coil, a decoder and a power supply signaling device.

To assemble such a metal detector with your own hands, you need the following set of elements:

• controller;
• resonator;
• capacitors of various types, including film ones;
• resistors;
• sound emitter;
• Voltage regulator.

The simplest do-it-yourself metal detector

The metal detector circuit is not complicated, and you can find it either in the vastness of the global network, or in specialized literature. Above is a list of radio elements that are useful for assembling a metal detector with your own hands at home. A simple metal detector can be assembled by hand using a soldering iron or other accessible way. The main thing at the same time, the parts should not touch the body of the device. To ensure the operation of the assembled metal detector, power supplies of 9-12 volts are used.

To wind the coil, a wire with a cross-sectional diameter of 0.3 mm is used, of course, this will depend on the selected circuit. By the way, the wound coil must be protected from the effects of extraneous radiation. To do this, it is screened with your own hands using ordinary food foil.

To flash the controller, special programs are used, which can also be found on the Internet.

Metal detector without chips

If a novice "treasure hunter" has no desire to get involved with microcircuits, there are schemes without them.

There are more simple circuits based on the use of traditional transistors. Such a device can find metal at a depth of several tens of centimeters.

Deep metal detectors are used to search for metals at great depths. But it is worth noting that they are not cheap and therefore it is quite possible to assemble it with your own hands. But before you start making it, you need to understand how a typical circuit works.

The scheme of a deep metal detector is not the simplest and there are several options for its execution. Before assembling it, it is necessary to prepare the following set of parts and elements:

• capacitors different type- film, ceramic, etc.;
• resistors of different ratings;
• semiconductors - transistors and diodes.

Nominal parameters, quantity depend on the selected circuit diagram device. To assemble the above elements, you will need a soldering iron, a set of tools (screwdriver, pliers, wire cutters, etc.), material for making the board.

The process of assembling a deep metal detector is approximately as follows. First, a control unit is assembled, the basis of which is a printed circuit board. It is made from textolite. Then the assembly scheme is transferred directly to the surface of the finished board. After the drawing has been transferred, the board must be etched. To do this, use a solution that includes hydrogen peroxide, salt, electrolyte.

After the board is etched, holes must be made in it to install the circuit components. After the board has been tinned. The most important step is coming. Do-it-yourself installation and soldering of parts on a prepared board.

To wind the coil with your own hands, use a wire of the PEV brand with a diameter of 0.5 mm. The number of turns and the diameter of the coil depend on the chosen scheme of the deep metal detector.

A little about smartphones

There is an opinion that it is quite possible to make a metal detector from a smartphone. This is not true! Yes, there are applications that install under the Android OS.

But in fact, after installing such an application, he will really be able to find metal objects, but only pre-magnetized ones. He will not be able to search and, moreover, discriminate against metals.

They can work steadily with the passage of tens of people per minute. Typically, stationary metal detectors have passage dimensions up to 2 m in height, up to 1 m in width. Thus, METOR 200 HS metal detectors have passage dimensions of 2010 x 710 x 594 mm (Fig. 1).

Modern stationary metal detectors are equipped with microprocessors, which set the operating modes (it is possible to set the detection of certain metals, the mass of an object, etc.) and indication (usually they have light, sound indication, data output to the display), count the number of passengers who have passed, determine the side and the zone of carrying metal objects, perform automatic self-control, etc. You can set the device to detect a razor blade but not an aluminum can of beer or a handful of coins. Some metal detectors are equipped with remote controls to control and display the results of their work. A computer can be used as such a remote control.

For example, in the stationary Intelliscan 12000 metal detector (Fig. 2), the space under the arch is divided into 18 independent detection zones - 6 horizontal and 3 vertical. When carrying metal objects on the display, which shows the contour of the human body, the zones of localization of these objects are displayed.

Rice. 1. Stationary metal detector METOR 200 HS

Rice. 2. Stationary metal detector Intelliscan 12000 with display

The Intelliscan 12000 metal detector has the following features:

20 programs for selective detection of metal objects;

99 levels of sensitivity when working on any of the programs;

alarm signals in each of the zones are generated if the dimensions of metal objects exceed the specified ones;

per-zone sensitivity setting in the range from -99% to + 99% relative to the base;

adjustment of the lower zones to compensate for the influence of metal reinforcement in the floor;

automatic testing and auto-calibration;

detuning from noise using digital filtering;

permanent status display on a color display;

protection of installations from outside interference with a six-digit access code;

the number of user-selectable operating frequencies - 16;

Complies with all 1991 US Federal Aviation Administration requirements for airport applications and standards National Institute justice NILECJ (USA);

compliance with the hygienic certificate of the Ministry of Health of the Russian Federation 77.01.09.346.P.10046.04.0.

There are options for stationary metal detectors that can be powered by batteries, operate at sub-zero temperatures of several tens of degrees in the field.

3. Metal detectors on the principle of "reception-transmission"

The principle of operation of metal detectors of this type is based on exposing the object (target) under study to an alternating magnetic field of a transmitting (radiating) coil and recording a signal that appears as a result of induction of eddy currents in a metal object (target). Thus, they belong to devices of the location type and must have at least two coils - transmitting (emitting) and receiving. In foreign terminology, metal detectors operating on this principle are often referred to as IB (Induction Balance) or VLF - (Very Low Frequency ).

Both the transmitted and received signals are continuous and coincide in frequency.

The fundamental point for metal detectors of this type is the choice of the mutual arrangement of the coils. They must be located so that, in the absence of foreign metal objects, the magnetic field of the transmitting coil induces a zero signal in the receiving coil.

Figure 3 a) and b) shows the arrangement of coils with perpendicular and crossed axes, in which there is no current induction in the receiving coil. On fig. 3 c) shows a system of one emitting (in the center) and two receiving coils. The latter are turned on counter by a signal induced by a radiating coil, and in the absence of metal objects at their output, the total emf equals zero.

Rice. 3. The location of the coils, in which there is no guidance

currents in the receiving coil

Coils that emit radiation and/or receive a signal are made in the form of some design called a search frame. The parallel arrangement of the coils is called coplanar (Fig. 4). This arrangement of the coils makes it possible to reduce the dimensions of the search frame, since it can be made in a flat (in the form of a "pancake") protective case.

Rice. 4. Variants of coplanar arrangement of coils

There are several ways to arrange the coils in the same plane, providing a zero signal in the receiving coil. The coils can be superimposed on each other so that the total flux of the magnetic induction vector through the plane of the receiving coil is equal to zero (Fig. 4 a). Rice. 4 b) illustrates the method when the receiving coil in the form of a "figure eight" is placed inside the emitting one. At the same time, emfs are induced in different halves of the "eight". with different signs and compensate each other. It is possible to place a receiving coil of the usual form inside the emitting coil, but then a special compensating device is used.

Usually, in metal detectors of this type, the search frame is formed by two coils located in the same plane and balanced so that when a signal is applied to the transmitting coil, the receiving coil has a minimum signal at the output. The operating frequency of radiation is from one to several tens of kHz.

Consider one of the variants of the structural diagram of a metal detector operating on the principle of "reception-transmission" (Fig. 5).

The generator creates an alternating (rectangular or sinusoidal) voltage, which is fed through the power amplifier to the radiating coil.

Rice. 5. Functional diagram metal detector,

operating on the principle of "reception-transmission"

When a metal target appears near the metal detector, currents from the radiating coil are induced in it, which are the cause of secondary electromagnetic radiation. The latter acts on the receiving coil and a variable emf is induced in it. (voltage). The frequency of the induced signal is the same as in the radiating coil.

The signal induced in the receiving coil has some phase shift with respect to the signal from the emitting coil, since it arrives at the receiving coil with some delay. When a metal object appears near the search frame, the signal amplitude in the receiving coil increases, and the phase shift changes depending on the conductivity of the metal (black, non-ferrous).

A synchronous detector separates a useful alternating signal coming from the output of a receiving amplifier and created by radiation from a metal object into a constant signal. Synchronization of its operation with the operation of the source (generator) of the radiating signal makes it possible to increase the efficiency of its operation against the background of noise and interference, which significantly exceed the amplitude of the useful signal.

The output signal of the synchronous detector is amplified and fed to an indicator, such as sound or light, which signals the appearance of a metal object near the metal detector.

The indication is turned on only for signals exceeding a certain threshold in amplitude. Thus, weak signals, mainly associated with the movement of the metal detector and external electromagnetic interference, do not trigger the indicator.

A metal detector is a very tempting device, it can be used for a variety of purposes, such as searching for old wiring, water pipes, well and in the end treasure. The concept of a metal detector is very broad, metal detectors themselves are different, the principle of metal search laid down in classic metal detectors is used in a variety of devices ranging from simple detectors to radar stations.

AT recent times the so-called impulse metal detectors are gaining great popularity, which contain only one coil in their composition and have a relatively simple design, while providing quite good sensitivity and high reliability. A pulsed metal detector works on the principle of receiving and transmitting, the search coil in such a metal detector can operate in two modes - receiving and transmitting. The signal emitted by the coil generates or excites eddy currents in the metal, which are picked up by the coil itself.

Different metals have different electrical conductivity and many metal detectors are able to recognize this with a fairly high accuracy, determining what kind of metal is in the ground.

The above scheme of a metal detector is very common on the net, but there are very few photos of real designs and reviews, so it was decided to repeat the scheme and try it out in practice.

The printed circuit board turned out to be quite compact, it was made using the loot method.

The scheme has many advantages:

• the presence of only one coil;
• extremely simple and not capricious scheme, which practically does not require additional settings;
• the whole circuit is built on just one chip;
• low sensitivity to the ground;
• if desired, the metal detector can be configured so that it sees only non-ferrous metals and ignores black ones, i.e. some kind of metal discrimination function.

Of the shortcomings:

• shallow search depth - the detector notices large metal objects at a distance of up to 30 cm, medium coins up to 5 and 8 cm.

This is not enough, but depending on the purpose for which... For example, to search for old water pipes in the wall, the scheme copes 100%.

The circuit is built on a single CD4011 CMOS chip, which contains 4 2I-NOT logic elements. It consists of 4 parts, reference and search oscillators, mixer and signal amplifier, which is made on a single transistor. As a dynamic head, it is preferable to use headphones with an impedance of 16 to 64 ohms, because. the output stage is not designed for low-resistance loads.

The metal detector works as follows. Initially, the search and reference oscillators are set to the same frequency, so we do not hear anything from the speaker. The frequency of the reference oscillator is fixed with the possibility of manual adjustment by rotating the variable resistor. The frequency of the search generator is highly dependent on the parameters of the LC circuit. If a metal object appears in the field of view of the search coil, the frequency of the LC circuit is disturbed, as a result of which the frequency of the search generator changes relative to the reference one. The mixer separates the frequency difference of these generators, which, in the form of an audio signal, is filtered and fed to the amplifier stage, the load for which is the earpiece.

Coil

The larger the coil diameter, the more sensitive the metal detector, but large coils have their drawbacks, so you need to choose the optimal parameters. For this circuit, the most optimal diameter lies in the range from 15 to 20 cm, the wire diameter is 0.4-0.6 mm, the number of turns is 40-50, if the coil diameter is within 20 cm. In my case, the coil trimmed, the turns and the diameter are smaller than necessary, so the sensitivity of the circuit is not so hot. If you plan to use the metal detector in conditions high humidity, the coil must be sealed.

Setting

All adjustment work is done in the absence of metal in the field of view of the coil!

If at the first connection the circuit does not react to metal, but all components are in good order, most likely the frequency difference from the generators is outside the audio range and the sound is simply not perceived by a person. In this case, it is worth twisting the variable resistor until an audible signal appears. Next, we slowly rotate the same resistor until we hear a low-frequency signal from the speaker, then rotate it a little more in the same direction until the signal disappears completely. This completes the setup.

For finer tuning, I advise you to use a multi-turn resistor, or two ordinary variables, one of which is for coarse tuning, and the second for smoother tuning. After setting up, we check the metal detector by bringing a metal object to its coil and make sure that the tone of the sound signal changes, that is, the circuit reacts to metal.

The effect of metal discrimination is observed if both generators operate at a frequency of about 130-135 kHz, while there is almost no sensitivity to ferrous metals.

The circuit can be powered from a constant source with a voltage of 3 to 15 volts, the best option is to use a 9-volt 6F22 battery, the current consumption of the circuit in this case will be in the range from 15 to 30 mA, depending on the load resistance.

B. SOLONENKO, Genichesk, Kherson region, Ukraine

It would not be an exaggeration to say that metal detectors invariably attract the attention of radio amateurs. Many such devices have been published in the journal "Radio". Today we offer readers a description of another design created in the radio engineering circle of the Technical Station of Young Technicians (see the article about it in Radio, 2005, No. 4, 5). The members of the circle were given the task of developing an easy-to-manufacture device based on an accessible element base, for the establishment of which one multimeter is enough. How well the guys succeeded, you, the readers, can judge.

The proposed metal detector works on the principle of "transmission-reception". A multivibrator was used as a transmitter, and an audio frequency amplifier (34) was used as a receiver. Coils of the same size and winding data are connected to the output of the first of these devices and the input of the second,

In order for the system of such a transmitter and receiver to become a metal detector, their coils must be positioned so that in the absence of foreign metal objects there is practically no connection between them, i.e. the transmitter signal does not go directly to the receiver. As you know, the inductive coupling between the coils is minimal if their axes are mutually perpendicular. If the transmitter and receiver coils are positioned in this way, then the transmitter signal in the receiver will not be heard. When a metal object appears in the vicinity of this balanced system, under the influence of an alternating magnetic field so-called eddy currents arise in the transmitting coil and, as a result, its own magnetic field, which induces an alternating EMF in the receiving coil. The signal received by the receiver is converted by phones into sound. Its volume depends on the size of the object and the distance to it.

Specifications of the metal detector: operating frequency - about 2 kHz; detection depth of a coin with a diameter of 25 mm - about 9 cm; iron and aluminum seaming caps - 23 and 25 cm, respectively; steel and aluminum sheets with dimensions of 200x300 mm - 40 and 45 cm; manhole- 60 cm.

Transmitter. The transmitter circuit is shown in fig. 1. As mentioned, this is a symmetrical multivibrator based on transistors VT1, VT2. The frequency of the oscillations it generates is determined by the capacitance of the capacitors CI, C2 and the resistance of the resistors R2, R3. The signal 34 from the collector load of the transistor VT2 - resistor R4 - through the decoupling capacitor C3 is fed to the coil L1, which converts electrical oscillations into an alternating magnetic field AF.

Fig.2

Receiver is a three-stage amplifier 34, made according to the circuit shown in Fig. 2. At its input, the same L1 coil is connected as in the transmitter. The output of the amplifier is loaded with telephones BF1.1, BF1.2 connected in series.

Fig.3

An alternating magnetic field of the transmitter, induced in a metal object, acts on the receiver coil, as a result of which a electricity a frequency of about 2 kHz. Through the separating capacitor C1, the signal is fed to the input of the first stage of the amplifier, made on the transistor VT1. The amplified signal from its load - resistor R2 - is fed through the decoupling capacitor C3 to the input of the second stage, assembled on the transistor VT2. The signal from its collector through the capacitor C5 is fed to the input of the third stage - the emitter follower on the transistor VT3. It amplifies the current signal and allows you to connect low-ohm phones as a load.

To reduce the effect of temperature environment on the stability of the amplifier, a negative DC voltage feedback was introduced into the first and second stages by turning on the resistor R1 between the collector and the base of the transistor VT1 and the resistor R3 between the collector and the base VT2. Gain reduction at frequencies below 2 kHz is achieved by an appropriate choice of the capacitance of separating capacitors C1, C3, at frequencies above this frequency - by introducing a frequency-dependent negative into the first and second stages. feedback by alternating voltage through capacitors C2 and C4. These measures made it possible to increase the noise immunity of the receiver. Capacitor C6 prevents the amplifier from self-exciting when the internal resistance of the battery increases as it discharges.

Fig.4

Details and construction. Transmitter and receiver details are located on printed circuit boards ah, made by cutting insulating tracks on blanks from one-sided foil-coated fiberglass. A drawing of the transmitter board is shown in fig. 3, the receiver - in fig. 4. The boards are designed for the use of MLT resistors with a power of 0.125 or 0.25 W and capacitors K73-5 (C2, C4 in the receiver) and K73-17 others. Oxide capacitor C6 in the receiver - K50-35 or similar foreign production. Instead of those indicated on the diagram, in the transmitter, you can use any other transistors of the KT503 series, and in the receiver - transistors of the KT315 series with any letter index or KT3102 series with indices A-B. The use of the latter is preferable, since they have a lower noise figure, and the signal from small objects will be less masked by the noise of the amplifier. Switches SA1 can be of any design, but preferably smaller. Phones BF1, BF2 - small inset, for example, from an audio player.

The receiver and transmitter coils, as already mentioned, are the same. They are made like this. At the corners of a rectangle measuring 115x75 mm, four nails with a diameter of 2 ... 2.5 and a length of 50 ... 60 mm are driven into the board, after putting on them PVC or polyethylene tubes 30 ... 40 mm long. 300 turns of PEV-2 wire with a diameter of 0.12 ... 0.14 mm are wound onto the nails isolated in this way. Upon completion of the winding, the turns are wrapped around the entire perimeter with a narrow strip of insulating tape, after which any two adjacent nails are bent towards the center of the rectangle and the coil is removed.

Polystyrene button boxes (internal dimensions - 120x80 mm) are used as receiver and transmitter cases. Battery compartments, PCB racks and coil fasteners are made of the same material and glued to the housings with a P-647 solvent (P-650 can also be used). The location of parts in the transmitter housing is shown in fig. 5, the details of the receiver are similarly arranged.

Fig.5

All metal structural elements located inside the receiver and transmitter coils (power battery, parts board, power switch) affect their magnetic field. To exclude a possible change in their position during operation, all of them must be securely fastened. This is especially true of the Krona battery as a replaceable structural element.

Establishment. To check the operation of the transmitter, instead of the L1 coil, connect phones and make sure that sound is heard in the phones when the power is turned on. Then, having connected the coil in place, the current consumed by the transmitter is controlled, it must be within 5 ... 7 mA.

The receiver is tuned with the input short-circuited. By selecting a resistor R1 in the first stage and R3 in the second, a voltage is set on the collectors of the transistors VT1 and VT2, respectively, equal to about half the supply voltage. Then, by selecting the resistor R5, they ensure that the collector current of the transistor VT3 becomes equal to 5 ... 7 mA. After that, having opened the input, connect the L1 receiver coil to it and, receiving the transmitter signal at a distance of about 1 m, make sure that the system as a whole is working.

Before assembling the nodes into a single structure, it makes sense to conduct several experiments. Having installed the transmitter and receiver on the table vertically at a distance of 1 m (in such a way that the axes of the coils seem to continue one another) and controlling the signal level in the phones, slowly turn the receiver around the vertical axis to a position in which the planes of the coils are perpendicular to one another. In this case, the signal will first slowly decrease, then disappear completely and, with further rotation, will begin to increase. Conduct the experiment several times so that when assembling and adjusting the metal detector, it is easy to determine the minimum signal in the receiver.

Fig.6

Then, on a table that does not contain metal structural elements, place the transmitter vertically, and at a distance of 10 cm from it, place the receiver horizontally on a stand (one or more books) so that the plane of the receiver coil is perpendicular to the plane of the transmitter coil and is slightly below its center. While monitoring the signal level in the phones, lift the side of the receiver facing the transmitter and get the signal to drop. By selecting gaskets between the receiver and the stand, find its position, at which the slightest movement of the gasket made from a paper postcard allows you to set the minimum signal in the receiver, which corresponds to the maximum sensitivity of the metal detector.

Bringing seam covers made of tin and aluminum alternately into the coverage area of ​​the metal detector, make sure that the zone of maximum sensitivity of the metal detector is located under and above the receiver coil (the magnetic fields of the receiver and transmitter coils are symmetrical). Pay attention to the fact that the metal detector reacts differently to covers of the same size made of different metals.

If, with a minimum connection of the coils, the signal is heard a little and when the cover is inserted on one side, it first decreases until it disappears completely, and then begins to grow, and when it is introduced on the other side, it increases without a dip, then this indicates either an inaccurate minimum setting or distortion of the magnetic coil fields of the receiver or transmitter. At the same time, this fact suggests that by introducing an additional metal object, it is possible to adjust the system until the signal completely disappears at a minimum, i.e., to achieve the maximum sensitivity of the device. If the signal disappears completely from a distance of 15...20 cm when the seaming cap is inserted, then by introducing a smaller object into the metal detector field, the same effect can be obtained when placed on the body of the receiver or transmitter. In the author's version, such an object turned out to be a coin with a diameter of 25 mm made of yellow metal (a similar effect will be obtained when an aluminum plate of a similar size is introduced). There were three places where the coin performed the task assigned to it: from below under the transmitter, under the receiver near the battery, and on the handle between the receiver and transmitter.

Assembly. The design of the author's version of the device in a simplified form is shown in Fig. 6, a appearance- in fig. 7. Carrier rail 2 (see fig. 6) and handle 3 are made of wood. The upper part of the handle is pasted over with plastic for ease of use, and the lower part is inserted into a pre-made hole in the rail and secured with glue. After assembly, the wooden part of the handle 3 and the carrier rail 2 are varnished to protect against moisture. At the top of the handle there is a telephone jack 4, which is connected to the receiver by wires twisted in pairs.

During assembly, the transmitter 1 is rigidly fixed on the carrier rail 2 in such a way that the receiver 7, located at its other end, is slightly below the line corresponding to the minimum of the received signal. Then select the thickness of the gasket 5 (of any insulating material) until the minimum of the received signal is easily set by moving the adjusting plate 6. After that, the receiver 7 is fixed on the carrier rail 2 with two screws. The screw at the edge of the carrier rail 2 is screwed in until it stops, and the second (approximately in the middle of the bottom wall of the housing) is not screwed in by 1 ... 2 mm. This eliminates the movement of the receiver in a horizontal plane and at the same time allows you to slip the adjusting plate 6 under its body, raising the edge of the receiver. By moving it in this way in a vertical plane, a minimum of the received signal is achieved. After the final assembly, the location of the compensating object is specified and glued.

The principle of operation of the metal detector

The principle of operation of the metal detector

As you know, a metal detector is able to detect the presence of metal objects without any contact with them. The operator is informed about the presence of metal using special signals: sound, arrow movement, changes in indicator values, etc.

Depending on the principle of operation, the following types of metal detectors can be distinguished:

1. Metal detector with electronic frequency meter

The principle of operation of such a metal detector is based on the evaluation of the frequency of the measuring generator by an electronic frequency meter, when the sensor itself is still far from the target. The resulting value is "remembered" by the register. After that, in the process of searching for objects of interest, the electronic frequency meter is engaged in continuous measurement of the frequency of the receiving generator. The reference frequency index is subtracted from the received data, and the result is displayed on the display screen.

Scheme of a metal detector with an electronic frequency meter

2. Beat metal detector

The principle of operation of a metal detector on beats is based on the combination of the difference in frequencies coming from two generators. One of these generators has a stable frequency, and the second one includes a sensor, which is an inductor. If metal objects are not located near the metal detector, the frequency values ​​of the generators in the device are practically the same. The presence of metal near the sensor leads to a sharp change in the frequency of the generator.

Scheme of a metal detector on beats

Registration of the frequency difference can occur in a variety of ways. The easiest way is to listen to the signal using headphones or a loudspeaker. Digital methods for measuring frequency fluctuations are also often used.

3. Metal detectors with the principle of "transmission-reception"

The principle of operation of such a metal detector is to register a signal that is reflected from a metal object. The occurrence of the reflected signal is the result of the action of a magnetic field with a variable flux of the device coil on the target (metal object). At the same time, the structure of the device includes at least two coils, one of which is “responsible” for signal transmission, and the other for its reception.

The work of the metal detector "transmission-reception" is based on a certain mutual arrangement of coils, excluding the impact of one on the other. Thus, if there are no foreign metal objects, the radiating coil induces a zero signal to the receiving system. The appearance of metal objects near the coils leads to a special signal.

4. Single coil induction metal detector

The design of the sensor of this device includes only one coil that monitors frequency changes. If a target appears close to the metal detector, a reflected signal occurs. In the coil, it is "induced" by an additional electrical signal. The operator only needs to isolate this signal. You can register the reflected signal by calculating from the electrical indicator present in the coil a signal of a similar phase, frequency, amplitude, which was observed in the absence of metal nearby.

In general, a single-coil induction metal detector combines the characteristics of devices operating on a beat with devices of the "transmit-receive" principle. Thus, a single-coil metal detector is characterized by high sensitivity and simple design.

5. Pulse metal detector

The pulse metal detector is characterized by high sensitivity and can be used to search for various objects even at great depths. The operation of such a metal detector is based on the temporary method of separating radiation and reflection signals. This method is very often used in echo and pulse type radar.

The pulse generator generates short-term current pulses, which subsequently enter the radiating coil. Here they are already being converted into magnetic induction pulses. Since the pulse generator, i.e. The radiating coil has an inductive character, “overloads” occur on the impulse fronts in the form of voltage drops. These bursts can reach amplitudes of tens or even hundreds of volts. However, it is still better not to use protective limiters, because. the front of the pulsed current and magnetic induction can be pulled. As a result, the process of separating the reflective type signal becomes more difficult.

Diagram of a pulsed metal detector

It should be noted that the emitting and receiving coils can be placed in an absolutely arbitrary order. This is due to the fact that the penetration of the emitted signal and the influence of the reflected signal on the coil are separated over certain time intervals. In addition, the same coil can perform any of the roles: both to receive a signal and to reflect it.

6. Magnetometers

Magnetometers are devices whose purpose is to change the indicators of the magnetic field. At the same time, magnetometers can also be used as metal detectors. This is possible due to the fact that the Earth's magnetic field can be distorted. various materials with ferromagnetic properties, such as iron. The detection of such objects occurs by registering deviations from the initial magnetic field module for a certain area. As a result, some magnetic inhomogeneity (anomalies) can be observed, which can be caused by metal objects.

Unlike the metal detectors discussed above, magnetometers cover a larger range of detecting iron objects. Probably, many have heard about finding, for example, a car located at a distance of 10 meters from the operator using a magnetometer. At the same time, the main disadvantage of magnetometers is their inability to detect objects made of non-ferrous metals. In addition, the magnetometer can respond not only to iron, but also to the so-called natural magnetic anomalies. These can be, for example, mineral deposits or individual minerals, etc.

Magnetometer circuit

7. Radars

The principle of operation of any radar is based on the method of studying electromagnetic energy, its reflection and reception from various objects in the air, on the sea or on land. The reflected signal is received for further processing and analysis. As a result, you can accurately determine the location of the object of interest, its speed and trajectory.

Radars have a number of undeniable advantages. So, they allow you to work with fairly large distances. The signal that was reflected can be considered such that it completely obeys the laws geometric optics, and its attenuation is proportional to only the second power of the distance. At the same time, a serious drawback of the radar is that by emitting electromagnetic waves, it allows you to find your location. However, now an intensive search is underway for methods to help hide the signatures of radars, and it is quite possible that in the near future it will be possible to get rid of this shortcoming.