The most famous type of magnetic prospecting equipment is magnetometer. Its modified version is gradiometer. The principles for measuring the magnetic field in these devices are the same - they can be proton, fluxgate, quantum, etc., only the design solutions are different, which allow solving slightly different problems.

Fig.1. Three-dimensional magnetic field of an ancient city.

Let's consider the most widely used types of magnetometers. First of all, these are, of course, proton, fluxgate and quantum magnetometers. They all have certain advantages and disadvantages. There are, of course, also cryogenic magnetometers, Hall effect magnetometers, and induction magnetometers. But pedestrian magnetometers that are of interest for archaeological research are, of course, proton, fluxgate and, to a lesser extent, quantum. Let's consider their comparative characteristics.

It would seem that the main characteristic of a magnetometer is sensitivity. However, this is not quite true. For example, cryogenic magnetometers they easily achieve a sensitivity of 0.0001 nT, but they are so inconvenient, bulky and capricious that they are not used even in the aero version (although there have been attempts).

Quantum magnetometers are also quite capable of showing an accuracy of 0.01 nT, but have very strict restrictions on the orientation of the sensors. They have been successfully used for aeromagnetic surveys for many years.

Fluxgate magnetometers, having a very high measurement accuracy and the ability to produce not discrete, like quantum and proton magnetometers, but a continuous signal, are sensitive to temperature changes, which gives designers some trouble with the “zero creep” of the device.

Proton magnetometers, being less sensitive, turned out to be very good in terms of stability, low susceptibility to temperature changes and orientation to the cardinal points (although the latter is still present). The disadvantages of proton sensors include the discreteness of measurements, which requires stopping at each point, the bulkiness and heavy weight of the sensors, as well as the impossibility of measurements in strong fields.

More about sensitivity. If you see a sensitivity of 0.1 nT in the device passport, this does not mean at all that you will be able to detect an anomaly of at least 1 nT! Firstly, this 0.1 nT is superimposed on the temperature drift of the instrument zero (several nT). Secondly, the influence of the spatial orientation of the device is another 2-4 nT. Well, and, naturally, the variations of the geomagnetic field that are already familiar to us.

In short, as long-term practice shows, it is impossible to identify an anomaly with an amplitude of less than 3-7 nT during a standard area pedestrian survey. During route surveying (when a search engine follows a certain route, often over rough terrain), trying to identify an anomaly based on the current readings of the device, it is very difficult to catch an anomaly even of 10-20 nT. So when searching, you can safely switch the sensitivity on your device from 0.1 to 1 nT and get to work without tiring yourself of looking at tenths on the display.

Another important characteristic of a magnetometer is the recording method. If the information is displayed only on the display in digital form and (or) on magnetic media, then, of course, this is a device intended for area survey work. These works are quite complex, require material and time costs, and the result, presented in the form of maps of the magnetic field of the site, is issued only after a certain time.

The search device must have a light (changing scale) and sound indication. This allows you to quickly, during field research, see an anomaly, find its center and immediately make a decision regarding its prospects. The most common search device is a hand-held metal detector, but its depth leaves much to be desired, although other characteristics (discrimination, target detection accuracy, etc.) have been brought to a high level by manufacturers.

The requirements for a more powerful deep-seeding device are met magnetometers-gradientometers. Being, in fact, two magnetometers combined into a single device, the gradiometer gives the owner information not about the numerical value of the field at the measurement point, but about the difference in the field between two points in space - about the gradient. Since the gradient of the Earth's field, geological structures and time variations is vanishingly small, the gradiometer ignores it. But the gradient from the results of human activity, on the contrary, is large. The field from small objects of human activity is small, but attenuates so quickly that this attenuation (gradient) is easily recorded by a gradiometer without first constructing magnetic field maps. An ordinary magnetometer will also catch this difference, but for this the operator will have to take not one, but two measurements at each point - below, at ground level, and 1-2 meters above, which, of course, is inconvenient. But to correctly measure the field with a magnetometer, it is necessary to stop at each point, and this is doubly inconvenient.

How to make a variometer (magnetometer) yourself. Is it possible to monitor disturbances in the Earth's magnetic field yourself? The answer is obvious - yes, it is possible, and the easiest way to do this is to regularly view the data of the nearest magnetic observatory on the Internet. Well, if you don’t have a computer or the Internet nearby, and you live in a region of Russia where there is no magnetic observatory nearby, you can make a device yourself that will help you judge the state of the Earth’s magnetic field. In addition to a household thermometer and barometer, a compass can be an equally simple and useful device for recording disturbances in the Earth's magnetic field. Do not try to see how the compass needle darts during a magnetic storm - this picture is on the conscience of the authors of works of art. One of the largest magnetic storms over the past 100 years at the latitude of Moscow was observed in October 2003 - the maximum deviation in the horizontal component reached a value of about 2000 nT, which, with the value of the H component itself being 17,000 nT, is only 10%. Taking into account the fact that such a change lasts for units and tens of minutes – i.e. The process of changing the magnetic field itself is quite slow - you need to keep your eyes on the compass needle for at least 15 minutes to notice such a deviation. It is clear that it is almost impossible to catch such a moment without having a system for continuous recording of magnetic field variations. It should be borne in mind that the regular solar-diurnal variation in a quiet field is in the range of 30-40 nT, i.e. 0.05%, with average magnetic storms the deviation is 200-300 nT, i.e. about 0.5%. It is clear from this that a device for monitoring magnetic field disturbances must be a sufficiently sensitive sensor with electronic recording. As an example, you can see the development of simple devices for observing magnetic field variations on your own on the website of the Lancaster University Ionospheric Physics Laboratory http://www.dcs.lancs.ac.uk/iono/aurorawatch/detectors/results.html or on the POETRY project website (PublicOutreach, Education, Teaching andReaching Youth), see http://image.gsfc.nasa.gov/poetry/. To begin with, you can try to assemble the simplest disturbance detector - a suspended magnet in a plastic bottle. To take readings, a mirror and an illuminator are used, so that the reflected bunny is fixed on a sheet of paper at some distance from the detector. By regularly noting the movements of the bunny on paper, you can notice disturbances in the magnetic field. On the websites of Lancaster University and the POETRY project, the entire structure is presented so clearly that there should be no problems with its repetition; the design details are very simple. But you need to keep in mind that the sensitivity of such a detector is low, and you will only be able to detect large storms, and such storms occur only a few times a year. A more sensitive detector can be built on the basis of a good compass. This design will require knowledge and ability to assemble electronic circuits. Design details are presented on the same website of Lancaster University, see http://www.dcs.lancs.ac.uk/iono/aurorawatch/detectors/compass.html A diagram of the magnetometer and recommendations for its assembly are presented on the website http://www. sam-europe.de/en/index_en.html. From the information presented, we can come to the conclusion that information about disturbances in the Earth’s magnetic field can be obtained from many sources, even to the point of making observations yourself. It is clear that such observations will be inferior to professional magnetic observatories, but for the purposes of amateur or educational projects this approach is quite justified. Club "Helios"

A magnetometer is a device that is used to explore the Earth's magnetic field or search for hidden objects. Based on the principle of operation, the device is a bit like a metal detector that reacts to metal surfaces, with the exception that it is sensitive to the natural magnetic field of the Earth, as well as large non-metallic objects that have their own residual field. The device has found its application in various branches of industry and science, as it allows you to record natural anomalies and also speeds up the search for objects.

Why is a magnetometer used?

Magnetometers respond to a magnetic field and express its strength in various physical units of measurement. In this regard, there are many types of these devices, each of which is adapted for a specific search purpose. Modifications of these devices are used in dozens of branches of science and industry:

• Geology.
• Archeology.
• Navigation.
• Seismology.
• Military intelligence.
• Geochronology.

IN geology Using a magnetometer, minerals can be found without the need for test drilling to take samples. The device allows you to record a mineral-rich vein and make a decision on the advisability of starting mining in the area. Also, using this equipment, you can determine where underground sources of drinking water are located, how they are located and their volume. Thanks to this, you can decide in advance where to build a well or borehole in order to get to the water without the need for maximum deepening.

Magnetometers are used in archeology during excavations. They allow you to respond to building foundations, statues and other objects hidden deep underground that have residual magnetization. First of all, it is burnt brick or stone. The device responds to ancient hearths and stoves hidden deep underground. It can be used to search for objects in ice or snow.

Magnetometer is also used in navigation. With its help, the Earth's magnetic field is determined, as a result of which it is possible to obtain data on the direction of movement in the event of disorientation. Such devices are used in aviation and maritime transport. Magnetometers are required equipment on space stations and shuttles.

IN seismology Magnetometers that react to the Earth’s magnetic field make it possible to predict an earthquake, since when the characteristics of tectonic plates change, the usual field indicators are disrupted. In this way, it is possible to identify fresh underground cracks through which an eruption may begin.

IN military intelligence This equipment allows you to search for military targets hidden from conventional radars. Using a magnetometer, you can identify a submarine lying on the sea or ocean floor.

IN geochronology The age of rocks can be determined by the strength of residual magnetization. There are more accurate methods, but with a magnetometer this can be done in seconds, without the need for expensive analysis.

Types of magnetometers according to operating principle

Based on their operating principle, magnetometers are divided into 3 types:

• Magnetostatic.
• Induction.
• Quantum.

Each variety reacts to an external magnetic field using a specific physical principle. Based on these three varieties, various highly specialized types of magnetometers have been created, which are more accurate for measurements under certain conditions.

Magnetostatic

Despite the external complexity of this device, it works according to a completely understandable physical principle. Inside the magnetometer is a small permanent magnet that responds to the magnetic field it comes into contact with. The magnet is suspended on an elastic suspension, allowing it to rotate. It has virtually no rigidity, so it does not hold it and allows it to scroll without resistance. When a permanent magnet reacts with a foreign field whose direction or strength is not the same as its own, an attraction or rejection reaction occurs. As a result, the suspended permanent magnet begins to rotate, which detects the sensitive sensor. In this way, the strength and direction of the external magnetic field is measured.

The sensitivity of a magnetostatic device depends on the reference magnet that is installed in it. The elasticity of the suspension also affects the measurement accuracy.

Induction

Induction magnetometers have a coil inside with a wire winding made of conductive material. It is energized from the battery power supply. The coil creates its own magnetic field, which begins to come into contact with third-party fields passing through its circuit. Sensitive sensors respond to changes that are displayed on the coil as a result of this interaction. They can respond to rotation or vibration. In more complex devices, sensors respond to changes in the magnetic permeability of the coil core. Regardless of how the change is recorded, the device displays indicators of external magnetic fields and allows you to determine the location of objects, their size and distance.

Quantum

A quantum magnetometer responds to the magnetic moment of electrons that move under the influence of external magnetic fields. This is expensive equipment that is used for laboratory research, as well as complex searches. The device records the magnetic moment of microparticles and the strength of the measured field. This equipment allows you to measure the strength of weak fields, including those found in outer space. It is this equipment that is used in geoexploration to search for deep mineral deposits.

Difference between models

A magnetometer is a highly technical equipment that may differ from other similar devices not only in the physical principle of response to changes in the magnetic field or sensitivity, but also in other characteristics. Devices may differ from each other according to the following criteria:

• Availability of display.
• Number of sensors.
• The presence of a sound indicator.
• Measurement errors.
• Indication method.
• Duration of continuous operation.
• Dimensions and weight.

As for the number of sensitive sensors, the more there are, the more accurate the equipment will be. The magnetometer can display its measurements either numerically or graphically. It is difficult to say which is better, since everything depends on the characteristics of the conditions in which the measurement is carried out. In certain cases, you just need to get a display of the magnetic field indicators in numbers, while sometimes you need more of a visual determination of the vector of its vortices. The best option is combined devices that allow you to visualize indicators in digital and graphic display.

Magnetometer designed to measure magnetic field induction. The magnetometer uses a reference magnetic field, which allows, through certain physical effects, convert the measured magnetic field into an electrical signal.
The applied use of magnetometers for detecting massive objects made of ferromagnetic (most often steel) materials is based on the local distortion of the Earth's magnetic field by these objects. The advantage of using magnetometers over traditional metal detectors is that longer detection range.

Fluxgate (vector) magnetometers

One type of magnetometer is . The fluxgate was invented by Friedrich Förster ( )

In 1937 and serves to determine magnetic field induction vector.

Fluxgate design

single rod fluxgate

The simplest fluxgate consists of a permalloy rod on which an excitation coil is placed (( drive coil), powered by alternating current, and a measuring coil ( detector coil).

Permalloy- an alloy with soft magnetic properties, consisting of iron and 45-82% nickel. Permalloy has high magnetic permeability (maximum relative magnetic permeability ~100,000) and low coercivity. A popular brand of permalloy for the manufacture of fluxgates is 80НХС - 80% nickel + chromium and silicon with a saturation induction of 0.65-0.75 T, used for cores of small-sized transformers, chokes and relays operating in weak fields of magnetic screens, for cores of pulse transformers, magnetic amplifiers and contactless relays, for magnetic head cores.
The dependence of relative magnetic permeability on field strength for some varieties of permalloy has the form -

If a constant magnetic field is applied to the core, then a voltage appears in the measuring coil even harmonics, the magnitude of which serves as a measure of the strength of a constant magnetic field. This voltage is filtered and measured.

double rod fluxgate

An example is the device described in the book Karalisa V.N."Electronic circuits in industry" -



The device is designed to measure constant magnetic fields in the range of 0.001 ... 0.5 oersted.
Sensor field windings L1 And L3 included counter. Measuring winding L2 wound over the field windings. The field windings are powered by a 2 kHz current from a push-pull generator with inductive feedback. The generator mode is stabilized by direct current by a resistor divider R8 And R9.

fluxgate with toroidal core
One of the popular design options for a fluxgate magnetometer is a fluxgate with a toroidal core ( ring core fluxgate) -

Compared to rod fluxgates, this design has less noise and requires creation much lower magnetomotive force.

This sensor is excitation winding, wound on a toroidal core, through which an alternating current flows with an amplitude sufficient to bring the core into saturation, and measuring winding, from which the alternating voltage is removed, which is analyzed to measure the external magnetic field.
The measuring winding is wound over the toroidal core, covering it entirely (for example, on a special frame) -


This design is similar to the original fluxgate design (a capacitor is added to achieve resonance at the second harmonic) -

Applications of proton magnetometers
Proton magnetometers are widely used in archaeological research.
The proton magnetometer is mentioned in the science fiction novel "Trapped in Time" by Michael Crichton. Timeline") -
He pointed down past his feet. Three heavy yellow housings were clamped to the front struts of the helicopter. “Right now we’re carrying stereo terrain mappers, infrared, UV, and side-scan radar.” Kramer pointed out the rear window, toward a six-foot-long silver tube that dangled beneath the helicopter at the rear. “And what’s that?” “Proton magnetometer.” "Uh-huh. And it does what?" “Looks for magnetic anomalies in the ground below us that could indicate buried walls, or ceramics, or metal.”

Cesium magnetometers

A type of quantum magnetometers are alkali metal atomic magnetometers with optical pumping.

cesium magnetometer G-858

Overhauser magnetometers

Solid State Magnetometers

The most accessible are magnetometers built into smartphones. For Android a good application using a magnetometer is . The page for this application is http://physics-toolbox-magnetometer.android.informer.com/.

Setting up magnetometers

To test the fluxgate you can use. Helmholtz coils are used to produce a nearly uniform magnetic field. Ideally, they represent two identical annular turns connected to each other in series and located at a distance of a turn radius from each other. Typically, Helmholtz coils consist of two coils on which a certain number of turns are wound, and the thickness of the coil should be much less than their radius. In real systems, the thickness of the coils can be comparable to their radius. Thus, we can consider a system of Helmholtz rings to be two coaxially located identical coils, the distance between the centers of which is approximately equal to their average radius. This coil system is also called a split solenoid ( split solenoid).

In the center of the system there is a zone of uniform magnetic field (magnetic field in the center of the system in a volume of 1/3 of the radius of the rings homogeneous within 1%), which can be used for measurement purposes, for calibrating magnetic induction sensors, etc.

Magnetic induction at the center of the system is defined as $B = \mu _0\,(\left((4\over 5)\right) )^(3/2) \, (IN\over R)$,
where $N$ is the number of turns in each coil, $I$ is the current through the coils, $R$ is the average radius of the coil.

Helmholtz coils can also be used to shield the Earth's magnetic field. To do this, it is best to use three mutually perpendicular pairs of rings, then their orientation does not matter.

The differential magnetometer we bring to your attention can be very useful for searching for large iron objects. It is almost impossible to search for treasures with such a device, but it is indispensable when searching for shallowly sunken tanks, ships and other types of military equipment.

The operating principle of a differential magnetometer is very simple. Any ferromagnetic object distorts the Earth's natural magnetic field. These items include anything made of iron, cast iron and steel. The distortion of the magnetic field can also be significantly influenced by the objects’ own magnetization, which often occurs. Having recorded the deviation of the magnetic field strength from the background value, we can conclude that there is an object made of ferromagnetic material near the measuring device.

The distortion of the Earth's magnetic field far from the target is small, and it is estimated by the difference in signals from two sensors separated by some distance. That is why the device is called differential. Each sensor measures a signal proportional to the magnetic field strength. The most widely used are ferromagnetic sensors and sensors based on the magnetonic precession of protons. The device in question uses sensors of the first type.

The basis of a ferromagnetic sensor (also called a fluxgate) is a coil with a core made of ferromagnetic material. A typical magnetization curve for such a material is well known from a school physics course and, taking into account the influence of the Earth’s magnetic field, has the following form, shown in Fig. 29.

Rice. 29. Magnetization curve

The coil is excited by an alternating sinusoidal carrier signal. As can be seen from Fig. 29, the displacement of the magnetization curve of the ferromagnetic core of the coil by the external magnetic field of the Earth leads to the fact that the field induction and the associated voltage on the coil begin to be distorted in an asymmetrical manner. In other words, the sensor voltage with a sinusoidal current of the carrier frequency will differ from the sinusoid by more “flattened” tops of the half-waves. And these distortions will be asymmetrical. In the language of spectral analysis, this means the appearance in the spectrum of the output voltage of the coil of even harmonics, the amplitude of which is proportional to the strength of the bias magnetic field (Earth's field). It is these even harmonics that need to be “caught”.

Rice. 30. Differential ferromagnetic sensor

Before mentioning a synchronous detector that naturally suggests itself for this purpose, operating with a reference signal of double the carrier frequency, let us consider the design of a complicated version of a ferromagnetic sensor. It consists of two cores and three coils (Fig. 30). At its core, this is a differential sensor. However, for simplicity, further in the text we will not call it differential, since the magnetometer itself is already differential :).

The design consists of two identical ferromagnetic cores with identical coils arranged in parallel next to each other. In relation to the exciting electrical signal of the reference frequency, they are connected counter-currently. The third coil is a winding wound on top of the first two core coils folded together. In the absence of an external biasing magnetic field, the electrical signals of the first and second windings are symmetrical and, ideally, act in such a way that there is no output signal in the third winding, since the magnetic fluxes through it are completely compensated.

In the presence of an external biasing magnetic field, the picture changes. First one or the other core at the peak of the corresponding half-wave “flies” into saturation deeper than usual due to the additional influence of the Earth’s magnetic field. As a result, a double frequency mismatch signal appears at the output of the third winding. Fundamental harmonic signals are ideally fully compensated there.

The convenience of the considered sensor lies in the fact that its coils can be included in oscillatory circuits to increase sensitivity. The first and second - into an oscillatory circuit (or circuits) tuned to the carrier frequency. The third - into an oscillating circuit tuned to the second harmonic.

The described sensor has a pronounced radiation pattern. Its output signal is maximum when the longitudinal axis of the sensor is located along the lines of force of the external constant magnetic field. When the longitudinal axis is perpendicular to the lines of force, the output signal is zero.

A sensor of the type considered, especially in conjunction with a synchronous detector, can successfully work as an electronic compass. Its output signal after rectification is proportional to the projection of the Earth's magnetic field strength vector onto the sensor axis. Synchronous detection makes it possible to find out the sign of this projection. But even without a sign - by orienting the sensor according to the minimum signal, we get a direction to the west or east. Orienting to the maximum, we obtain the direction of the magnetic field line of the Earth. In mid-latitudes (for example, in Moscow), it goes obliquely and “sticks” into the ground in the direction to the north. The angle of magnetic declination can be used to approximately estimate the geographic latitude of an area.

Differential ferromagnetic magnetometers have their advantages and disadvantages. The advantages include the simplicity of the device; it is no more complicated than a direct amplification radio receiver. The disadvantages include the laboriousness of manufacturing sensors - in addition to accuracy, an absolutely exact match of the number of turns of the corresponding windings is required. An error of one or two turns can greatly reduce the possible sensitivity. Another disadvantage is the “compass” nature of the device, i.e., the inability to fully compensate for the Earth’s field by subtracting signals from two spaced sensors. In practice, this leads to false signals when the sensor is rotated around an axis perpendicular to the longitudinal one.

Practical design

The practical design of a differential ferromagnetic magnetometer was implemented and tested in a prototype version without a special electronic part for sound indication, using only a microammeter with a zero in the middle of the scale. The sound indication circuit can be taken from the description of the metal detector based on the “transmission-reception” principle. The device has the following parameters.

Main technical characteristics

• Supply voltage - 15... 18 V
• Current consumption - no more than 50 mA

Detection depth:

• pistol - 2 m
• cannon barrel - 4 m
• tank - 6 m

Structural scheme

The block diagram is shown in Fig. 31. A quartz-stabilized master oscillator produces clock pulses for the signal conditioner.

Rice. 31. Block diagram of a differential ferromagnetic magnetometer

At one of its outputs there is a square wave of the first harmonic, which goes to the power amplifier, which excites the radiating coils of sensors 1 and 2. The other output generates a square wave of the reference double clock frequency with a 90° shift for a synchronous detector. The difference signal from the output (third) windings of the sensors is amplified in the receiving amplifier and rectified by a synchronous detector. The rectified constant signal can be recorded with a microammeter or with sound indication devices described in previous chapters.

Schematic diagram

The schematic diagram of a differential ferromagnetic magnetometer is shown in Fig. 32 - part 1: master oscillator, signal conditioner, power amplifier and radiating coils, fig. 33 - part 2: receiving coils, receiving amplifier, synchronous detector, indicator and power supply.

Rice. 32. Electrical circuit diagram - part 1

The master oscillator is assembled on inverters D1.1-D1.3. The generator frequency is stabilized by a quartz or piezoceramic resonator Q with a resonant frequency of 215 Hz = 32 kHz (“clock quartz”). Circuit R1C1 prevents the generator from being excited at higher harmonics. The OOS circuit is closed through resistor R2, and the POS circuit is closed through resonator Q. The generator is simple, has low current consumption, operates reliably at a supply voltage of 3...15 V, and does not contain tuned elements or overly high-resistance resistors. The output frequency of the generator is about 32 kHz.

Signal conditioner(Fig. 32)

The signal conditioner is assembled on a binary counter D2 and a D-flip-flop D3.1. The type of binary counter is not important; its main task is to divide the clock frequency by 2, 4 and 8, thus obtaining meanders with frequencies of 16, 8 and 4 kHz, respectively. The carrier frequency for excitation of the emitting coils is 4 kHz. Signals with frequencies of 16 and 8 kHz, acting on the D-flip-flop D3.1, form at its output a square wave doubled with respect to the carrier frequency of 8 kHz, shifted by 90° relative to the output signal of 8 kHz of the binary counter. Such a shift is necessary for the normal operation of a synchronous detector, since the same shift has a useful double-frequency mismatch signal at the sensor output. The second half of the microcircuit of two D-flip-flops - D3.2 is not used in the circuit, but its unused inputs must be connected to either logical 1 or logical 0 for normal operation, which is shown in the diagram.

Amplifier(Fig. 32)

The power amplifier does not seem like that at first glance and represents only powerful inverters D1.4 and D1.5, which in antiphase swing an oscillatory circuit consisting of series-parallel connected radiating coils of the sensor and capacitor C2. An asterisk next to the capacitor rating means that its value is indicated approximately and that it must be selected during setup. The unused inverter D1.6, in order not to leave its input unconnected, inverts the D1.5 signal, but practically works “idle”. Resistors R3 and R4 limit the output current of the inverters to an acceptable level and, together with the oscillating circuit, form a high-quality bandpass filter, due to which the shape of the voltage and current in the emitting coils of the sensor almost coincides with the sinusoidal one.

Receiving amplifier(Fig. 33)

The receiving amplifier amplifies the difference signal coming from the receiving coils of the sensor, which together with the capacitor SZ form an oscillatory circuit tuned to a double frequency of 8 kHz. Thanks to the tuning resistor R5, the signals from the receiving coils are subtracted with certain weighting coefficients, which can be changed by moving the slider of the resistor R5. This achieves compensation for non-identical parameters of the sensor's receiving windings and minimizing its "compass".

The receiving amplifier is two-stage. It is assembled using op-amps D4.2 and D6.1 with parallel voltage feedback. Capacitor C4 reduces the gain at higher frequencies, thereby preventing overload of the amplification path with high-frequency interference from power networks and other sources. Op-amp correction circuits are standard.

Synchronous detector(Fig. 33)

The synchronous detector is made using op-amp D6.2 according to a standard circuit. The D5 CMOS multiplexer-demultiplexer 8 by 1 chip is used as analog switches (Fig. 32). Its digital address signal is moved only in the least significant bit, providing alternate switching of points K1 and K2 to a common bus. The rectified signal is filtered by capacitor C8 and amplified by op amp D6.2 with simultaneous additional attenuation of unfiltered RF components by circuits R14C11 and R13C9. The op-amp correction circuit is standard for the type used.

Rice. 33. Circuit diagram - part 2. Receiving amplifier

Indicator(Fig. 33)

The indicator is a microammeter with zero in the middle of the scale. The indicator part can successfully use the circuitry of other types of metal detectors described earlier. In particular, the design of a metal detector based on the principle of an electronic frequency meter can be used as an indicator. In this case, its LC oscillator is replaced with an RC oscillator, and the measured output voltage is fed through a resistive divider to the frequency-setting circuit of the timer. You can read more about this on Yuri Kolokolov’s website.

The D7 chip stabilizes the unipolar supply voltage. The D4.1 op amp creates an artificial midpoint power supply, allowing the use of conventional bipolar op amp circuitry. Ceramic blocking capacitors C18-C21 are mounted in close proximity to the housings of digital microcircuits D1, D2, D3, D5.

Types of parts and design

The types of microcircuits used are indicated in table. 6.

Table 6. Types of chips used

Instead of K561 series microcircuits, it is possible to use K1561 series microcircuits. You can try to use some microcircuits of the K176 series or foreign analogues of the 40ХХ and 40ХХХ series.

Dual operational amplifiers (op-amps) of the K157 series can be replaced with any general-purpose op-amps of similar parameters (with appropriate changes in pinout and correction circuits).

There are no special requirements for the resistors used in the differential magnetometer circuit. They just need to have a durable and miniature design and be easy to install. Nominal power dissipation 0.125...0.25 W.

Potentiometers R5, R16 are preferably multi-turn for ease of precise adjustment of the device. The handle of potentiometer R5 must be made of plastic and must be of sufficient length so that the touch of the operator’s hand during adjustment does not cause changes in the indicator readings due to interference.

Capacitor C16 - electrolytic of any small-sized type.

Capacitors of oscillatory circuits C2* and SZ* consist of several (5-10 pcs.) capacitors connected in parallel. Tuning the circuit to resonance is carried out by selecting the number of capacitors and their rating. Recommended type of capacitors K10-43, K71-7 or foreign thermostable analogues. You can try to use conventional ceramic or metal film capacitors, however, if the temperature fluctuates, you will have to adjust the device more often.

Microammeter - any type for a current of 100 μA with zero in the middle of the scale. Small-sized microammeters, for example, type M4247, are convenient. You can use almost any microammeter, and even a milliammeter - with any scale limit. To do this, you need to adjust the values ​​of resistors R15-R17 accordingly.

Quartz resonator Q - any small-sized watch quartz (similar ones are also used in portable electronic games).

Switch S1 - any type, small-sized.

The sensor coils are made on round ferrite cores with a diameter of 8 mm (used in magnetic antennas of radio receivers in the CB and DV ranges) and a length of about 10 cm. Each winding consists of 200 turns of copper winding wire with a diameter of 0.31 mm, evenly and tightly wound in two layers in double lacquer-silk insulation. A layer of screen foil is attached over all windings. The edges of the screen are insulated from each other to prevent the formation of a short-circuited turn. The screen output is made with tinned single-core copper wire. In the case of an aluminum foil screen, this terminal is placed on the screen along its entire length and tightly wrapped with electrical tape. In the case of a screen made of copper or brass foil, the terminal is soldered.

The ends of the ferrite cores are fixed in fluoroplastic centering disks, thanks to which each of the two halves of the sensor is held inside a plastic pipe made of textolite, which serves as a housing, as is schematically shown in Fig. 34.

Rice. 34. Sensor-antenna design

The length of the pipe is about 60 cm. Each of the halves of the sensor is located at the end of the pipe and is additionally fixed with silicone sealant, which fills the space around the windings and their cores. Filling is carried out through special holes in the pipe body. Together with fluoroplastic washers, such a sealant gives the fastening of fragile ferrite rods the necessary elasticity, which prevents them from cracking during accidental impacts.

Setting up the device

1. Make sure installation is correct.

2. Check the current consumption, which should not exceed 100 mA.

3. Check the correct operation of the master oscillator and other elements of pulse signal generation.

4. Set up the oscillatory circuit of the sensor. Emitting - at a frequency of 4 kHz, receiving - at 8 kHz.

5. Make sure that the amplification path and the synchronous detector are operating correctly.

Working with the device

The procedure for setting up and operating the device is as follows. We go out to the search site, turn on the device and begin to rotate the sensor antenna. It is best in a vertical plane passing through the north-south direction. If the device sensor is on a rod, then you can not rotate it, but swing it as far as the rod allows. The indicator needle will deviate (compass effect). Using variable resistor R5 we try to minimize the amplitude of these deviations. In this case, the middle point of the microammeter readings will “move” and it will also need to be adjusted with another variable resistor R16, which is designed to set zero. When the “compass” effect becomes minimal, the device is considered balanced.

For small objects, the method of searching using a differential magnetometer does not differ from the method of working with a conventional metal detector. Near an object, the arrow can deviate in any direction. For large objects, the indicator needle will deviate in different directions over a large area.

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