Metal detector based on the transmit-receive principle - Theory

The terms "transmit-receive" and "reflected signal" in various detector devices are usually associated with methods such as pulse echo and radar, which is a source of confusion when it comes to metal detectors.

Unlike various types of locators, in metal detectors of this type both the transmitted signal (emitted) and the received signal (reflected) are continuous, they exist simultaneously and coincide in frequency.

Operating principle

The principle of operation of metal detectors of the “transmission-reception” type is to register a signal reflected (or, as they say, re-emitted) by a metal object (target), see, pp. 225-228. The reflected signal occurs due to the influence of the alternating magnetic field of the transmitting (emitting) coil of the metal detector on the target. Thus, a device of this type implies the presence of at least two coils, one of which is transmitting and the other receiving.

The main fundamental problem that is solved in metal detectors of this type is the choice of the relative arrangement of the coils, in which the magnetic field of the emitting coil, in the absence of foreign metal objects, induces a zero signal in the receiving coil (or in the system of receiving coils). Thus, it is necessary to prevent direct impact of the transmitting coil on the receiving coil. The appearance of a metal target near the coils will lead to the appearance of a signal in the form of a variable emf. in the receiving coil.

Sensor circuits

At first it may seem that in nature there are only two options for the relative arrangement of coils, in which there is no direct signal transmission from one coil to another (see Fig. 1 a and 16) - coils with perpendicular and crossing axes.

Rice. 1. Options for the relative arrangement of the metal detector dactic coils according to the “transmission-reception” principle.

A more thorough study of the problem shows that there can be as many of these different metal detector sensor systems as desired, but they will contain more complex systems with more than two coils, appropriately connected electrically. For example, Fig. 1c shows a system of one emitting (in the center) and two receiving coils connected counter-currently according to the signal induced by the emitting coil. Thus, the signal at the output of the system of receiving coils is ideally equal to zero, since the emf induced in the coils. mutually compensated.

Of particular interest are sensor systems with coplanar coils (i.e. located in the same plane). This is explained by the fact that metal detectors are usually used to search for objects located in the ground, and bringing the sensor closer to the minimum distance to the surface of the earth is possible only if its coils are coplanar. In addition, such sensors are usually compact and fit well into protective housings such as “pancake” or “flying saucer”.

The main options for the relative arrangement of coplanar coils are shown in Fig. 2a and 26. In the circuit in Fig. 2a, the relative arrangement of the coils is chosen such that the total flux of the magnetic induction vector through the surface limited by the receiving coil is equal to zero. In the circuit of Fig. 26, one of the coils (receiving) is twisted in the form of a figure of eight, so that the total emf induced on the halves of the turns of the receiving coil, located in one wing of the figure of eight, compensates for a similar total emf. s., directed in the other wing of the G8.

Rice. 2. Coplanar options for the relative arrangement of metal detector coils according to the “transmission-reception” principle.

Various other designs of sensors with coplanar coils are also possible, for example Fig. 2c. The receiving coil is located inside the emitting coil. The emf induced in the receiving coil. is compensated by a special transformer device that selects part of the signal from the emitting coil.

Practical Considerations

Sensitivity A metal detector depends primarily on its sensor. For the considered sensor options, sensitivity is determined by formulas (1.20) and (1.33). With the orientation of the sensor to the object at the roll angle y optimal for each case, it is determined by the same coefficient K 4 and the functions of normalized coordinates F(X,Y) and G(X,Y). For comparison, in the square X O[-4,4], Y O[-4,4], the modules of these functions are shown in the form of an axonometric set of sections on a logarithmic scale in Fig. 12 and Fig. 13.

The first thing that catches your eye is the pronounced maxima near the locations of the sensor coils (0,+1) and (0,-1). The maxima of the functions F(X,Y) and G(X,Y) are not of practical interest and, for the convenience of comparing functions, are cut off at the 0(dB) level. From the figures and from the analysis of the functions F(X,Y) and G(X,Y) it is also clear that in the indicated square the modulus of the function F almost everywhere slightly exceeds the modulus of the function G, with the exception of the most distant points at the corners of the square and with the exception of a narrow region near X=0, where the function F has a “ravine”.

The asymptotic behavior of these functions far from the origin can be illustrated at Y=0. It turns out that the modulus of the function F decreases with distance in proportion to x^(-7), and the modulus of the function G decreases in proportion to x^(-6). Unfortunately, the advantage of the G function in sensitivity appears only at large distances exceeding the practical range of the metal detector. The same values ​​of the modules F and G are obtained at X>>4.25.

Rice. 12. Graph of the function F(X,Y).

Fig. 13. Graph of the function G(X,Y).

The “ravine” function F has very important practical significance. Firstly, it indicates that the sensor of a coil system with perpendicular axes has minimal (theoretically zero) sensitivity to metal objects located on its longitudinal axis. Naturally, these items also include many elements of the design of the sensor itself. Consequently, the useless signal reflected from them will be much less than that of a cross-axis coil system sensor. The latter is very important, given that the reflected signal from the metal elements of the sensor itself can exceed the useful signal by several orders of magnitude (due to the proximity of these elements to the sensor coils). It's not that the useless signal from the metal elements of the sensor structure is difficult to compensate for. The main difficulty lies in the slightest changes in these signals, which are usually caused by thermal and especially mechanical deformations of these elements. These slightest changes may already be comparable to the useful signal, which will lead to incorrect readings or false alarms of the device. Secondly, if some small object has already been detected using a metal detector of a coil system with perpendicular axes, then the direction of its exact location can be easily “taken in direction” by the zero value of the metal detector signal with the exact orientation of its longitudinal axis to the object (for any roll orientation) . Considering that the “capture” area of ​​the sensor during search can be several square meters, the latest quality of the system

the topic of coils with perpendicular axes is very useful in practice (less useless excavations).

The next feature of the graphs of the functions F(X,Y) and G(X,Y) is the presence of a ring-shaped “crater” of zero sensitivity passing through the centers of the coils (a circle of unit radius centered at the point (0,0)). In practice, this feature allows you to determine the distance to small objects. If it turns out that at a certain finite distance the reflected signal vanishes (with optimal roll orientation), it means that the distance to the object is half the base of the device, that is, the value L/2.

It should also be noted that the directional patterns along the roll angle y for metal detector sensors with different relative positions of the coils also differ. Fig. 14b shows the radiation pattern of the device with perpendicular axes at the coils, and Fig. 14a - with crossed axes. Obviously, the second diagram is more preferable, as it has fewer roll dead zones and fewer lobes.

In order to evaluate the dependence of the voltage induced in the receiving coil on the parameters of the metal detector and the object, it is necessary to analyze expression (1.19) for the coefficient K 4. The voltage induced in the receiving coil is proportional to (L/2)^6. The arguments of the functions F and G are also normalized to the value L/2, decreasing with the 6th - 7th degree of distance. Therefore, to a first approximation, other things being equal, the sensitivity of a metal detector does not depend on its base.

Directional patterns for roll sensors of coil systems:
- with crossing axes (a)
— with perpendicular axes (b).

In order to analyze selectivity metal detector, that is, its ability to distinguish objects made of different metals or alloys, it is necessary to refer to expression (1.23). The metal detector can distinguish objects by the phase of the reflected signal. In order for the resolution of the device to be

talls was maximum, it is necessary to select the signal frequency of the emitting coil accordingly, so that the phase of the signal reflected from the objects is about 45°. This is the middle of the range of possible changes in the phase of the first term of expression (1.23), and there the slope of the phase-frequency characteristic is maximum. We consider the second term of expression (1.23) to be zero, since when searching, we are primarily interested in selectivity for non-ferromagnetic metals. Naturally, the optimal choice of signal frequency implies knowledge of the typical size of the intended objects. Almost all foreign industrial metal detectors use the coin size as this size. The optimal frequency is:

With a typical coin diameter of 25 (mm), its volume is about 10^(-6) (m^3), which according to formula (1.25) corresponds to an equivalent radius of about 0.6 (cm). From here we obtain an optimal frequency value of about 1 (kHz) with a conductivity of the coin material of 20 (n0m H m). In industrial devices, the frequency is usually an order of magnitude higher (for technological reasons).

conclusions

1. According to the author, a system of coils with perpendicular axes is preferable for searching for treasures and relics than a system of coils with crossing axes. All other things being equal, the first system has a slightly higher sensitivity. In addition, with its help it is much easier to determine (“direction finding”) the exact direction in which to look for a detected object.

2. The considered coil systems have an important property that allows one to estimate the distance to small objects by the nulling of the reflected signal at a distance to the object equal to half the base.

3. Other things being equal (dimensions and number of coil turns, sensitivity of the receiving path, current magnitude and frequency in the emitting coil), the sensitivity of the metal detector according to the “transmission-reception” principle practically does not depend on its base, that is, on the distance between the coils.

A metal detector is used to search for various types of metal. But few people know how it works. Let's figure out what principles underlie the operation of a metal detector, how it differs from a metal detector, and what types of metal detectors are known.

Metal detector and metal detector: is there a difference?

Strictly speaking, both of these concepts mean the same thing. Often, they are used as synonyms. True, in the minds of the speaker and listener, when the word “metal detector” is uttered, a picture of a person looking for treasure in the forest with a long tool with a sensor at the end more often appears. And in the case of a “metal detector,” one immediately imagines magnetic frames at the airport and people with special hand-held sensors that react to metal. As you can see, for the average person the difference is only in presentation.

If we turn to the origins, it will be clear that a metal detector is simply the Russian equivalent of the English term “metal detector”, and “metal detector”, in this case, is just a transliterated translation.

However, in the professional environment of Russian-speaking people who often use these devices, there is an idea of ​​a clear difference between them. A metal detector is an inexpensive device that can only detect the presence or absence of metal in a certain environment. Accordingly, a metal detector is a device with a similar purpose, but its advantage is that with its help it is additionally possible to determine the type of metal object. The price of such a tool is several orders of magnitude higher. The purposes of these devices are the same, but the nature of their implementation is different. Therefore, the question “what is the difference between a metal detector and a metal detector” can be answered with full confidence that this difference lies in the area of ​​additional functionality, while leaving unchanged the goals and objectives related to such technology.

But for convenience, we will adhere to a point of view that is understandable to everyone. Let us denote a device used for searching in the ground or under water by the term “metal detector”, and “metal detectors” will mean hand-held inspection and special arched devices used in the work of various security services.

How does a metal detector work?

It is quite difficult to answer this question unequivocally. There are many different options for the design of this device. And it can be difficult for a potential buyer to find “the one” among all the variety.

The most common is an electronic device operating at certain frequencies, capable of detecting metal objects according to specified parameters in a so-called neutral or weakly conducting environment. It is clear that it reacts to the conductivity of the materials from which objects are made. A device of this design is called pulsed. This is when the signals emitted by the device and reflected by the object are transmitted after a few fractions of seconds. They are the ones that are recorded by technology. The principle of operation of a pulse metal detector can be briefly described as follows: pulses from the current generator, as a rule, enter the emitting coil in milliseconds, where they are transformed into magnetic induction pulses. Sharp voltage surges are formed on the pulse components of the generator. They are reflected in the receiving coil (in more complex types of devices, one coil has the ability to perform both functions) at certain intervals. Then the signals arrive via a communication channel to the processing unit and are displayed in clear symbols for subsequent human perception.

But you need to be careful, because this popular type of technology has a number of disadvantages:

1. Difficulty in differentiating detected objects by metal type;
2. Large voltage amplitude;
3. Technical complexity of switching and generation;
4. Presence of radio interference.

Other types of metal detectors based on operating principle

Such devices consist of most well-known models. Some of them have already been discontinued, but are still used in practice.

1. BFO (Beat Frequency Oscillation). It is based on counting and recording the difference in oscillation frequency. Depending on the type of metal (ferrous or non-ferrous), the frequency either increases or decreases. Such devices are no longer produced; they are outdated. But previously produced models still work. The characteristics of such a metal detector leave much to be desired. It has a small detection depth, a strong dependence of search results on the type of soil (ineffective on acidic, mineralized soils), and low sensitivity.
2. TR (Transmitter Receiver). Equipment of the “receive-transmit” type. Also applies to obsolete. The problems are the same as the previous type (does not work on mineralized soils) with the exception of detection depth. She is quite big.
3. VLF (Very Low Frequency). Often such a device combines two operating schemes: “reception-transmission” and low-frequency research. During operation, the device analyzes the signal in phases. Its advantages are high sensitivity and the ability to search for ferrous and non-ferrous metals at depth. But objects lying near the surface are much more difficult for him to detect.
4. PI (Pulse Induction). It is based on the process of induction. The principle of operation of the metal detector is contained in the coil. She is the heart of the sensor. The appearance of extraneous currents from metal objects inside the electromagnetic field activates the reflected impulse. It reaches the coil in the form of an electrical signal. At the same time, the device clearly perceives mineralized and salty soil with metals. Currents from salts reach the sensor much faster and are not displayed graphically or audibly. This metal detector is considered the most sensitive of all. For conducting searches on the seabed, this is the most effective device option.
5. RF (Radio Frequency / RF two-box). It is a “receive-transmit” device, only operating at high frequencies. It has two coils (a receiving coil and, accordingly, a transmitting coil). The operation of this metal detector is based on a violation of the inductive balance: the receiving coil detects a signal that is reflected from the object. This signal was originally sent by the transmission coil. The characteristics of such a metal detector make it possible to use it to search for shallow deposits of ores, minerals at great depths, or to detect large objects. It has no equal in penetration depth (from 1 to 9 meters depending on the type of soil). Often used in industry. Diggers and treasure hunters do not ignore it. A significant disadvantage of such a device is its inability to detect small objects such as coins.

The principle of operation of a metal detector for searching non-ferrous metalsnot particularly different from the others. It also depends on the type and design of the device. If configured correctly, non-ferrous metal can be detected. The only difference between it and black is that eddy currents reflected from an object made of non-ferrous metal take longer to die out.

How else are metal detectors different?

In addition to the internal “filling”, there are other differences between metal detectors. Firstly, they are presented in different price categories. There are devices that are cheaper and more widespread, and there are also those that can be classified as premium.

Also, already in the description of metal detectors, the difference in the display of information for user access is visible. The devices can be programmed to display graphic information (displayed on a special display), sound devices that report the detection or absence of an object (they differ in that they emit different frequencies). More expensive models may feature displays with entire scales of discriminatory values.

The information itself is also different. For example, the most inexpensive models simply tell the user whether there is metal or not. Slightly more expensive devices determine what kind of metal it is - ferrous or non-ferrous. The most expensive models can provide complete information: information about the depth of the object, the probability ratio as a percentage relative to the metal, the type of object.

All types of metal detectors

The devices vary in:principle of operation, tasks performed, elements used. The principles have already been written above, so let’s see what they are by task:

1. Deep;

2. Ground;

3. Magnetometer;

4. Mine detector.

The elements can be microprocessor and analog.

About the characteristics

Different devices are characterized by variability of parameters.

The principle of operation of the metal detectorand its operating frequency are classification parameters. Determine the type of device, for example, professional or ground. Sensitivity determines depth. Target designation allows you to adjust the device to a given target size. The metal type is calculated by the discriminator. Weight, everything is simple here: a heavy device is inconvenient to use for a long time. The soil type is indicated when balancing soil parameters.

Working with a metal detector. Peculiarities

You need to first study your device and its weak points. You should not chase the latest models. If the user does not have basic skills and understanding of how the device works, then even the most sophisticated metal detector will not help him.

Each price category has its leaders. They should be chosen, since these are models that have been tested by generations of treasure hunters. The ability to operate the device can only be achieved through practice. By trying over and over again, a person begins to correctly decipher the signals that the technology gives him. And the main question depends on the correct decoding: to dig or not to dig?

For example, knowing what elements are installed inside your metal detector, you can understand exactly how to operate the metal detector. If it is a mono coil, then its electromagnetic radiation appears cone-shaped. Consequently, there are blind spots when searching. To eliminate them, you need to ensure that each passage with the device overlaps the previous one by 50%. Knowing such little things, you can use the metal detector most effectively.

Working with a metal detectorimplies obtaining a certain result. To do this, it is necessary that the metal detector meets some simple but absolutely necessary requirements:

1. The principle of operation of the metal detectorshould allow him to feel metal objects at maximum depth;
2. There must be a division into ferrous and non-ferrous metals;
3. The device must have an operating processor installed to ensure fast operation. This is important for recognizing two nearby objects.

How to work with a metal detector correctly?You need to start by setting up the device. As a rule, if we want to find a specific object, then the settings need to be set accordingly. But there are 2 general rules, the observance of which will definitely be useful for beginners.

1. Reduce the threshold value for the sensitivity parameter. Since increasing this indicator often leads to increased interference, it is better for beginners to sacrifice the device’s ability to detect objects lying nearby in order to more accurately localize a single target.
2. Use the “all metals” discrimination parameter.

This was just some general information on how to properly use a metal detector. Let's look at this in more detail. The most important thing is to never rush! The search area is divided into zones and sections. Each of them should be passed slowly and carefully. The catcher must be kept as close to the ground as possible; Operation of the metal detector should be smooth, without jerking. Carefully move the device from side to side. If metal is detected in the ground, then, as a rule, you will hear a sound signal: clear - evidence of the detection of a small object of the correct shape, fuzzy, intermittent - the shape of the detected object is incorrect. Learning to determine the size of a find and its depth by sound can only be done experimentally. The type of metal found is classified according to a scale (the device reflects an electrical impulse, and the processor, based on this data, calculates the density of the material from which the object is made).

There are two modes: dynamic (main) and static, they affect how to properly operate a metal detector. Static is the independent movement of the coil over the object; used to accurately determine the center of a target. The exploration of the territory occurs according to a certain scheme:

1. The coil should be parallel to the ground;
2. It is important to maintain a constant distance between the ground and the coil;
3. Take small steps. Don't skip sections!
4. The speed of movement should be about half a meter per second;
5. The height of the device above the ground is 3 or 4 cm.

Searches are carried out in dynamic mode. When a stable signal is detected, switch the device to static mode: move it in a cross-shaped motion over the intended location; where the signal gets maximum volume and dig. Switch the metal detector back to dynamic mode. Dig down half a bayonet, cutting off an even square or round lump. If the object is still in the hole, dig further. It is better to extract the find from the turf using the halving method. After completing your search, be sure to put the sod back in the hole! Now you know exactly how to use a metal detector.

A little about metal detectors

Operating principles of metal detectorsabsolutely the same as for metal detectors, the differences are only in the environments of use and the power of the coil. Because of this, the effectiveness of metal detectors is less; they would not be able to detect anything in the ground. The main types of metal detectors are: manual inspection (detection range up to 25 meters) and arched (frame).

To briefly describe how a hand-held metal detector works, you can do this: the device is absolutely ready for operation when turned on, no configuration is required, when metal is detected, a direct current pulse is recorded, sound and indication are turned on.

The principle of operation of metal detectors of this type is based on the influence of the alternating magnetic field of a transmitting coil on the object under study and the registration of the signal that appears as a result of the induction of eddy currents in the target. Thus, they belong to location-type devices and must have at least 2 coils - transmitting and receiving.

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

A fundamental point for metal detectors of this type is the choice of coil location. They must be located so that, in the absence of foreign metal objects, the magnetic field of the emitting coil induces a zero signal in the receiving coil.

The coils that create radiation or receive a signal are made in the form of a structure called a search frame. The parallel arrangement of the coils is called coplanar.

Typically, in metal detectors of this type, the search frame is formed by 2 coils, located in the same plane and balanced so that when a signal is applied to the previous coil, the output of the receiving coil is minimal. The operating frequency of radiation is from one to several tens of kHz.

Metal detectors on beats

Beating is a phenomenon that occurs when two periodic signals with similar frequencies and amplitudes are multiplied. The resulting signal will ripple with a frequency equal to the frequency difference. If a low frequency signal is applied to the speaker, we will hear a characteristic “gurgling” sound.

The metal detector contains two generators: reference and measuring. The first has a stable frequency, while the second can change frequency when approaching a metal object. Its sensitive element is an inductance coil made in the form of a search frame.

Signals from the generators are sent to a detector, at the output of which an alternating voltage is released with a frequency equal to the difference between the frequencies of the reference and measuring generators. Next, this signal increases in amplitude and is sent to a light and sound indicator.

The presence of metal near the measuring frame leads to a change in the parameters of the surrounding magnetic field and to a change in the frequency of the corresponding generator. A frequency difference arises, which is isolated and used to generate a signal.

The greater the mass of the metal and the closer the metal object, the more the frequencies of the generators differ and the higher the frequency of the generator output voltage.

Can be considered as some modification of beat-based metal detectors metal detectors - frequency meters . They only have a measuring generator. When the metal detector's measuring frame approaches a metal object, the frequency of the generator changes. Then the length of the period in the absence of metal is subtracted from it.

Single-coil induction metal detectors

This metal detector has one coil, which is both emitting and receiving.

An electromagnetic field is created around the coil, which, upon reaching a metal object, creates eddy currents in it, which cause changes in the magnetic induction of the field around the coil.

The currents arising in the object change the magnitude of the magnetic induction of the electromagnetic field around the coil. The compensating device maintains a constant current through the coil. Therefore, when the inductance changes, the indicator will work.

Pulse metal detectors

A pulse metal detector consists of a current pulse generator, receiving and emitting coils, a switching device and a signal processing unit. Based on the principle of operation, it is a location-type metal detector.

Using a switching unit, the current generator periodically generates short current pulses that enter the emitting coil, which creates pulses of electromagnetic radiation. When this radiation is exposed to a metal object, a damped current pulse appears in the latter and persists for some time. This current creates radiation from the metal object, which induces current in the coil of the measuring frame. Based on the magnitude of the induced signal, one can judge the presence or absence of conductive objects near the measuring frame.

The main problem with this type of metal detector is to separate the weak secondary radiation from the much more powerful radiation.

Most pulse-type metal detectors have a low repetition rate of current pulses supplied to the emitting coil.

Magnetometers

For magnetically sensitive metal detectors, sensitivity is usually denoted by the magnitude of the magnetic field induction that the device is capable of registering. Sensitivity is usually measured in nanoteslas.

In addition to sensitivity, to determine the qualities of a magnetometer, resolution is used, which determines the minimum difference in induction.

Devices whose operating principle is based on the use of nonlinear properties of ferromagnetic materials have become widespread.

Sensitive elements that implement this principle are called fluxgates .

A typical magnetometer design includes a rod with a battery power supply and an electronic unit placed on it, as well as a fluxgate converter on an axis perpendicular to the rod.

Before use, the device is pre-calibrated to compensate for the effects of the Earth's field in the absence of ferromagnetic test objects.

There are magnetometers that operate on other physical principles. Thus, quantum devices are known based on the effect of nuclear magnetic resonance and the Zeeman effect, with optical pumping. They have great sensitivity.

Handheld metal detectors

They are not large in size and weight. During the search process, they manually move along the control object.

The ability of an object to perceive metal objects is determined by its sensitivity. Hand-held metal detectors can detect an object the size of a small coin from a distance of 5-10 to several tens of centimeters.

Sensitivity depends on the orientation of the metal detector frame relative to the test object. It is recommended to carry out a search frame along the test object several times at different angles.

Examples of hand-held metal detectors:

selective metal detector AKA 7215 :

The alarm tone depends on the type of metal detected

Has a potentiometer for smooth sensitivity adjustment, as well as a switch - ferrous and non-ferrous metals

Continuous operating time from a fresh 9V battery – at least 40 hours

Weight 280 g.

Handheld metal detector GARRETT:

There is a switch to reduce sensitivity

Automatic monitoring of battery level

Alarm indication – sound and LED

Shockproof housing

Headphone/battery jack

Meets hygiene certificates

Continuous operation time - up to 80 hours

Developments in recent years have been characterized by an increase in the “electronic complexity” of devices. They are equipped with microprocessors, displays, etc. All this allows you to expand the functionality of the devices.

The displays show information about the detected object and its conductivity.

Metal detectors are often needed, for example, when searching for lost metal objects or pipes, cables, tanks buried underground. Metal detectors are also associated with treasure hunters and miners :)

Types of metal detectors

The most complex and sensitive, but also the most expensive, are built on the principle radio signal transmission/reception. The complexity and high cost lies not only in the abundance of electronic components of the circuit, but also in the need for qualified configuration of the circuits.

There are several more types based on different principles: induction, frequency meters, pulse, generation attenuation, beat method, pulse induction, resonance disruption...

The meaning of all metal detectors is one: change in generator frequency when a metal object enters the field of the coil. This change in frequency is usually very insignificant, and the second essence of this or that circuit is to catch this slightest change and convert it into something.

The diagram of a simple metal detector is presented below.

By making such a metal detector compact and taking it with you on a trip to the sea, it will help you when searching for gold jewelry lost by you or your relatives on the beach. But what is closer to you is searching for hidden wiring in the wall or some kind of stud. We will look at such a simple and proven metal detector circuit for similar purposes here so that we can assemble it with our own hands.

Circuit of a simple metal detector using transistors

The circuit diagram of this simple metal detector can be repeated by an amateur without much experience.

Metal detector characteristics:

• Coin detection - 10-15 cm (with good adjustment, some grab it up to 50 cm!);
• Steel scissors - 20-25 cm;
• Large objects - 1-1.5 meters.

The circuit consists of two high-frequency generators, each with one transistor (VT1 and VT2). The frequency of the left generator (VT1) changes when metal enters the L1 field, and the frequency of the right one (VT2) remains unchanged. The values ​​of the elements of both generators are selected so that the frequencies of the generators differ only slightly. The generators operate at a radio frequency (more than 100 kHz), and such a sound is neither audible to our ears nor reproduced by a speaker. But their small difference, for example, 160 kHz and 161 kHz is equal to 1 kHz - these are vibrations already audible to the ear. And both generator coils (L1, L2) are inductively coupled (located close), so both signals from the generators with a difference of 1 kHz are combined and we hear the so-calledamplitude beats frequency 1 kHz.

Setting up a metal detector

BEST METAL DETECTOR

Why was Volksturm named the best metal detector? The main thing is that the scheme is really simple and really working. Of the many metal detector circuits that I have personally made, this is the one where everything is simple, thorough and reliable! Moreover, despite its simplicity, the metal detector has a good discrimination scheme - determining whether iron or non-ferrous metal is in the ground. Assembling the metal detector consists of error-free soldering of the board and setting the coils to resonance and to zero at the output of the input stage on the LF353. There is nothing super complicated here, all you need is desire and brains. Let's look at the constructive metal detector design and a new improved Volksturm diagram with description.

Since questions arise during the assembly process, in order to save you time and not force you to flip through hundreds of forum pages, here are the answers to the 10 most popular questions. The article is in the process of being written, so some points will be added later.

1. The operating principle and target detection of this metal detector?
2. How to check if the metal detector board is working?
3. Which resonance should I choose?
4. Which capacitors are better?
5. How to adjust resonance?
6. How to reset the coils to zero?
7. Which wire is better for coils?
8. What parts can be replaced and with what?
9. What determines the depth of target search?
10. Volksturm metal detector power supply?

How the Volksturm metal detector works

I will try to briefly describe the principle of operation: transmission, reception and induction balance. In the search sensor of the metal detector, 2 coils are installed - transmitting and receiving. The presence of metal changes the inductive coupling between them (including the phase), which affects the received signal, which is then processed by the display unit. Between the first and second microcircuits there is a switch controlled by pulses of a generator phase-shifted relative to the transmitting channel (i.e. when the transmitter is working, the receiver is turned off and vice versa, if the receiver is turned on, the transmitter is resting, and the receiver calmly catches the reflected signal in this pause). So, you turned on the metal detector and it beeps. Great, if it beeps, it means many nodes are working. Let's figure out why exactly it beeps. The generator on the u6B constantly generates a tone signal. Next, it goes to an amplifier with two transistors, but the amplifier will not open (it will not let a tone pass) until the voltage at the output u2B (7th pin) allows it to do so. This voltage is set by changing the mode using this same thrash resistor. They need to set the voltage so that the amplifier almost opens and passes the signal from the generator. And the input couple of millivolts from the metal detector coil, having passed through the amplification stages, will exceed this threshold and it will finally open and the speaker will beep. Now let's trace the passage of the signal, or rather the response signal. At the first stage (1-у1а) there will be a couple of millivolts, up to 50. At the second stage (7-у1B) this deviation will increase, at the third (1-у2А) there will already be a couple of volts. But there is no response everywhere at the outputs.

How to check if the metal detector board is working

In general, the amplifier and switch (CD 4066) are checked with a finger at the RX input contact at maximum sensor resistance and maximum background on the speaker. If there is a change in the background when you press your finger for a second, then the key and opamps work, then we connect the RX coils with the circuit capacitor in parallel, the capacitor on the TX coil in series, put one coil on top of the other and begin to reduce to 0 according to the minimum reading of the alternating current on the first leg of the amplifier U1A. Next, we take something large and iron and check whether there is a reaction to metal in the dynamics or not. Let's check the voltage at y2B (7th pin), it should change with a thrash regulator + a couple of volts. If not, the problem is in this op-amp stage. To start checking the board, turn off the coils and turn on the power.

1. There should be a sound when the sense regulator is set to maximum resistance, touch the RX with your finger - if there is a reaction, all op-amps work, if not, check with your finger starting from u2 and change (inspect the wiring) of the non-working op-amp.

2. The operation of the generator is checked by the frequency meter program. Solder the headphone plug to pin 12 of the CD4013 (561TM2), carefully removing p23 (so as not to burn the sound card). Use In-lane on the sound card. We look at the generation frequency and its stability at 8192 Hz. If it is strongly shifted, then it is necessary to unsolder the capacitor c9, if even after it is not clearly identified and/or there are many frequency bursts nearby, we replace the quartz.

3. Checked the amplifiers and generator. If everything is in order, but still does not work, change the key (CD 4066).

Which coil resonance to choose?

When connecting the coil into series resonance, the current in the coil and the overall consumption of the circuit increases. The target detection distance increases, but this is only on the table. On real ground, the ground will be felt the more strongly, the greater the pump current in the coil. It is better to turn on parallel resonance, and increase the sense of input stages. And the batteries will last much longer. Despite the fact that sequential resonance is used in all branded expensive metal detectors, in Sturm it is parallel that is needed. In imported, expensive devices, there is a good detuning circuitry from the ground, so in these devices it is possible to allow sequential.

Which capacitors are best installed in the circuit? metal detector

The type of capacitor connected to the coil has nothing to do with it, but if you experimentally changed two and saw that with one of them the resonance is better, then simply one of the supposedly 0.1 μF actually has 0.098 μF, and the other 0.11. This is the difference between them in terms of resonance. I used Soviet K73-17 and green imported pillows.

How to adjust coil resonance metal detector

The coil, as the best option, is made from plaster floats, glued with epoxy resin from the ends to the size you need. Moreover, its central part contains a piece of the handle of this very grater, which is processed down to one wide ear. On the bar, on the contrary, there is a fork with two mounting ears. This solution allows us to solve the problem of coil deformation when tightening the plastic bolt. The grooves for the windings are made with a regular burner, then zero is set and filled. From the cold end of the TX, leave 50 cm of wire, which should not be filled initially, but make a small coil from it (3 cm in diameter) and place it inside the RX, moving and deforming it within small limits, you can achieve an exact zero, but do this It’s better outside, placing the coil near the ground (as when searching) with GEB turned off, if any, then finally fill it with resin. Then the detuning from the ground works more or less tolerably (with the exception of highly mineralized soil). Such a reel turns out to be light, durable, little subject to thermal deformation, and when processed and painted it is very attractive. And one more observation: if the metal detector is assembled with ground detuning (GEB) and with the resistor slider located centrally, set zero with a very small washer, the GEB adjustment range is + - 80-100 mV. If you set zero with a large object - a coin of 10-50 kopecks. the adjustment range increases to +- 500-600 mV. Do not chase the voltage when setting up the resonance - with a 12V supply, I have about 40V with a series resonance. To make discrimination appear, we connect the capacitors in the coils in parallel (series connection is only necessary at the stage of selecting capacitors for resonance) - for ferrous metals there will be a drawn-out sound, for non-ferrous metals - a short one.

Or even simpler. We connect the coils one by one to the transmitting TX output. We tune one into resonance, and after tuning it, the other. Step by step: Connected, poked a multimeter in parallel with the coil with a multimeter at the alternating volts limit, also soldered a 0.07-0.08 uF capacitor parallel to the coil, look at the readings. Let's say 4 V - very weak, not in resonance with the frequency. We poked a second small capacitor in parallel with the first capacitor - 0.01 microfarads (0.07+0.01=0.08). Let's look - the voltmeter has already shown 7 V. Great, let's increase the capacitance further, connect it to 0.02 µF - look at the voltmeter, and there is 20 V. Great, let's move on - we'll add a couple thousand more peak capacitance. Yeah. It has already started to fall, let's roll back. And so achieve maximum voltmeter readings on the metal detector coil. Then do the same with the other (receiving) coil. Adjust to maximum and connect back to the receiving socket.

How to zero metal detector coils

To adjust the zero, we connect the tester to the first leg of the LF353 and gradually begin to compress and stretch the coil. After filling with epoxy, the zero will definitely run away. Therefore, it is necessary not to fill the entire coil, but to leave places for adjustment, and after drying, bring it to zero and fill it completely. Take a piece of twine and tie half of the spool with one turn to the middle (to the central part, the junction of the two spools), insert a piece of stick into the loop of the twine and then twist it (pull the twine) - the spool will shrink, catching the zero, soak the twine in glue, after almost complete drying adjust the zero again by turning the stick a little more and fill the twine completely. Or simpler: The transmitting one is fixed in plastic, and the receiving one is placed 1 cm over the first one, like wedding rings. There will be an 8 kHz squeak at the first pin of U1A - you can monitor it with an AC voltmeter, but it’s better to just use high-impedance headphones. So, the receiving coil of the metal detector must be moved or shifted from the transmitting coil until the squeak at the output of the op-amp subsides to a minimum (or the voltmeter readings drop to several millivolts). That's it, the coil is closed, we fix it.

Which wire is better for search coils?

The wire for winding the coils does not matter. Anything from 0.3 to 0.8 will do; you still have to slightly select the capacitance to tune the circuits to resonance and at a frequency of 8.192 kHz. Of course, a thinner wire is quite suitable, it’s just that the thicker it is, the better the quality factor and, as a result, the instinct. But if you wind it 1 mm, it will be quite heavy to carry. On a sheet of paper, draw a rectangle 15 by 23 cm. From the upper and lower left corners, set aside 2.5 cm and connect them with a line. We do the same with the upper right and lower corners, but set aside 3 cm each. We put a dot in the middle of the lower part and a point on the left and right at a distance of 1 cm. We take plywood, apply this sketch and drive nails into all the points indicated. We take a PEV 0.3 wire and wind 80 turns of wire. But honestly, it doesn’t matter how many turns. Anyway, we will set the frequency of 8 kHz to resonance with a capacitor. As much as they reeled in, that's how much they reeled in. I wound 80 turns and a capacitor of 0.1 microfarads, if you wind it, say 50, you will have to put a capacitance of about 0.13 microfarads. Next, without removing it from the template, we wrap the coil with a thick thread - like how wire harnesses are wrapped. Afterwards we coat the coil with varnish. When dry, remove the spool from the template. Then the coil is wrapped with insulation - fum tape or electrical tape. Next - winding the receiving coil with foil, you can take a tape from electrolytic capacitors. The TX coil does not need to be shielded. Remember to leave a 10mm GAP in the screen, down the middle of the reel. Next comes winding the foil with tinned wire. This wire, together with the initial contact of the coil, will be our ground. And finally, wrap the coil with electrical tape. The inductance of the coils is about 3.5mH. The capacitance turns out to be about 0.1 microfarads. As for filling the coil with epoxy, I didn’t fill it at all. I just wrapped it tightly with electrical tape. And nothing, I spent two seasons with this metal detector without changing the settings. Pay attention to the moisture insulation of the circuit and search coils, because you will have to mow on wet grass. Everything must be sealed - otherwise moisture will get in and the setting will float. Sensitivity will worsen.

What parts can be replaced and with what?

Transistors:
BC546 - 3 pcs or KT315.
BC556 - 1 piece or KT361
Operators:

LF353 - 1 piece or exchange for the more common TL072.
LM358N - 2pcs
Digital chips:
CD4011 - 1 piece
CD4066 - 1 piece
CD4013 - 1 piece
Resistors are constant, power 0.125-0.25 W:
5.6K - 1 piece
430K - 1 piece
22K - 3pcs
10K - 1 piece
390K - 1 piece
1K - 2pcs
1.5K - 1 piece
100K - 8pcs
220K - 1 piece
130K - 2 pieces
56K - 1 piece
8.2K ​​- 1 piece
Variable resistors:
100K - 1 piece
330K - 1 piece
Non-polar capacitors:
1nF - 1 piece
22nF - 3pcs (22000pF = 22nF = 0.022uF)
220nF - 1 piece
1uF - 2pcs
47nF - 1 piece
10nF - 1 piece
Electrolytic capacitors:
220uF at 16V - 2 pcs

The speaker is miniature.
Quartz resonator at 32768 Hz.
Two ultra-bright LEDs of different colors.

If you cannot get imported microcircuits, here are domestic analogues: CD 4066 - K561KT3, CD4013 - 561TM2, CD4011 - 561LA7, LM358N - KR1040UD1. The LF353 microcircuit has no direct analogue, but feel free to install LM358N or better TL072, TL062. It is not at all necessary to install an operational amplifier - LF353, I simply increased the gain to U1A by replacing the resistor in the negative feedback circuit of 390 kOhm with 1 mOhm - the sensitivity increased significantly by 50 percent, although after this replacement the zero went away, I had to glue it to the coil in a certain place tape a piece of aluminum plate. Soviet three kopecks can be sensed through the air at a distance of 25 centimeters, and this is with a 6-volt power supply, the current consumption without indication is 10 mA. And don’t forget about the sockets - the convenience and ease of setup will increase significantly. Transistors KT814, Kt815 - in the transmitting part of the metal detector, KT315 in the ULF. It is advisable to select transistors 816 and 817 with the same gain. Replaceable with any corresponding structure and power. The metal detector generator has a special clock quartz at a frequency of 32768 Hz. This is the standard for absolutely all quartz resonators found in any electronic and electromechanical watches. Including wrist and cheap Chinese wall/table ones. Archives with a printed circuit board for the variant and for (variant with manual detuning from the ground).

What determines the depth of target search?

The larger the diameter of the metal detector coil, the deeper the instinct. In general, the depth of target detection by a given coil depends primarily on the size of the target itself. But as the diameter of the coil increases, there is a decrease in the accuracy of object detection and sometimes even the loss of small targets. For objects the size of a coin, this effect is observed when the coil size increases above 40 cm. Overall: a large search coil has a greater detection depth and greater capture, but detects the target less accurately than a small one. The large coil is ideal for searching for deep and large targets such as treasure and large objects.

According to their shape, coils are divided into round and elliptical (rectangular). An elliptical metal detector coil has better selectivity compared to a round one, because the width of its magnetic field is smaller and fewer foreign objects fall into its field of action. But the round one has a greater detection depth and better sensitivity to the target. Especially on weakly mineralized soils. The round coil is most often used when searching with a metal detector.

Coils with a diameter of less than 15 cm are called small, coils with a diameter of 15-30 cm are called medium, and coils over 30 cm are called large. A large coil generates a larger electromagnetic field, so it has a greater detection depth than a small one. Large coils generate a large electromagnetic field and, accordingly, have greater detection depth and search coverage. Such coils are used to view large areas, but when using them, a problem may arise in heavily littered areas because several targets may be caught in the field of action of large coils at once and the metal detector will react to a larger target.

The electromagnetic field of a small search coil is also small, so with such a coil it is best to search in areas heavily littered with all sorts of small metal objects. The small coil is ideal for detecting small objects, but has a small coverage area and a relatively shallow detection depth.

For universal searching, medium coils are well suited. This search coil size combines sufficient search depth and sensitivity to targets of different sizes. I made each coil with a diameter of approximately 16 cm and placed both of these coils in a round stand from under an old 15" monitor. In this version, the search depth of this metal detector will be as follows: aluminum plate 50x70 mm - 60 cm, nut M5-5 cm, coin - 30 cm, bucket - about a meter. These values ​​​​were obtained in the air, in the ground it will be 30% less.

Metal detector power supply

Separately, the metal detector circuit draws 15-20 mA, with the coil connected + 30-40 mA, totaling up to 60 mA. Of course, depending on the type of speaker and LEDs used, this value may vary. The simplest case is that the power was taken from 3 (or even two) lithium-ion batteries connected in series from a 3.7V mobile phone and when charging discharged batteries, when we connect any 12-13V power supply, the charging current starts from 0.8A and drops to 50mA per an hour and then you don’t need to add anything at all, although a limiting resistor certainly wouldn’t hurt. In general, the simplest option is a 9V crown. But keep in mind that the metal detector will eat it in 2 hours. But for customization, this power option is just right. Under any circumstances, the crown will not produce a large current that could burn something on the board.

Homemade metal detector

And now a description of the process of assembling a metal detector from one of the visitors. Since the only instrument I have is a multimeter, I downloaded O.L. Zapisnykh’s virtual laboratory from the Internet. I assembled an adapter, a simple generator and ran the oscilloscope at idle. It seems to show some kind of picture. Then I started looking for radio components. Since signets are mostly laid out in the “lay” format, I downloaded “Sprint-Layout50”. I found out what laser-iron technology is for manufacturing printed circuit boards and how to etch them. Etched the board. By this time, all the microcircuits had been found. Whatever I couldn’t find in my shed, I had to buy. I started soldering jumpers, resistors, microcircuit sockets, and quartz from a Chinese alarm clock onto the board. Periodically checking the resistance on the power buses to ensure there are no snot. I decided to start by assembling the digital part of the device, as it would be the easiest. That is, a generator, a divider and a commutator. Collected. I installed a generator chip (K561LA7) and a divider (K561TM2). Used ear chips, torn out from some circuit boards found in a shed. I applied 12V power while monitoring the current consumption using an ammeter, and the 561TM2 became warm. Replaced 561TM2, applied power - zero emotions. I measure the voltage on the generator legs - 12V on legs 1 and 2. I am changing 561LA7. I turn it on - at the output of the divider, on the 13th leg there is generation (I observe it on a virtual oscilloscope)! The picture is really not that great, but in the absence of a normal oscilloscope it will do. But there is nothing on legs 1, 2 and 12. This means the generator is working, you need to change TM2. I installed a third divider chip - there is beauty on all outputs! I came to the conclusion that you need to desolder the microcircuits as carefully as possible! This completes the first step of construction.

Now we set up the metal detector board. The "SENS" sensitivity regulator did not work, I had to throw out the capacitor C3 after that the sensitivity adjustment worked as it should. I didn’t like the sound that appeared in the extreme left position of the “THRESH” regulator - threshold, I got rid of it by replacing resistor R9 with a chain of series-connected 5.6 kOhm resistor + 47.0 μF capacitor (negative terminal of the capacitor on the transistor side). While there is no LF353 microcircuit, I installed the LM358 instead; with it, Soviet three kopecks can be sensed in the air at a distance of 15 centimeters.

I turned on the search coil for transmission as a series oscillatory circuit, and for reception as a parallel oscillatory circuit. I set up the transmitting coil first, connected the assembled sensor structure to the metal detector, an oscilloscope parallel to the coil, and selected capacitors based on the maximum amplitude. After this, I connected the oscilloscope to the receiving coil and selected the capacitors for RX based on the maximum amplitude. Setting the circuits to resonance takes several minutes if you have an oscilloscope. My TX and RX windings each contain 100 turns of wire with a diameter of 0.4. We start mixing on the table, without the body. Just to have two hoops with wires. And to make sure of the functionality and possibility of mixing in general, we will separate the coils from each other by half a meter. Then it will be zero for sure. Then, having overlapped the coils by about 1 cm (like wedding rings), move and push apart. The zero point can be quite accurate and it is not easy to catch it right away. But it is there.

When I raised the gain in the RX path of the MD, it began to work unstably at maximum sensitivity, this was manifested in the fact that after passing over the target and detecting it, a signal was issued, but it continued even after there was no target in front of the search coil, this manifested itself in the form of intermittent and fluctuating sound signals. Using an oscilloscope, the reason for this was discovered: when the speaker is operating and the supply voltage drops slightly, “zero” goes away and the MD circuit goes into a self-oscillating mode, which can only be exited by coarsening the sound signal threshold. This didn’t suit me, so I installed a KR142EN5A + super bright white LED for power supply to raise the voltage at the output of the integrated stabilizer; I didn’t have a stabilizer for a higher voltage. This LED can even be used to illuminate the search coil. I connected the speaker to the stabilizer, after that the MD immediately became very obedient, everything started working as it should. I think the Volksturm is truly the best homemade metal detector!

Recently, this modification scheme was proposed, which would turn the Volksturm S into the Volksturm SS + GEB. Now the device will have a good discriminator as well as metal selectivity and ground detuning; the device is soldered on a separate board and connected instead of capacitors C5 and C4. The revision scheme is also in the archive. Special thanks for the information on assembling and setting up the metal detector to everyone who took part in the discussion and modernization of the circuit; Elektrodych, fez, xxx, slavake, ew2bw, redkii and other fellow radio amateurs especially helped in preparing the material.