Homemade metal detectors: simple and more complex - for gold, ferrous metal, for construction. Metal detectors operating on the principle of pulse induction Block diagram of a pulse metal detector

1.1. Work principles

Metal detector based on the "transmission-reception" principle

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 (emitted) and received (reflected) signals are continuous, they exist simultaneously and coincide in frequency.

The operating principle of transmit-receive metal detectors is to register a signal reflected (or, as they say, re-emitted) by a metal object (target), see, pp. 225-228. The reflected signal arises 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 is 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 an alternating electromotive force (emf) in the receiving coil.

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 transmission of a signal from one coil to another (see Fig. 1, a and b) - coils with perpendicular and crossing axes.

Rice. 1. Options for the relative arrangement of metal detector sensor coils based on the “transmission-reception” principle

A more thorough study of the problem shows that there can be as many different systems of metal detector sensors as desired. But these are more complex systems with more than two coils, electrically connected accordingly. For example, in Fig. 1, c 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. 2, a and b. In the diagram in Fig. 2, and the relative position 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 diagram of Fig. 2, b 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 induced in the other wing of the G8. Various other designs of sensors with coplanar coils are also possible, for example Fig. 2, e.

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

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.

Beat metal detector

The name “beat metal detector” is an echo of the terminology adopted in radio engineering since the days of the first superheterodyne receivers. Beats are a phenomenon that most noticeably manifests itself when two periodic signals with similar frequencies and approximately equal amplitudes are added and consists of a pulsation in the amplitude of the total signal. The ripple frequency is equal to the difference in frequencies of the two added signals. By passing such a pulsating signal through a rectifier (detector), it is possible to isolate the difference frequency signal. Such circuitry has been traditional for a long time, but currently it is no longer used either in radio engineering or in metal detectors. In both cases, amplitude detectors were replaced by synchronous detectors, but the term “on beats” has remained to this day.

The principle of operation of a beat metal detector is very simple and consists in recording the frequency difference from two generators, one of which is stable in frequency, and the other contains a sensor - an inductor in its frequency-setting circuit. The device is adjusted in such a way that, in the absence of metal near the sensor, the frequencies of the two generators coincide or are very close in value. The presence of metal near the sensor leads to a change in its parameters and, as a consequence, to a change in the frequency of the corresponding generator. This change is usually very small, but the change in the frequency difference between the two oscillators is already significant and can be easily recorded.

The frequency difference can be recorded in a variety of ways, from the simplest, when the difference frequency signal is listened to on headphones or through a loudspeaker, to digital methods of frequency measurement. The sensitivity of a metal detector to beats depends, among other things, on the parameters for converting changes in the impedance of the sensor into frequency.

Typically, the conversion consists of obtaining the difference frequency of a stable generator and a generator with a sensor coil in the frequency-setting circuit. Therefore, the higher the frequencies of these generators, the greater the frequency difference will be in response to the appearance of a metal target near the sensor. Registration of small frequency deviations presents a certain difficulty. Thus, by ear you can confidently register a shift in the frequency of the tone signal of at least 10 Hz. Visually, by blinking the LED, you can register a frequency shift of at least 1 Hz. In other ways, it is possible to achieve registration of a smaller frequency difference, however, this registration will require considerable time, which is unacceptable for metal detectors that always operate in real time.

Selectivity for metals at such frequencies, which are very far from optimal, is very weak. In addition, it is almost impossible to determine the phase of the reflected signal from the generator frequency shift. Therefore, the metal detector has no selectivity on beats.

Metal detector based on the principle of an electronic frequency meter

A positive side for practice is the simplicity of the design of the sensor and the electronic part of metal detectors based on beats and on the principle of a frequency meter. Such a device can be very compact. It is convenient to use when something has already been detected by a more sensitive device. If the discovered object is small and located deep enough in the ground, then it can “get lost” and be moved during the excavation. In order not to “look through” the excavation site many times with a bulky, sensitive metal detector, it is advisable to control its progress at the final stage with a compact device of short range, which can be used to more accurately determine the location of the object.

Single Coil Induction Metal Detector

The word “induction” in the name of metal detectors of this type fully reveals the principle of their operation, if you remember the meaning of the word “inductio” (Latin) - guidance. A device of this type contains a sensor of one coil of any convenient shape, excited by an alternating signal. The appearance of a metal object near the sensor causes the appearance of a reflected (re-emitted) signal, which “induces” an additional electrical signal in the coil. All that remains is to highlight this additional signal.

The induction-type metal detector has gained the right to life, mainly due to the main drawback of devices based on the “transmission-reception” principle - the complexity of the sensor design. This complexity leads either to the high cost and complexity of manufacturing the sensor, or to its insufficient mechanical rigidity, which causes false signals to appear when moving and reduces the sensitivity of the device.

Rice. 3. Block diagram of the input unit of an induction metal detector

If you set yourself the goal of eliminating this drawback from devices based on the “transmission-reception” principle by eliminating its very cause, then you can come to an unusual conclusion - the emitting and receiving coils of the metal detector must be combined into one! In fact, in this case there are no very undesirable movements and bends of one coil relative to the other, since there is only one coil and it is both emitting and receiving. The sensor is also extremely simple. The price for these advantages is the need to isolate the useful reflected signal from the background of a much larger excitation signal of the emitting/receiving coil.

The reflected signal can be isolated by subtracting from the electrical signal present in the sensor coil a signal of the same shape, frequency, phase and amplitude as the signal in the coil in the absence of metal nearby. *How this can be implemented in one of the ways is shown in Fig. 3.

The generator produces an alternating voltage of a sinusoidal shape with a constant amplitude and frequency. The voltage-to-current converter (VCT) converts the generator voltage Ur into current Ig, which is supplied to the oscillatory circuit of the sensor. The oscillating circuit consists of a capacitor C and a sensor coil L. Its resonant frequency is equal to the frequency of the generator. The PNT conversion coefficient is selected so that the voltage of the oscillatory circuit id is equal to the generator voltage Ur (in the absence of metal near the sensor). Thus, the adder subtracts two signals of the same amplitude, and the output signal - the result of the subtraction - is equal to zero. When metal appears near the sensor, a reflected signal occurs (in other words, the parameters of the sensor coil change), and this leads to a change in the voltage of the oscillating circuit 11d. A non-zero signal appears at the output.

In Fig. Figure 3 shows only the simplest version of one of the diagrams of the input part of metal detectors of the type under consideration. Instead of a PNT in this circuit, it is in principle possible to use a current-setting resistor. Various bridge circuits can be used to turn on the sensor coil, adders with different transmission coefficients for inverting and non-inverting inputs, partial connection of an oscillating circuit, etc.

In the diagram in Fig. 3 an oscillatory circuit is used as a sensor. This is done for simplicity in order to obtain zero phase shift between the Ur and 11d signals (the circuit is tuned to resonance). You can abandon the oscillatory circuit with the need to fine-tune it for resonance and use only the sensor coil as a PNT load. However, the PNT gain for this case must be complex to correct for the 90° phase shift resulting from the inductive nature of the PNT load.

Pulse metal detector

In the types of electronic metal detectors discussed earlier, the reflected signal is separated from the emitted one either geometrically - due to the relative position of the receiving and emitting coils, or using special compensation circuits. Obviously, there may also be a temporary method for separating the emitted and reflected signals. This method is widely used, for example, in pulse echo and radar. During location, the mechanism of delay of the reflected signal is due to the significant time it takes for the signal to propagate to the object and back.

In relation to metal detectors, such a mechanism may be the phenomenon of self-induction in a conductive object. How to use this in practice? After exposure to a magnetic induction pulse, a damped current pulse appears in a conducting object and is maintained for some time (due to the phenomenon of self-induction), causing a time-delayed reflected signal. It carries useful information and should be registered.

Thus, another scheme for constructing a metal detector can be proposed, fundamentally different from those discussed earlier in the method of signal separation. This type of metal detector is called a pulse detector. It consists of a current pulse generator, receiving and emitting coils, which can be combined into one, a switching device and a signal processing unit.

The current pulse generator generates short current pulses in the millisecond range that enter the emitting coil, where they are converted into magnetic induction pulses. Since the emitting coil - the load of the pulse generator - has a pronounced inductive nature, overloads in the form of voltage surges occur at the generator at the pulse fronts. Such bursts can reach tens to hundreds (!) of volts in amplitude, but the use of protective limiters is unacceptable, since it would lead to a delay in the front of the current pulse and magnetic induction and, ultimately, to complicate the separation of the reflected signal.

The receiving and emitting coils can be positioned relative to each other quite arbitrarily, since the direct penetration of the emitted signal into the receiving coil and the effect of the reflected signal on it are separated in time. In principle, one coil can serve as both a receiving and an emitting coil, but in this case it will be much more difficult to decouple the high-voltage output circuits of the current pulse generator from the sensitive input circuits.

The switching device is designed to perform the above-mentioned separation of the emitted and reflected signals. It blocks the input circuits of the device for a certain time, which is determined by the duration of the current pulse in the emitting coil, the discharge time of the coil and the time during which short responses of the device from massive weakly conductive objects such as soil are possible. After this time, the switching device must ensure the transmission of the signal from the receiving coil to the signal processing unit.

The signal processing unit is designed to convert the input electrical signal into a form convenient for human perception. It can be designed based on solutions used in other types of metal detectors. The disadvantages of pulse metal detectors include the difficulty of implementing in practice the discrimination of objects by type of metal, the complexity of the equipment for generating and switching current and voltage pulses of large amplitude, and the high level of radio interference.

Magnetometers

Magnetometers are a broad group of devices designed to change the parameters of a magnetic field (for example, the module or components of the magnetic induction vector). The use of magnetometers as metal detectors is based on the phenomenon of local distortion of the Earth's natural magnetic field by ferromagnetic materials, such as iron. Having detected with the help of a magnetometer a deviation from the module or direction of the magnetic induction vector of the Earth's field that is usual for a given area, we can confidently say that there is some magnetic inhomogeneity (anomaly) that can be caused by an iron object.

Compared to the previously discussed metal detectors, magnetometers have a much greater detection range of iron objects. It is very impressive to know that using a magnetometer you can register small shoe nails from a shoe at a distance of 1 m, and a car at a distance of 10 m! Such a large detection range is explained by the following. An analogue of the emitted field of conventional metal detectors for magnetometers is the uniform (on the search scale) magnetic field of the Earth. Therefore, the response of the device to an iron object is inversely proportional not to the sixth, but only to the third power of the distance.

The fundamental disadvantage of magnetometers is the inability to detect objects made of non-ferrous metals with their help. In addition, even if we are only interested in iron, the use of magnetometers for searching is difficult - in nature there is a wide variety of natural magnetic anomalies of various scales (individual minerals, mineral deposits, etc.). However, when searching for sunken tanks and ships, such devices are unrivaled!

Radars

It is a well-known fact that with the help of modern radars it is possible to detect an aircraft at a distance of several hundred kilometers. The question arises: does modern electronics really not allow us to create a compact device that allows us to detect objects of interest to us at least at a distance of several meters?9 The answer is a number of publications in which such devices are described.

Typical of them is the use of the achievements of modern microwave microelectronics and computer processing of the received signal. The use of modern high technologies makes it almost impossible to independently manufacture these devices. In addition, their large overall dimensions do not yet allow them to be widely used in field conditions.

The advantages of radars include a fundamentally higher detection range - the reflected signal, in a rough approximation, can be considered to obey the laws of geometric optics and its attenuation is proportional not to the sixth or even the third, but only to the second power of the distance.

The decay time of this electrical pulse depends on the magnitude of the electrical resistance of the coil with the wire. A complete absence of resistance, or, on the contrary, a very high value of it, will cause the impulse to fluctuate. It's like throwing a rubber ball onto a very hard surface and having it bounce off multiple times before finally settling down. With sufficient electrical resistance, the pulse decay time is shortened and the reflected pulse is “smoothed out.” This is similar to throwing a rubber ball at a pillow. A pulse induction detector coil is said to be critically damped when the reflected pulse quickly decays to zero without oscillation. Excessive or insufficient suppression will introduce instability and mask signals from highly conductive metals such as gold and reduce detection depth. When a metal object is close to the search coil, it stores some of the pulse energy, which leads to a delay in the process of attenuation of this pulse to zero. The change in the width of the reflected pulse is measured and signals the presence of a metallic object. In order to isolate the signal of such an object, we must measure that part of the pulse where it decreases to zero (tail). At the input of the coil receiver there is a resistor and a limiting diode circuit, which cut the input pulse voltage to 1 volt so as not to overload the circuit input. The signal at the receiver consists of a pulse from the transmitter and a reflected pulse. Typically the receiver gain is 60 decibels. This means that the area where the reflected signal drops to zero can be increased by a factor of 1000.

Gate circuit.
The amplified signal from the receiver enters a circuit that measures the time the voltage drops to zero. The reflected pulse is converted into a train of pulses. When a metal object approaches the coil, the shape of the transmitter pulse will not change, but the reflected pulse will become slightly longer. Increasing the duration of the pulse tail by just a few millionths of a second (microsecond) is enough to determine the presence of metal under the coil. Pulses (strobes) synchronized with the beginning of the transmitter pulse are superimposed on this reflected pulse, and at the output of the electronic circuit a series of strobes is obtained, the number of which is proportional to the length of the “tail” of the pulse. The most sensitive pulse is located as close as possible to the end of the tail, where the voltage is very close to zero. Typically this is a time domain of about 20 microseconds after the transmitter is turned off and the reflected pulse begins. Unfortunately, this is also an area where the operation of a pulsed induction metal detector becomes unstable. For this reason, most models of pulse induction metal detectors continue to produce gate pulses for another 30-40 microseconds after the reflected pulse has completely decayed.

Integrator.
Next, the gated signal must be converted to DC voltage. This is accomplished by an integrator circuit that averages the sequence of pulses and converts them into a corresponding voltage, which increases when the object is close to the frame and decreases when the object moves away. The voltage is further amplified and drives the audio control circuit.
The period of time during which the integrator collects incoming gates is called the integrator time constant (TI). It determines how quickly the metal detector responds to a metal object. A long PVI (on the order of seconds) has the advantage of reducing noise and simplifying detector tuning, but it also requires very slow movement of the search coil, since the object may be missed if it moves quickly. A short PVI (on the order of tenths of a second) reacts faster to the target, which allows you to move the coil faster, but noise immunity and operational stability deteriorate.

DISCRIMINATION (recognition).
Pulse induction metal detectors are not capable of the same degree of discrimination as VLF devices. By measuring the increasing period of time between the end of the transmitter pulse and the point at which the reflected pulse dissipates to zero (delay time), objects composed of certain metals can be filtered out. In terms of this characteristic, aluminum foil comes first, followed by small nickel coins, buttons and gold. Some coins can be identified from a very long pulse tail, however the iron is NOT identified in this way.
Many attempts have been made to create a pulse induction metal detector capable of detecting iron, but all these attempts have had very limited success. Although iron has a long tail, silver and copper have the same characteristics. Such a long delay has a bad effect on determining the depth. The mineral content of the soil will also lengthen the reflected pulse, changing the point at which an object is detected or rejected. If the integrator time constant is adjusted so that the golden ring is not detected in the air, the same ring can “glow” in soil saturated with salts. Thus, salt-saturated soil changes everything about the dwell time and selectivity of a pulsed induction metal detector.

DEVELOPMENT FROM THE GROUND.
Ground offset is very critical for VLF devices, but not for pulse induction metal detectors. On average, the soil does not store any significant amount of energy from the search coil and usually does not produce any signal itself. The soil will not mask the signal from the object and, on the contrary, soil mineralization slightly lengthens the signal in proportion to the increase in the depth of the object. In relation to MDs with pulse induction, the term “automatic ground balance” is often used; they usually do not react to excessive soil mineralization and do not require external adjustment for different soil types. The exception is one of the most unpleasant soil components - magnetite (Fe3O4), or magnetic iron oxide. It overloads the input coils of VLF type detectors, greatly reducing their sensitivity; pulse induction metal detectors will work, but may show false targets if the coil is brought too close to the ground. You can minimize this detrimental effect by lengthening the delay between the end of the transmitter pulse and the start of gating. By adjusting this time constant, you can tune out interference caused by soil mineralization.

AUTOMATIC AND MANUAL SETUP.
Most pulse induction metal detectors have manual settings. This means that the operator must turn the setting until a clicking or itching sound is heard in the headphones. If the soil in the search area varies from and to neutral sand or from dry soil to seawater, then adjustments are necessary. If you don't do this, you may lose detection depth and miss some objects. Manual tuning is very difficult when using a short integrator time constant (TITC). Therefore, many manually tuned instruments have a long PIR and require the search coil to be moved slowly.
There are no problems with using pulse induction MD for underwater searching because it does not move the search coil quickly. When used in the surf, the coil will be either in the water or under the water, and in such conditions, using devices with manual settings can be very disappointing, since you will have to constantly adjust the response threshold. In this case, some operators immediately set the device just below the response threshold. But this can lead to a decrease in detection depth when soil characteristics change.
Automatic adjustment (SAT-self adjusting Threshold) gives a significant advantage when searching in and over salt water or soil with high salt content. It allows you to use the detector at maximum sensitivity without constant adjustment. This improves operating stability, noise immunity and allows the use of higher gain. Pulse induction MDs do not emit strong negative signals like VLF devices. Therefore, they do not go off scale on pits with minerals. It is necessary to continuously move the coil of a metal detector equipped with an auto-tuning system; if you stop the coil, the setting is lost or the device stops responding.

Audio control.
Pulse induction MD audio signaling circuits fall into two categories: variable frequency and variable volume. Variable-frequency circuits based on a voltage-controlled oscillator are good for recording small objects, since changes in frequency are easier to detect by ear than changes in volume, especially at low volume levels, especially for instruments with manual threshold adjustment. However, the sound of a fire siren quickly becomes tiresome, and some people are unable to distinguish high-pitched tones. One good option is mechanical vibration, which was originally used for underwater vehicles. Such a device produces sounds and vibration, which increases to a buzzing sound when an object is detected. The signals from such a mechanical device are easy to recognize and are not drowned out by the air supply system.
Many people prefer a more traditional audio tone that increases in volume rather than frequency. Such sound control systems work well in devices with fast frame movement, those in devices with automatic adjustment, and they sound similar to devices with VLF.

Conclusions on MD with pulse induction.
These are specialized tools. They are of little use for searching for coins in urban environments, since they cannot filter out iron and ferrocontaining debris. They can be used for archaeological searches in rural areas where there is no iron debris in large quantities, searching for gold nuggets and for searching at maximum depth in extreme conditions, such as sea coasts or places where the ground is highly mineralized. Such metal detectors show excellent results in such conditions and are generally comparable to VLF devices, especially in their ability to tune out such soils and “pierce” them to the maximum depth.

Radio amateurs - national economy 1992.

Creating sufficiently sensitive metal detectors is a rather difficult and thankless task. Radio amateurs periodically take up the challenge and present exhibits for exhibition, but few of them meet the required parameters. Thus, for a long time, metal detectors were designed based on two high-frequency generators tuned to similar frequencies, one of which was stable in frequency (usually stabilized by a quartz resonator), and the other - the working one - was connected to the receiving frame and changed its frequency when approaching metals . The signals of the two generators were summed, the low-frequency beat signal was isolated, and the presence of metal was judged from it. After the emergence of a new element base, instead of reference signal generators, they began to design a metal detector with a voltage-frequency converter, analog-to-digital converters, frequency synthesizers and other possible new products.

Archaeologists and criminologists could be advised to use a different measurement scheme - geophysical. In the area where metal inclusions are searched, a loop of wire with a diameter of 5...25 m or more should be laid out, powered by an autonomous generator with a frequency of 500 Hz (the higher the frequency, the shallower the depth). It is very convenient to use aviation DC-AC voltage converters with a frequency of 400 Hz (umformers). They have sufficient power. You can also use DC-AC converters made with powerful transistors. They can be made at several frequencies, and thereby carry out “frequency probing,” i.e., determine the depth of the suspected metal object. To conduct searches, in addition to the generator, you must have a receiver, which can be a selective amplifier tuned to the frequency (frequencies) of the generator and have a receiving magnetic antenna at the input, also tuned to the frequency (frequencies) of the generator. The idea of ​​this search method is that in the area of ​​influence of the electromagnetic field of a wire loop, any metal bodies of continuous conductivity begin to emit their field, shifted in phase relative to the primary one, ideally by 90°. The receiving frame relative to the primary field is usually oriented so that in the absence of metallic inclusions the signal at the receiver output would be minimal or absent altogether, and in the presence of metallic inclusions it would reach a maximum. By taking measurements at several frequencies, it is possible to determine the approximate depth of the deposits, and using receiving frames differently oriented in space, and the location of objects. The main advantage of this measurement method is that the desired metal object becomes the source of radiation itself.

Equipment of this kind can be used for tracing underground pipes, laying cables, tracing hidden wiring and other purposes. To do this, the generator is connected at one end to the traced metal system, and the other end is grounded (if the search is carried out on the street, in a field) or connected to the pipes of the heating network or water supply (if the tracing is carried out in a building).

The loop induction method was widely presented at the VRV in its application to inductive non-contact methods for turning on household electrical appliances (contactless headphones for listening to radio, television, etc., contactless telephone sets not connected by wires to the telephone network, which can be freely carried in your hands while moving around room). It would seem that the problem is different, but the solution principle is the same: inductive coupling between the loop in which the signal is generated and the receiver that picks up this signal.

Pulse metal detector(Fig. 27). The author of the design is radio amateur V. S. Gorchakov. At the 33rd World Exhibition, the exhibit was awarded the Third Prize of the exhibition.

The device is designed to locate metal objects in the ground. Its tests have shown that it can detect an aluminum plate 100 x 100 x 2 mm at a depth of 75 cm, the same plate measuring 200 x 200 x 2 mm at a depth of 100 cm, a long steel pipe with a diameter of 300 mm at a depth of 200 cm, a sewer manhole well at a depth of 200 cm, a long steel pipe with a diameter of 50 mm at a depth of 120 cm, a copper washer with a diameter of 25 mm at a depth of 35 cm.

The device (Fig. 27, a) consists of a master oscillator 1 at a frequency of 100 Hz, a pulse current amplifier 2, a radiating frame 3, a delay generator 4 at 100 μs, a gate pulse generator 5, a matching amplifier 6, an electronic switch 7, a receiving frame 8 , two-way limiter 9, signal amplifier 10, integrator 11, DC amplifier 12, indicator 13, voltage stabilizer 14.

The metal detector works as follows. The master oscillator emits a pulse of duration T and (Fig. 27, b), the decline of which triggers the delay generator. The master oscillator pulse is amplified in power by a current amplifier and supplied to the radiating frame. The delay generator generates a pulse with a duration of 100 μs, the fall of which triggers the gating pulse generator. This generator produces a strobe pulse with a duration of 30 μs, which, through a matching amplifier, controls the operation of the electronic switch. The switch opens the signal amplifier for the duration of the strobe pulse and passes the signal from amplifier 10 to the integrator. The signal from the integrator output is fed through a DC amplifier to a dial indicator.

In Fig. Figure 27, b shows the time distribution of signals on the transmitting (emitting) frame (curve 1), on the receiving frame in the absence (curve 2) and in the presence of metal (curve 5). As a result of the experiments, it was found that in the absence of metal, the received pulse within a time of 100 μs rather sharply decreases in amplitude. If there are metal inclusions in the control zone, the duration of the decrease in amplitude of the received pulse is significantly delayed, mainly due to the action of Foucault currents. The property of deformation of the shape of the received signal due to the influence of metallic inclusions is the basis for the design of this device.

The design of the device sensor is shown in Fig. 27, v. The emitting and receiving frames are wound on a dielectric frame with an outer diameter of 300 mm. The receiving frame is wound inside the emitting frame. Its internal diameter is 260 mm. The transmitting frame contains 300 turns of PEV-2 0.44 wire, and the receiving frame contains 60 turns of PEV-2 0.14 wire. The fastening of handle 1 is arbitrary and does not require any special explanation.

In Fig. Figure 28 shows a schematic diagram of the device. The master oscillator is made on DD1.1 and DD1.2 microcircuits. The signal from the output of the generator through resistor R9 is supplied to the input of the pulse current amplifier - transistors VT3-VT5, the load of which is the radiating frame L1.1. Through capacitor C3, the pulse from the master oscillator is supplied to the input of the delay generator, made using elements DD1.3, DD1.4 according to the Schmidt trigger circuit. The decay of the delay pulse triggers the gating pulse generator, made on elements DD2.1-DD2.3. The gating pulse through the matching amplifier (transistors VT1, VT2) is supplied to the electronic switch DA1, which controls the operation of the signal amplifier (DA1.1 and DA1.2) and the integrator (C12, R30), passing the DC signal to the DC amplifier (DA2) during the strobe pulse. The load of the DC amplifier is the pointer device PA1. To increase measurement stability, the power supply to the amplifier stages is additionally stabilized. Electronic stabilizers are made on transistors VT6, VT7.

The pulse metal detector brought to your attention is a joint development of the author and engineer from Donetsk (Ukraine) Yuri Kolokolov (Internet address - http://home.skif.net/~yukol/index.htm), through whose efforts it was possible to turn the idea into a finished product product based on a programmable single-chip microcontroller. He developed the software, and also carried out full-scale testing and extensive debugging work.

Currently, the Moscow company "Master Kit" (see also the Advertising Appendix at the end of the book) is planning to produce kits for radio amateurs for self-assembly of the described metal detector. The kit will contain a printed circuit board and electronic components, including a pre-programmed controller. Perhaps, for many lovers of searching for treasures and relics, purchasing such a kit and its subsequent simple assembly will be a convenient alternative to purchasing an expensive industrial device or making a metal detector entirely on their own.

For those who feel confident and are ready to try to manufacture and program a microprocessor pulse metal detector, Yuri Kolokolov’s personal page on the Internet contains the code for an evaluation version of the controller firmware in Intel HEX format and other useful information. This version of the firmware differs from the full version in the absence of some operating modes of the metal detector.

The operating principle of a pulsed or eddy current metal detector is based on the excitation of pulsed eddy currents in a metal object and the measurement of the secondary electromagnetic field that these currents induce. In this case, the exciting signal is supplied to the transmitting coil of the sensor not constantly, but periodically in the form of pulses. In conducting objects, damped eddy currents are induced, which excite a damped electromagnetic field. This field, in turn, induces a damped current in the receiving coil of the sensor. Depending on the conductive properties and size of the object, the signal changes its shape and duration. In Fig. 24. The signal on the receiving coil of a pulse metal detector is schematically shown.

Rice. 24. Signal at the input of a pulse metal detector
Oscillogram 1 - signal in the absence of metal targets; oscillogram 2 - signal when the sensor is near a metal object

Pulse metal detectors have their advantages and disadvantages. The advantages include low sensitivity to mineralized soil and salt water, the disadvantages are poor selectivity by metal type and relatively high energy consumption.

Practical design

Most practical designs of pulsed metal detectors are built using either a two-coil circuit or a single-coil circuit with an additional power source. In the first case, the device has separate receiving and emitting coils, which complicates the design of the sensor. In the second case, there is only one coil in the sensor, and to amplify the useful signal, an amplifier is used, which is powered by an additional power source. The meaning of this construction is as follows - the self-induction signal has a higher potential than the potential of the power source that is used to supply current to the transmitting coil. Therefore, to amplify such a signal, the amplifier must have its own power source, the potential of which must be higher than the voltage of the signal being amplified. This also complicates the design of the device.

The proposed single-coil design is built according to an original scheme, which is devoid of the above disadvantages.
Main technical characteristics
Supply voltage 7.5... 14 V
Current consumption no more than 90 mA

Detection depth:
coin with a diameter of 25 mm 20 cm
pistol 40 cm
helmet 60 s

Attention!

Despite the relative simplicity of the design of the proposed pulse metal detector, preparing it at home can be difficult due to the need to enter a special program into the microcontroller. This can only be done if you have the appropriate qualifications and software and hardware to work with the microcontroller.

Structural scheme

The block diagram is shown in Fig. 25 The basis of the device is a microcontroller. With its help, time intervals are formed to control all components of the device, as well as indication and general control of the device. Using a powerful switch, energy is pulsedly accumulated in the sensor coil, and then the current is interrupted, after which a self-induction pulse occurs, exciting an electromagnetic field in the target.

Rice. 25. Block diagram of a pulse metal detector

The highlight of the proposed circuit is the use of a differential amplifier in the input stage. It serves to amplify a signal whose voltage is higher than the supply voltage and bind it to a certain potential (+5 V). For further amplification, a receiving amplifier with a high gain is used. The first integrator is used to measure the useful signal. During forward integration, the useful signal is accumulated in the form of voltage, and during reverse integration, the result is converted into pulse duration. The second integrator has a large integration constant (240 ms) and serves to balance the amplification path with respect to direct current.

Schematic diagram

The schematic diagram of a pulse metal detector is shown in Fig. 26 - differential amplifier, receiving amplifier, integrators and powerful switch. S1 2200M

Rice. 26. Schematic diagram of a pulse metal detector. Amplification path, powerful key, integrators

Rice. 27. Schematic diagram of a pulse metal detector. Microcontroller

In Fig. Figure 27 shows the microcontroller and controls and indications. The proposed design is developed entirely on imported element base. The most common components from leading manufacturers are used. You can try to replace some elements with domestic ones, this will be discussed below. Most of the elements used are not in short supply and can be purchased in large cities in Russia and the CIS through companies that sell electronic components.

The differential amplifier is assembled using op amp D1.1. Chip D1 is a quad operational amplifier type TL074. Its distinctive properties are high speed, low consumption, low noise level, high input impedance, and the ability to operate at input voltages close to the supply voltage. These properties determined its use in a differential amplifier in particular and in the circuit in general. The gain of the differential amplifier is about 7 and is determined by the values ​​of resistors R3, R6-R9, R11.

Receiving amplifier D1.2 is a non-inverting amplifier with a gain of 56. During the action of the high-voltage part of the self-induction pulse, this coefficient is reduced to 1 using the analog switch D2.1. This prevents overloading of the input amplification path and ensures rapid entry into mode to amplify a weak signal. Transistor VT3, as well as transistor VT4, are designed to match the levels of control signals supplied from the microcontroller to analog switches.

Using the second integrator D1.3, the input amplifier circuit is automatically balanced for direct current. The integration constant of 240 ms is chosen to be large enough so that this feedback does not affect the gain of the rapidly changing desired signal. Using this integrator, the output of amplifier D1.2 maintains a level of +5 V in the absence of a signal.

The measuring first integrator is made on D1.4. During the integration of the useful signal, key D2.2 opens and, accordingly, key D2.4 closes. A logical inverter is implemented on switch D2.3. After signal integration is completed, key D2.2 closes and key D2.4 opens. Storage capacitor C6 begins to discharge through resistor R21. The discharge time will be proportional to the voltage that has established on capacitor C6 by the end of integration of the useful signal.

This time is measured using a microcontroller that performs analog-to-digital conversion. To measure the discharge time of capacitor C6, an analog comparator and timers are used, which are built into the microcontroller D3.

LEDs VD3...VD8 provide light indication. Button S1 is intended for the initial reset of the microcontroller. Using switches S2 and S3, the operating modes of the device are set. Using variable resistor R29, the sensitivity of the metal detector is adjusted.

Functioning algorithm

Rice. 28. Oscillograms

To explain the operating principle of the described pulse metal detector in Fig. Figure 28 shows oscillograms of signals at the most important points of the device.

During interval A, key VT1 opens. A sawtooth current begins to flow through the sensor coil - oscillogram 2. When the current reaches about 2 A, the key closes. At the drain of transistor VT1, a surge of self-induction voltage occurs - oscillogram 1. The magnitude of this surge is more than 300 V (!) and is limited by resistors R1, R3. To prevent overload of the amplification path, limiting diodes VD1, VD2 are used. Also, for this purpose, during interval A (accumulation of energy in the coil) and interval B (release of self-induction), key D2.1 is opened. This reduces the end-to-end gain of the path from 400 to 7. Oscillogram 3 shows the signal at the output of the amplification path (pin 8 of D1.2). Starting from interval C, switch D2.1 closes and the path gain becomes large. After the completion of the guard interval C, during which the amplification path enters the mode, key D2.2 opens and key D2.4 closes - integration of the useful signal begins - interval D. After this interval, key D2.2 closes and key D2.4 opens - “reverse” integration begins. During this time (intervals E and F), capacitor C6 is completely discharged. Using a built-in analog comparator, the microcontroller measures the value of the interval E, which turns out to be proportional to the level of the input useful signal. Firmware version 1.0 has the following interval values:

A-60...200 μs, C - 8 μs,

B - 12 µs, D - 50 µs,

A+B+C+D+E+F - 5 ms - repetition period.

The microcontroller processes the received digital data and indicates, using LEDs VD3-VD8 and a sound emitter Y1, the degree of impact of the target on the sensor. The LED indication is an analogue of a dial indicator - if there is no target, the VD8 LED lights up, then, depending on the level of impact, VD7, VD6, etc. light up sequentially.

Types of parts and design

Instead of the operational amplifier D1 TL074N, you can try using the TL084N or two dual op-amps of the TL072N, TL082N types. The D2 chip is a quad analog switch of the CD4066 type, which can be replaced with the domestic K561KTZ chip. The D4 AT90S2313-10PI microcontroller has no direct analogues. The circuit does not provide circuits for its in-circuit programming, so it is advisable to install the controller on a socket so that it can be reprogrammed.

The 78L05 stabilizer can, as a last resort, be replaced with KR142EN5A.

You can try replacing transistor VT1 type IRF740 with IRF840. Transistors VT2-VT4 type 2N5551 can be replaced with KT503 with any letter index. However, you should pay attention to the fact that they have different pinouts. LEDs can be of any type; it is advisable to take VD8 in a different color. Diodes VD1, VD2 type 1N4148.

Resistors can be of any type, R1 and R3 should have a power dissipation of 0.5 W, the rest can be 0.125 or 0.25 W. It is advisable to select R9 and R11 so that their resistance differs by no more than 5%.

It is advisable to use a multi-turn trimmer resistor R7.

Capacitor C1 is electrolytic, for a voltage of 16 V, the rest of the capacitors are ceramic. It is advisable to take capacitor C6 with a good TKE.

Button S1, switches S2-S4, variable resistor R29 can be of any type that fits the dimensions. You can use a piezo emitter or headphones from the player as a sound source.

The design of the device body can be arbitrary. The rod near the sensor (up to 1 m) and the sensor itself should not have metal parts or fastening elements. It is convenient to use a plastic telescopic fishing rod as the starting material for making a rod.

The sensor contains 27 turns of wire with a diameter of 0.6...0.8 mm, wound on a 190 mm mandrel. The sensor does not have a screen and must be attached to the rod without the use of massive screws, bolts, etc. (!) Otherwise, the technology for its manufacture can be the same as for an induction metal detector. A shielded cable cannot be used to connect the sensor and the electronic unit due to its high capacitance. For these purposes, you need to use two insulated wires, for example MGShV type, twisted together.

Setting up the device

Attention! The device contains high, potentially life-threatening voltage - at the VT1 collector and at the sensor. Therefore, when setting up and operating, electrical safety precautions should be observed.

1. Make sure installation is correct.

2. Apply power and make sure that the current consumption does not exceed 100 (mA).

3. Using tuning resistor R7, achieve such balancing of the amplification path so that the oscillogram at pin 7 of D1.4 corresponds to oscillogram 4 in Fig. 28. In this case, it is necessary to ensure that the signal at the end of the interval D is unchanged, i.e. The oscillogram at this point should be horizontal.

A properly assembled device does not require further adjustment. It is necessary to bring the sensor to a metal object and make sure that the indicators are working. A description of the operation of the controls is given in the software description.

Software

At the time of writing this chapter, software versions 1.0 and 1.1 have been developed and tested. The firmware code version 1.0 in Intel HEX format can be found on the Internet on Yuri Kolokolov’s personal page.

The commercial version 1.1 of the software is planned for delivery in the form of already programmed microcontrollers as part of kits produced by Master Kit. Version 1.0 implements the following features:

Supply voltage control - when the supply voltage is less than 7 V, the VD8 LED begins to light up intermittently;

Fixed sensitivity level;

Static search mode.

Software version 1.1 is different in that it allows you to adjust the sensitivity of the device using a variable resistor R29.

Work on new versions of the software continues, and it is planned to introduce additional modes. Switches S1, S2 are reserved to control new modes. New versions, after extensive testing, will be available in Master Kits. Information about new versions will be published on the Internet on Yuri Kolokolov’s personal page.

Instrumental search is simply enormously popular. Adults and children, amateurs and professionals are looking for it. They are looking for treasures, coins, lost things and buried scrap metal. And the main search tool is metal detector.

There are a great variety of different metal detectors to suit every taste and color. But for many people, buying a ready-made branded metal detector is simply financially expensive. And some people want to assemble a metal detector with their own hands, and some even build their own small business on their assembly.

Homemade metal detectors

In this section of our website about homemade metal detectors, I will be collected: best metal detector circuits, their descriptions, programs and other data for manufacturing DIY metal detector. There are no metal detector circuits from the USSR or circuits with two transistors here. Since such metal detectors are only suitable for visually demonstrating the principles of metal detection, but are not at all suitable for real use.

All metal detectors in this section will be quite technologically advanced. They will have good search characteristics. And a well-assembled homemade metal detector is not much inferior to its factory-made counterparts. Basically, there are various schemes presented here pulse metal detectors And metal detector circuits with metal discrimination.

But to make these metal detectors, you will need not only desire, but also certain skills and abilities. We tried to break down the diagrams of the given metal detectors by level of complexity.

In addition to the basic data required to assemble a metal detector, there will also be information about the required minimum level of knowledge and equipment for making a metal detector yourself.

To assemble a metal detector with your own hands, you will definitely need:

This list will contain the necessary tools, materials and equipment for self-assembly of all metal detectors without exception. For many schemes you will also need various additional equipment and materials, here are just the basics for all schemes.

1. Soldering iron, solder, tin and other soldering supplies.
2. Screwdrivers, pliers, wire cutters and other tools.
3. Materials and skills for making a printed circuit board.
4. Minimum experience and knowledge in electronics and electrical engineering as well.
5. And also straight hands will be very useful when assembling a metal detector with your own hands.

Here you can find diagrams for self-assembly of the following models of metal detectors:

	Principle of operation	I.B.
	Metal discrimination	There is
	Maximum search depth
		There is
	Operating frequency	4 - 17 kHz
	Difficulty level	Average

	Principle of operation	I.B.
	Metal discrimination	There is
	Maximum search depth	1-1.5 meters (Depends on the size of the coil)
	Programmable microcontrollers	There is
	Operating frequency	4 - 16 kHz
	Difficulty level	Average

	Principle of operation	I.B.
	Metal discrimination	There is
	Maximum search depth	1 - 2 meters (Depends on the size of the coil)
	Programmable microcontrollers	There is
	Operating frequency	4.5 - 19.5 kHz
	Difficulty level	High