Butterfly metal detector scheme from all sides. Metal detector "Butterfly": we collect manually

15.08.202327.05.2017

It is built on the principle of failure of synchronization of two coils. It is because of such an interesting design, outwardly resembling a butterfly, that the device got its name. Homemade is easy to assemble, and all components are easy to get.

And now about the most important thing, to what depth this miracle of technology is able to see. According to the author, a five-kopeck coin of the time of the USSR is found at a depth of up to 15 cm, which is a pretty good indicator. The device sees a metal lid from a jar at a depth of 30 cm, and the device detects such a massive object as a metal hatch at a height of 60 cm.

The metal detector can work in water, and one battery charge is enough for 20-30 hours of operation. The current consumption is 15 mA, the device weighs only 500 grams. The device can be adjusted to any soil. There is also a simple discriminator, the type of metal can be determined by what sound is fed into the headphones.

Materials and tools for manufacturing:
- two transistors KT315 (VS182, VS546...);
- two capacitors 1000 pf (1 nf or 102);
- two capacitors 10000 pf (10 nf or 103);
- stereo headphones;
- two resistances per 100 kOhm;
- one 3.7V mobile battery;
- wire type PEV or PEL with a diameter of 0.5-0.7 mm in varnish insulation;
- soldering iron and other tools;
- materials for the manufacture of the body.

Metal detector manufacturing process:

Step one. Device diagram
The circuit is not too rigid, so it is not necessary to accurately observe the number of turns, the rating of parts and the power source. The most important condition is the identity of the right and left sides. Interference during work is compensated, and the soil does not affect the quality of work. If the circuit is assembled symmetrically, it works immediately. The signals from the generators are fed to the X and Y plates.

The picture shows that the frequencies are the same.

When disruptions occur in the headphones, clicks begin to appear.

Now the generators are out of sync.

Step two. Board Assembly
To connect the parts, any glue that does not conduct current is used. The battery must always be charged and have a good capacity, otherwise the setting will constantly go astray. For these purposes, you can use one or two batteries from the mobile.

Capacitors should be used mica, not sensitive to temperature changes. If there is no textolite, the board can even be made of cardboard.

Step three. We wind the coil
The first turn of the coil is soldered to the minus from the battery. Next, you need to wind the coil, for example, on a pan, after ten turns you need to make a loop. Here the wire must be stripped and soldered to the emitter of the transistor, that is, the middle tap in the circuit. Next, you need to wind the last 20 turns, subsequently this wire is soldered to the connection of two capacitors 10,000 pF and 1000 pF. In this exact way, you need to assemble the second coil.

Step four. Metal detector housing
For the manufacture of the rod, you can use a metal-plastic water pipe, the rod must be made collapsible. Pipes 20 and 26 fit tightly into one another. Coils and circuits for protection against moisture are covered with oil varnish. Coils are glued at a distance of 10 cm.

Step five. Setting up a metal detector
When the metal detector is turned on, a squeak should appear in the headphones. If it so happens that the circuits are not assembled the same, some adjustment will be required. There are two ways to set up the metal detector, that is, to synchronize the generators. The first is that a large sheet of metal is brought to each coil, while under one of the coils the squeak should stop. On this coil, you need to bend the last turn inward. Or you can simply increase or decrease the distance between the coils until the sound is very quiet or stops altogether. On top of each coil, you need to glue tubes with ferrite, then you can adjust the metal detector with a ferrite rod to complete silence.

Searching for artifacts underground is a fairly popular activity. For some, this is a profession, someone is simply fond of archeology. There are numerous groups of treasure hunters: both romantics and pragmatic treasure hunters. All these people are united by one passion: the search for metal objects hidden at various depths.

If you have an accurate map showing the location of the treasure, or plans for fighting during the war, this does not guarantee success. You can shovel tons of soil, and the object you are looking for will lie quietly a couple of meters from the place of active search.

To search for gold, and less valuable metals, you will need a DIY metal detector.

Important information: The use of such devices is not prohibited by law. However, there are penalties for the consequences of such a search, concerning excavation, as well as the extraction of discovered objects.

We will not go into subtleties, this is the topic of another article. Simply put: if you find a golden ring on the beach, or a handful of Soviet coins in the forest, there will be no problems associated with the use of electronic search tools.

But for the extracted bronze spoons aged 100 years and older, you can get a real term or a large fine.

Nevertheless, devices for searching for metal objects in the thickness of the earth are freely sold, and those who want to save money can make a metal detector with their own hands at home.

The principle of operation of the device

Unlike ground detectors, which operate using waves of various frequencies or ultrasound, a metal detector (factory-made or self-made) operates with an inductance.

The coil emits an electromagnetic field, which is then analyzed by the receiver. If any object that conducts electric current or has ferromagnetic properties is in the coverage area, the field format is distorted. More precisely, under the influence of the active field of the coil, the object forms its own. This event is recorded by the receiver, and an alert is generated: the arrow of the device moves, a tone sounds, the indicator lights light up.

Knowing the method of work, you can calculate the electrical circuit, and create a powerful metal detector with your own hands. The complexity of the design depends only on the availability of the element base and your desire. Consider several popular options for how to assemble a homemade metal detector:

The so-called "butterfly"

This nickname is derived from the characteristic shape of the platform on which the inductors are located.

The location of the elements is related to the principle of operation. The circuit is made in the form of two generators operating at the same frequency. When connecting identical coils to them, an induction balance is created. As soon as a foreign object with electrical conductivity gets into the electromagnetic field, the field balance is destroyed.

Generators are implemented on NE555 chips. The illustration shows a typical diagram of such a device.

A coil for a metal detector (there are two of them, in the diagram: L1 and L2) is made by hand from a wire with a cross section of 0.5–0.7 mm². The ideal option is a transformer winding copper core in varnish insulation (removed from any unnecessary transformer). Characteristics do not have to be maintained with jewelry accuracy, on one condition: the coils must be the same.

Approximate parameters: diameter 190 mm, each coil has exactly 30 turns. The assembled product must be monolithic. To do this, the turns are seized with a mounting thread, and filled with transformer varnish. If this is not done, the vibration of the turns will throw the circuit off the tuned balance.

Wiring diagram

There are two manufacturing options:

given the small number of elements, you can assemble it on a breadboard by connecting the legs of the parts using conductors;
for accuracy and reliability, it is better to etch the board according to the proposed drawing.

Any soldering "on snot" can fail in the field, and you will be offended for wasted time.

Just like a transistor metal detector, the NE555 needs to be fine-tuned before use. The diagram shows three variable resistors:

R1 is designed to adjust the frequency of the generator and achieve that same balance;
R2 coarsely adjusts the sensitivity;
using resistor R3, you can set the sensitivity with an accuracy of 1 cm.

Info: This circuit cannot discriminate against metals. The seeker only makes it clear that the object exists. And by the tone of the signal (based on your experience), you can determine the approximate volume and depth of occurrence.

The power supply is quite universal: 9-12 volts. You can pick up a battery from an uninterruptible power supply, or assemble a power supply from AAA batteries. A good option is 18650 batteries (they are also used for vaping).

Butterfly setting

The principle of operation is described above, so let's just analyze the technology. We set all the resistors to the middle position, and ensure the disruption of the synchronization of the generators. To do this, we add the coils in a figure-of-eight, and move them relative to each other until the squeak develops into crackling. This is the breakdown of synchronization.

We fix the rings, and rotate the resistor R1 until a steady crackle appears at even intervals.

Bringing metal objects to the place of overlap of the coils (this is the search point), achieve a stable squeak. Sensitivity is adjustable by resistor R2.

It remains to adjust the resistor R3, which is used rather to correct the voltage drop in the power supply.

Mechanical

A do-it-yourself metal detector rod is made from a light plastic pipe or from wood. The use of aluminum is undesirable as it will interfere with the work. The circuit and controls can be hidden in a sealed enclosure (for example, a junction box for wiring).

The butterfly finder is ready to go.

Pirate

Another popular impulse model for beginner treasure hunters is the Pirate metal detector. It is also easy to do it yourself, detailed instructions in two versions:

It is desirable to bring the power closer to 12 volts, since the quality of work depends on the voltage. Printed circuit boards have already been tested, both options are shown in the illustration.

The coil (in this case, one) is made from the same 0.5 mm transformer wire. The optimal diameter is 20 mm, the number of turns is 25. Since we make the Pirate metal detector with our own hands, the external design fades into the background. Any materials that you were ready to throw away will do.

The handle is best made detachable, for ease of transportation. Remember that the use of metals is unacceptable.

Sensitivity is adjusted by two variable resistors in real time while searching. No fine tuning of the generator is required.

And if you manage to properly seal the hull, you can search for "treasures" in the beach surf, and even at the bottom of the reservoir.

Do-it-yourself underwater metal detector is more difficult to make, but it will give an undeniable advantage over competitors.

Performance Improvement

A do-it-yourself deep metal detector can be made from a ready-made Pirate at no additional cost. To do this, you can go in two ways:

Increasing the diameter of the inductor. This significantly increases the downward permeability, but reduces the sensitivity to small objects.
Reducing the number of turns of the coil with simultaneous adjustment of the circuit. To do this, you will have to sacrifice one coil for experiments. We remove (and cut off) turn by turn until we see that the sensitivity began to decrease. We remember the number of turns at the maximum parameters, and make a new coil for this circuit. Then we change the resistor R7 to a variable one, with similar power parameters. After conducting several experiments with sensitivity, we fix the resistance, change the variable to a constant resistor.

The Pirate metal detector can be assembled on the popular Arduino controller.

It is more convenient to use such a device, but there will still be no discrimination against metals.

Having figured out how to make a metal detector with your own hands for amateur tasks, we will briefly analyze several serious models.

Do-it-yourself Clone PI W metal detector

In fact, this is a cheaper version of the professional Clone PI-AVR finder, only a line of LEDs is used instead of an LCD display. This is not so convenient, but still allows you to control the depth of artifacts.

The best-priced option is based on the CD4066 chip and the ATmega8 microcontroller.

Of course, there is also a printed circuit board layout for this solution, only the control buttons are placed on a separate panel.

Programming ATmega8 is a topic for a separate article, if you have worked with such controllers, no difficulties will arise.

The powerful do-it-yourself Clone PI W metal detector allows you to find metal no more than a meter deep, though without discrimination.

Seeker "Chance"

A similar circuit on the ATmega8 controller is called "Chance". The principle of operation is similar, only the possibility of screening (partial discrimination) of ferrous metals has appeared.

A printed circuit board drawing has also been worked out, which can be successfully replaced with a classic “breadboard” for Arduino

"Terminator 3" do it yourself

If you need a homemade metal detector with metal discrimination, pay attention to this model. The scheme is quite complicated, but your work pays off with the coins found, which may turn out to be gold.

The peculiarity of the "Terminator" is the separation of the receiving and transmitting coils. To emit a signal, a 200 mm ring is made. 30 turns of wire are laid for it, then it is cut, as a result we get 2 half-coils with a total capacity of 60 turns (see diagram).

The receiving coil is located inside, 48 turns with a diameter of 100 mm.

The adjustment is made using an oscilloscope, after reaching the optimal results in amplitude, the windings are fixed in the housing by pouring epoxy resin.

Then an experimental practical setting of the discrimination switch is made. For this, real objects from various metals are used, and their type is applied on the mode switch (after verification).

Radio amateurs are working on an improved version of the Terminator 4, but there is no practical copy yet.

Simple metal detectors from ready-made electrical appliances

Outcome

Regardless of the complexity of the scheme, making a homemade metal detector will require enough time and effort from you. Therefore, out of curiosity, such devices are not made. But for professional use - this is a great alternative to factory copies.

Selection of parts and material

Thanks to the TL074 chip, which performs the entire process of the metal detector, we do not need many details. Let's describe each of them and the necessary material:

Chip MC1 TL074CN. The numbering of contacts is made from the key on the housing counterclockwise, when viewed from above (Fig. 2).

Variable capacitor C1 100 pF with a dielectric cap - to eliminate capacitance distortion during adjustment. When buying, try to get heat-resistant types.
As switch SA1, you can use any toggle switch or microswitch.
Battery krone G1. You can also use a more powerful power source, such as 8 AA batteries, which will give a total of 12 V. But this will significantly increase the size of the circuit.
Battery contact connector (Fig. 3). We find in unnecessary equipment, powered by a crown or 8 AA batteries.

Piezoelectric speaker PZT1 (Fig. 4). You can get it in cheap testers, children's toys and other devices with a tweeter.

Box for the scheme. Any dielectric box will do, up to a cardboard one.
Dielectric base for coils and rod attachment. Choose the length in the region of 35-40 cm, and select the width based on the method of fastening the rod. A plastic or plywood sheet will do.
The bar can be made from a light wooden stick or a PVC pipe. An important point is the manufacture of a dielectric material. Fastening should also be done - without metal components, such as glue or plastic fasteners.
About 60 meters of PEL wire with a diameter of 0.3 - 0.35 mm. We will wind the coils with them.
Any foil for winding shielding.
Well-shielded cable with at least two cores (fig. 5).

Electrical insulating varnish, threads and electrical tape.

Assembling a metal detector

The scheme of the metal detector is shown in Figure 6.

When assembling on a printed circuit board, the circuit board diagram and the program for its processing and printing can be downloaded from the link. When printing, put a tick on the mirror image. The printed circuit board is also shown in Figure 7.

When assembling by surface mounting, you can glue the microcircuit with the case to the box. Next, we connect the parts according to the diagram with a thin insulated wire. For the switch, speaker and variable capacitor, we bring out the wires to fix the parts on the box. Solder the connector for the battery to the circuit. We fix the scheme with glue.

On the box we make holes for a variable capacitor and a switch. We fix them with glue or silicone. We glue the speaker from the inside to the case and make holes for good audibility. We make the connection of body parts with the circuit. We fix the body on the bar in any convenient way. You can use metal fasteners.

We make a connection of the rod with a dielectric base for windings. You can use liquid nails or plastic fasteners.

The next step is to assemble the sensor - a part of a metal detector that includes inductors and a mount for them. If you follow the article, then you can easily assemble a sensor with your own hands that does not differ in quality from the sensors of complex home-made metal detectors. Both windings must be completely identical and have a mirror image. Let us describe the assembly of the first coil. Then, by analogy with the first, assemble and connect the second.

We find a round object with a diameter of about 12 cm. We wind 70 turns with a prepared wire. We wind carefully - without bends, fractures and damage to the wire insulation. Leave the two ends of the winding aside. We fix the winding at a distance of 3-5 cm with threads. We impregnate the winding with varnish. After drying, we isolate the winding with electrical tape and wrap it with foil. The foil must never close at the end of the circle! We solder the wire around the circumference to the foil and bring it out (Fig. 8), after which we again isolate the winding with electrical tape. We make the winding conclusions of the minimum length.

We take the prepared cable. We solder both ends of the coil with the cable cores, and solder the wire from the foil with the cable screen. We reliably isolate and insert the cable into the box with the board, after which we make connections according to the diagram. The shielded output must be connected to the minus of the board. We fix the cable along the entire length of the rod except for the base. This will eliminate interference and failures in setting up the metal detector. After we figured out with one coil, we make the second one in the same way.

The coils will be fixed when setting up the metal detector.

Device adjustment

The last step in the assembly is to fix the coils. The Butterfly metal detector is very sensitive, so we remove all metal objects within the range. Do not forget about jewelry on the body - watches, earrings and more. We insert the battery and turn on the metal detector. We start with the location of the coils on the base. The optimal distance between them is 10 cm. If the metal detector is silent, this indicates a malfunction of the circuit or successful synchronization of the frequency generators. We bring a metal object to the sensor. The appearance of distortion or clicks in the dynamics indicates a working circuit. If the metal detector does not react in any way to metal objects, then we check the correctness of the connection of the parts and the serviceability of the battery.

Our task is to fix the windings in a position in which the speaker is silent or produces single rare clicks. After achieving this result, we fix the coils with glue in this position. We fix the sensor cables to the base of the rod to ensure their immobility. You can cut the edges of the base along the contour of the coils.

The last step is to add a device to adjust the timing in the field. We fix plastic tubes on top of both coils and insert iron rods into them. Rods and tubes are selected in such a way as to exclude unauthorized movement of iron. After installing them, we again achieve the ideal setting by moving the rod in one of the tubes (Fig. 9).

While working with a metal detector, you can experiment with a variable capacitor C1 to select the optimal detection distances and response quality. If synchronization fails, we use the movement of ferrite rods to adjust.

For beginners, a simple two-coil metal detector circuit is based on the principle of disrupting the synchronization of these same generators. There are many different metal detector circuits on the Internet, but it is quite difficult for a novice radio amateur to properly configure and adjust the assembled circuit without an oscilloscope. We propose to assemble a simple metal detector with your own hands with simple settings.

Metal detector characteristics

a coin the size of 5 kopecks of the USSR detects up to 15 cm;
metal lid from a jar - up to 30 cm;
metal manhole - up to 60 cm.
can work in water.
One battery charge is enough for 20 - 30 hours of operation.
current consumption - 15 mA.
Weight 500 grams.

It adapts to the appropriate ground and even with good sealing of the coils it can work in water. By a certain sound in the headphones, you can determine the type of metal.

The circuit is not critical to the power supply, the number of turns and the ratings of the parts.

There is only one condition - the left and right parts of the circuit must be the same!

Interference is compensated, the ground does not affect. Sensitivity does not depend on the scheme, but on the very physics of measurements. We accumulate phase shift.

Symmetrically assembled circuit works immediately. But it's interesting to see.

When powered by 4 V on the coils, about 40 V can be applied directly to the tube.
Signals from generators are fed to plates X and Y

Frequencies match

Here are the breakdowns.
There were clicks in the headphones.

The generators are out of sync.

We assemble the parts on any non-conductive glue.

If the battery is dead and runs out, then the setting will float away. The phone battery fits well.

For the construction, we use a metal-plastic water pipe. The rod can be detachable, pipes with a diameter of 26 and 20 fit tightly into one another. Capacitors with good temperature stability, mica - this is important.

Cover the coils and circuit with oil varnish. Between the coils 10 cm. We put the coils at a distance of 10 cm and glue it - no need to move

The scheme of the metal detector "Butterfly" with one speaker

Source: majstin.narod.ru

Parts list:

Transistor - KT315 (VS182, VS546, VS547, etc.) - 2pcs;
Capacitor - 1000 pf (1 nf or 102) - 2 pcs;
Capacitor - 10000 pf (10 nf or 103) - 2 pcs;
Resistance - 100 kOhm - 2pcs;
Ordinary stereo headphones - 1 pc;
Power supply (from a cell phone) - 3.7 V - 1 pc;
Wire in lacquer insulation PEV, PEL, PETV, etc. with a diameter of 0.4-0.7 mm.

If you do not have all the necessary parts and board,

You can order them from us

all necessary printed circuit board parts.

How to wind coils?

Detailed explanation of coil manufacturing:0 - oh turn- this is the beginning of the coil (soldered to the track lower according to the scheme (battery minus). Then you wind the coil (for example, on a pan), as 10 turns made, twist the loop, strip the insulation and solder it to the middle outlet according to the scheme, i.e. to the emitter of the transistor ( it will be a withdrawal). Then roll some more 20 turns, cut off and solder the end of the wire to the connection of two capacitors 1000 pF and 10000 pF ( this is the end of the coil). The second coil is done in the same way.

Setting up the metal detector

When it squeaks, it's good! But still, it is not always possible (for various reasons) to make the same generators, therefore it is necessary to adjust one of them to the frequency of the second.

Coarse tuning with the last turn of the coils

We take a piece of aluminum or a larger tin can. We bring to the coils in turn. Above one, the squeak is even higher, above the other it falls silent. On the coil where it stops, we bend the last turn inward (or move the coils, reduce or increase the distance between them), achieve the quietest squeak or until the signal stops. Ferrite tubes must be glued on top of each coil. You can then adjust the ferrite rod to complete silence.

Ferrite core tuning

To do this, take a tube and a ferrite rod, fasten next to one of the coils. Move the ferrite rod in the tube until you synchronize the generators. Ferrite rod with a coil from an old radio (maybe a tube one). Wind up all the wires, leave the frame with the ferrite core.

Video instruction for setting up a metal detector

Can be adjusted with aluminum strips glued to the coil inside or outside. For tuning, we put a strip of aluminum or just canning lids on both coils and move them, achieving the lowest buzz tone.

If clicks are clearly heard in the headphones, this means that the sensitivity of the device is at its maximum.

Another option for frequency tuning is to install a variable capacitor parallel to the 1000 pF capacitor, for example, from an old radio.

Mounting a metal detector

In the absence of a board made of foil textolite or getinax, boards can be made of cardboard.

Photo of manufacturing options for the printed circuit board of this metal detector

Photo of a metal detector by Vladimir Nosikov

Photo of a metal detector by Dmitry Kosnov

A metal detector or metal detector is designed to detect objects that differ in their electrical and / or magnetic properties from the environment in which they are located. Simply put, it allows you to find metal in the ground. But not only metal, and not only in the ground. Metal detectors are used by inspection services, criminologists, military, geologists, builders to search for profiles under the skin, fittings, reconciliation of underground utility plans, and people of many other specialties.

Do-it-yourself metal detectors are most often made by amateurs: treasure hunters, local historians, members of military historical associations. They, beginners, are primarily intended for this article; the devices described in it make it possible to find a coin with a Soviet penny at a depth of up to 20-30 cm or a piece of iron with a sewer manhole about 1-1.5 m below the surface. However, this home-made device can also be useful on the farm during repairs or at a construction site. Finally, having found a centner or two of an abandoned pipe or metal structures in the ground and handing over the find for scrap, you can get a decent amount. And there are definitely more such treasures in the Russian land than pirate chests with doubloons or boyar-robber egg-pods with efimki.

Note: if you are not well versed in electrical engineering with radio electronics, do not be afraid of diagrams, formulas and special terminology in the text. The very essence is stated simply, and at the end there will be a description of the device, which can be made in 5 minutes on the table, not knowing how to not only solder, but twist the wires. But it will allow you to "feel" the features of the search for metals, and if interest arises, knowledge and skills will come.

A little more attention compared to the rest will be given to the Pirate metal detector, see fig. This device is quite simple for beginners to repeat, but in terms of its quality indicators it is not inferior to many branded models priced up to $300-400. And most importantly, it showed excellent repeatability, i.e. full performance when manufactured according to descriptions and specifications. Circuitry and the principle of operation of the "Pirate" are quite modern; There are plenty of guides on how to set it up and how to use it.

Operating principle

The metal detector operates on the principle of electromagnetic induction. In general, the metal detector circuit consists of an electromagnetic oscillation transmitter, a transmitting coil, a receiving coil, a receiver, a useful signal extraction circuit (discriminator) and an indication device. Separate functional units are often combined in circuitry and design, for example, the receiver and transmitter can work on one coil, the receiving part immediately highlights the useful signal, etc.

The coil creates an electromagnetic field (EMF) of a certain structure in the medium. If an electrically conductive object is in the area of \u200b\u200bits action, pos. And in the figure, eddy currents or Foucault currents are induced in it, which create its own EMF. As a result, the structure of the coil field is distorted, pos. B. If the object is not electrically conductive, but has ferromagnetic properties, then it distorts the original field due to shielding. In both cases, the receiver captures the difference between the EMF and the original one and converts it into an acoustic and/or optical signal.

Note: in principle, for a metal detector it is not necessary that the object be electrically conductive, the ground is not. The main thing is that their electrical and / or magnetic properties are different.

Detector or scanner?

In commercial sources, expensive highly sensitive metal detectors, e.g. Terra-N are often called geoscanners. This is not true. Geoscanners operate on the principle of measuring the electrical conductivity of the soil in different directions at different depths, this procedure is called lateral logging. According to the logging data, the computer builds on the display a picture of everything in the earth, including geological layers of various properties.

Varieties

Common parameters

The principle of operation of a metal detector can be implemented in technically different ways, according to the purpose of the device. Metal detectors for beach gold digging and construction and repair searches may look similar in appearance, but differ significantly in design and technical data. To properly make a metal detector, you need to clearly understand what requirements it must meet for this type of work. Based on this, the following parameters of search metal detectors can be distinguished:

Penetration, or penetrating power - the maximum depth to which the EMF of the coil extends in the ground. Deeper, the device will not detect anything at any size and properties of the object.
The size and dimensions of the search area is an imaginary area in the ground in which the object will be found.
Sensitivity is the ability to detect more or less small objects.
Selectivity is the ability to respond more strongly to desirable findings. The sweet dream of beach miners is a detector that only beeps for precious metals.
Noise immunity - the ability not to respond to EMF of extraneous sources: radio stations, lightning discharges, power lines, electric vehicles and other sources of interference.
Mobility and efficiency are determined by power consumption (how many batteries are enough), the device’s weight and dimensions, and the size of the search area (how much you can “probe” in 1 pass).
Discrimination, or resolution - gives the operator or control microcontroller the ability to judge the nature of the object found by the reaction of the device.

Discrimination, in turn, is a composite parameter, since there are 1, maximum 2 signals at the output of the metal detector, and there are more values that determine the properties and location of the find. However, taking into account the change in the reaction of the device while approaching the object, 3 components are distinguished in it:

Spatial - indicates the location of the object in the search area and the depth of its occurrence.
Geometric - makes it possible to judge the shape and size of an object.
Qualitative - allows you to make assumptions about the properties of the material of the object.

Operating frequency

All parameters of the metal detector are connected in a complex way and many relationships are mutually exclusive. So, for example, lowering the oscillator frequency makes it possible to achieve greater penetration and search area, but at the cost of increased power consumption, and worsens sensitivity and mobility due to an increase in coil size. In general, each parameter and their complexes are somehow tied to the frequency of the generator. That's why The initial classification of metal detectors is based on the operating frequency range:

Super-low-frequency (VLF) - up to the first hundreds of Hz. Absolutely non-amateur devices: power consumption from tens of watts, without computer processing, it is impossible to judge anything from a signal, vehicles are needed to move.
Low-frequency (LF) - from hundreds of Hz to several kHz. They are simple in circuitry and design, noise-resistant, but not very sensitive, discrimination is poor. Penetration - up to 4-5 m with power consumption from 10 W (the so-called deep metal detectors) or up to 1-1.5 m when powered by batteries. They react most sharply to ferromagnetic materials (ferrous metal) or large masses of diamagnetic materials (concrete and stone building structures), therefore they are sometimes called magnetic detectors. They are not very sensitive to the properties of the soil.
Increased frequency (IF) - up to several tens of kHz. More difficult than bass, but the requirements for the coil are low. Penetration - up to 1-1.5 m, C grade noise immunity, good sensitivity, satisfactory discrimination. Can be universal when used in pulsed mode, see below. On flooded or mineralized soils (with fragments or particles of rock that shield EMF), they work poorly or do not smell anything at all.
High, or radio frequency (HF or RF) - typical metal detectors "for gold": excellent discrimination to a depth of 50-80 cm in dry non-conductive and non-magnetic soils (beach sand, etc.) Power consumption - as before. n. The rest is on the verge of "failure". The efficiency of the device largely depends on the design and quality of the coil(s).

Note: mobility of metal detectors according to paragraphs. 2-4 is good: from one set of salt cells ("batteries") AA and without overworking the operator, you can work up to 12 hours.

Pulse metal detectors stand apart. Their primary current flows into the coil in pulses. By setting the pulse repetition rate within the LF, and their duration, which determines the spectral composition of the signal corresponding to the IF-HF ranges, you can get a metal detector that combines the positive properties of LF, IF and HF or is tunable.

Search method

There are at least 10 EMF search methods. But such as, say, the method of direct digitization of the response signal with computer processing is the lot of professional use.

A home-made metal detector is schematically built most of all in the following ways:

Parametric.
Receiving-transmitting.
With phase accumulation.
On the beat.

Without receiver

Parametric metal detectors in some way fall outside the definition of the principle of operation: they have neither a receiver nor a receiving coil. For detection, the direct influence of the object on the parameters of the generator coil is used - inductance and quality factor, and the structure of the EMF does not matter. Changing the parameters of the coil leads to a change in the frequency and amplitude of the generated oscillations, which is fixed in various ways: by measuring the frequency and amplitude, by changing the current consumption of the generator, by measuring the voltage in the PLL loop (phase locked loop, "pulling" it to a given value), etc.

Parametric metal detectors are simple, cheap and noise-resistant, but their use requires certain skills, because. the frequency "floats" under the influence of external conditions. Their sensitivity is weak; most of all are used as magnetic detectors.

With receiver and transmitter

The device of the transceiver metal detector is shown in fig. at the beginning, to an explanation of the principle of operation; the principle of operation is also described there. Such devices allow to achieve the best efficiency in their frequency range, but are complex in circuitry, require a particularly high-quality coil system. Transceiver metal detectors with a single coil are called induction. Their repeatability is better, because the problem of the correct arrangement of the coils relative to each other disappears, but the circuitry is more complicated - you need to highlight a weak secondary signal against the background of a strong primary.

Note: in pulsed transceiver metal detectors, the emission problem can also be eliminated. This is explained by the fact that as a secondary signal they “catch” the so-called. "tail" of the pulse re-radiated by the object. The primary pulse spreads due to dispersion during reemission, and a part of the secondary pulse is in the gap between the primary ones, from where it can be easily distinguished.

Click to Click

Metal detectors with phase accumulation, or phase-sensitive, are either single-coil pulsed, or with 2 generators, each working on its own coil. In the first case, the fact is used that during re-emission pulses not only spread out, but are also delayed. In time, the phase shift increases; when it reaches a certain value, the discriminator is triggered and a click is heard in the headphones. As you get closer to the object, the clicks become more frequent and merge into a higher pitched sound. It is on this principle that Pirate is built.

In the second case, the search technique is the same, but 2 strictly symmetrical electrically and geometrically generators work, each on its own coil. At the same time, due to the interaction of their EMF, mutual synchronization occurs: the generators work in time. When the overall EMF is distorted, synchronization breaks begin, audible as the same clicks, and then a tone. Two-coil metal detectors with a synchronization breakdown are simpler than impulse ones, but less sensitive: their penetration is 1.5-2 times less. Discrimination in both cases is close to excellent.

Phase-sensitive metal detectors are the favorite tools of resort miners. Aces of the search adjust their devices so that exactly above the object the sound disappears again: the frequency of the clicks goes into the ultrasonic region. In this way, on a shell beach, it is possible to find gold earrings the size of a fingernail at a depth of up to 40 cm. However, on soil with small inhomogeneities, watered and mineralized, metal detectors with phase accumulation are inferior to others, except for parametric ones.

By squeak

Beats of 2 electrical signals - a signal with a frequency equal to the sum or difference of the main frequencies of the original signals or multiples of them - harmonics. So, for example, if signals with frequencies of 1 MHz and 1,000,500 Hz or 1.0005 MHz are applied to the inputs of a special device - a mixer, and headphones or a speaker are connected to the output of the mixer, then we will hear a pure tone of 500 Hz. And if the 2nd signal is 200 100 Hz or 200.1 kHz, the same thing will happen, because 200 100 x 5 = 1,000,500; we "caught" the 5th harmonic.

There are 2 generators in the beat detector: reference and working. The reference oscillatory circuit coil is small, protected from extraneous influences, or its frequency is stabilized by a quartz resonator (simply, quartz). The contour coil of the working (search) generator is a search coil, and its frequency depends on the presence of objects in the search area. Before searching, the working generator is tuned to zero beats, i.e. until the frequencies match. As a rule, they don’t achieve a complete zero sound, but tune it to a very low tone or wheezing, so it’s more convenient to search. By changing the tone of the beats, the presence, size, properties and location of the object are judged.

Note: most often, the frequency of the search generator is taken several times lower than the reference one and works on harmonics. This allows, firstly, to avoid the mutual influence of generators, which is harmful in this case; secondly, to tune the device more precisely, and thirdly, to search at the optimal frequency in this case.

In general, metal detectors based on harmonics are more complicated than impulse ones, but they work on any ground. Properly made and tuned, they are not inferior to impulse ones. This can be judged at least by the fact that beach gold diggers do not agree in any way on what is better: impulse or beat?

Coil and more

The most common misconception of novice radio amateurs is the absolutization of circuitry. Like, if the scheme is "cool", then everything will be tip-top. With regard to metal detectors, this is doubly untrue, because. their operational advantages strongly depend on the design and workmanship of the search coil. As a resort prospector put it: "The findability of a detector should pull the pocket, not the legs."

When developing a device, its circuit and coil parameters are adjusted to each other until an optimum is obtained. A certain scheme with a “foreign” coil, if it works, will not reach the declared parameters. Therefore, when choosing a prototype for repetition, see first of all the description of the coil. If it is incomplete or inaccurate, it is better to build another device.

About coil dimensions

A large (wide) coil radiates EMF more efficiently and “enlightens” the ground deeper. Its search area is wider, which allows you to reduce the "finding by feet". However, if there is a large unwanted object in the search area, its signal will be "hammered" by a weak one from the desired trifle. Therefore, it is advisable to take or make a metal detector designed to work with coils of different sizes.

Note: typical coil diameters are 20-90 mm for finding rebar and profiles, 130-150 mm "for beach gold" and 200-600 mm "for big iron".

Monoloop

The traditional type of metal detector coil is the so-called. thin coil or Mono Loop (single loop): a ring of many turns of enameled copper wire with a width and thickness of 15-20 times less than the average diameter of the ring. The advantages of a monoloop coil are the weak dependence of parameters on the type of soil, the search area narrowing downwards, which allows, by moving the detector, to more accurately determine the depth and location of the find, and structural simplicity. Disadvantages - low quality factor, which is why the tuning “floats” during the search, susceptibility to interference and a vague reaction to the object: working with a monoloop requires considerable experience in using this particular instance of the device. It is recommended for beginners to make homemade metal detectors with a mono-loop in order to get a workable design without any problems and gain search experience with it.

Inductance

When choosing a circuit, in order to verify the authenticity of the author's promises, and even more so when designing or refining it yourself, you need to know the inductance of the coil and be able to calculate it. Even if you are making a metal detector from a purchased kit, you still need to check the inductance by measurements or by calculation, so as not to rack your brains later: why, everything seems to be in order, and not beeping.

Calculators for calculating the inductance of coils are available on the Internet, but a computer program cannot foresee all cases of practice. Therefore, in fig. given an old, decades-tested nomogram for calculating multilayer coils; a thin coil is a special case of a multilayer coil.

To calculate the search monoloop, the nomogram is used as follows:

We take the value of the inductance L from the description of the device and the dimensions of the loop D, l and t from there or at our choice; typical values: L = 10 mH, D = 20 cm, l = t = 1 cm.
According to the nomogram, we determine the number of turns w.
We set the laying coefficient k = 0.5, by the dimensions l (coil height) and t (its width) we determine the cross-sectional area of \u200b\u200bthe loop and find the area of pure copper in it as S = klt.
Dividing S by w, we get the cross section of the winding wire, and along it - the diameter of the wire d.
If it turned out d = (0.5 ... 0.8) mm, everything is OK. Otherwise, we increase l and t at d>0.8 mm or decrease at d<0,5 мм.

Noise immunity

The monoloop "catches" interference well, because arranged in exactly the same way as a loop antenna. You can increase its noise immunity, firstly, by placing the winding in the so-called. Faraday shield: a metal tube, braid or foil winding with a break so that a short-circuited coil does not form, which will “eat” all the EMI of the coil, see fig. on right. If there is a dotted line near the designation of the search coil on the original diagram (see the diagrams below), this means that the coil of this device must be placed in the Faraday shield.

Also, the screen must be connected to the common wire of the circuit. There is a catch for beginners here: the grounding conductor must be connected to the screen strictly symmetrically to the section (see the same figure) and connected to the circuit also symmetrically with respect to the signal wires, otherwise the interference will still “penetrate” into the coil.

The screen also absorbs some of the search EMF, which reduces the sensitivity of the device. This effect is especially noticeable in pulsed metal detectors; their coils cannot be shielded at all. In this case, an increase in noise immunity can be achieved by balancing the winding. The bottom line is that for a remote source of EMF, the coil is a point object, and emf. interference in its halves will overwhelm each other. A symmetrical coil may also be needed in circuitry if the generator is a push-pull or inductive three-point.

However, in this case, it is impossible to symmetricalize the coil with the usual bifilar method (see Fig.): when conducting and / or ferromagnetic objects are in the field of the bifilar coil, its symmetry is violated. That is, the noise immunity of the metal detector will disappear just when it is most needed. Therefore, the monoloop coil must be symmetrical by cross winding, see the same fig. Its symmetry is not broken under any circumstances, but winding a thin coil with a large number of turns in a cross way is hellish work, and then it is better to make a basket coil.

Basket

Basket coils have all the advantages of mono-loops to an even greater extent. In addition, basket coils are more stable, their quality factor is higher, and the fact that the coil is flat is a double plus: sensitivity and discrimination will increase. Basket coils are less susceptible to interference: harmful emfs. in crossing wires they cancel each other out. The only negative is that basket coils need a precisely made rigid and durable mandrel: the total tension force of many turns reaches large values.

Basket coils are structurally flat and voluminous, but electrically voluminous "basket" is equivalent to flat, i.e. creates the same EMF. The volumetric basket coil is even less sensitive to interference and, which is important for pulsed metal detectors, the pulse dispersion in it is minimal, i.e. easier to catch the variance caused by the object. The advantages of the original "Pirate" metal detector are largely due to the fact that its "native" coil is a voluminous basket (see Fig.), but its winding is complex and time-consuming.

It is better for a beginner to wind a flat basket on his own, see fig. below. For metal detectors "for gold" or, say, for the "butterfly" metal detector described below and a simple transceiver 2-coil, unusable computer disks will be a good mandrel. Their plating will not hurt: it is very thin and nickel. An indispensable condition: an odd, and nothing else, the number of slots. A nomogram is not required for calculating a flat basket; calculation is carried out in this way:

They are set with a diameter D2 equal to the outer diameter of the mandrel minus 2-3 mm, and take D1 = 0.5D2, this is the optimal ratio for search coils.
According to formula (2) in fig. calculate the number of turns.
From the difference D2 - D1, taking into account the flat laying factor of 0.85, the diameter of the wire in insulation is calculated.

How not to and how to wind baskets

Some amateurs take it upon themselves to wind bulky baskets in the manner shown in fig. below: make a mandrel from insulated nails (pos. 1) or self-tapping screws, wind according to the scheme, pos. 2 (in this case, pos. 3, for the number of turns, a multiple of 8; every 8 turns the “pattern” is repeated), then foam, pos. 4, the mandrel is pulled out, and the excess foam is cut off. But it soon turns out that the stretched coils cut the foam and all the work went soft. That is, in order to wind securely, you need to glue pieces of durable plastic into the holes of the base, and only then wind it. And remember: an independent calculation of a volumetric basket coil without appropriate computer programs is impossible; the flat basket technique is not applicable in this case.

DD coils

DD in this case does not mean long-range, but a double or differential detector; in the original - DD (Double Detector). This is a coil of 2 identical halves (shoulders), folded with some intersection. With an accurate electrical and geometric balance of the DD arms, the search EMF is pulled into the intersection zone, on the right in Fig. on the left - a monoloop coil and its field. The slightest inhomogeneity of space in the search area causes an imbalance, and a sharp strong signal appears. The DD-coil allows an inexperienced searcher to detect a shallow, deep, well-conducting object when a rusty can lies next to it and above.

Coils DD are clearly oriented "on gold"; all metal detectors with the GOLD marking are equipped with them. However, on finely heterogeneous and / or conductive soils, they either fail completely, or often give false signals. The sensitivity of the DD coil is very high, but the discrimination is close to zero: the signal is either marginal or not at all. Therefore, metal detectors with DD coils are preferred by seekers who are only interested in "being in the pocket."

Note: more details about DD coils can be found later in the description of the corresponding metal detector. They wind their shoulders DD or in bulk, like a monoloop, on a special mandrel, see below, or with baskets.

How to attach a coil

Ready-made frames and mandrels for search coils are sold in a wide range, but sellers are not shy about cheating. Therefore, many amateurs make the base of the plywood coil, on the left in the figure:

Multiple designs

Parametric

The simplest metal detector for searching for fittings, wiring, profiles and communications in walls and ceilings can be assembled according to fig. The ancient transistor MP40 changes without any change to KT361 or its analogues; to use pnp transistors, you need to reverse the polarity of the battery.

This metal detector is a parametric type magnetic detector operating at low frequencies. The tone of the sound in the headphones can be changed by selecting the capacitance C1. Under the influence of the object, the tone drops, unlike all other types, so initially you need to achieve a “mosquito squeak”, and not wheezing or grumbling. The device distinguishes wiring under current from “empty”, a hum of 50 Hz is superimposed on the tone.

The circuit is a pulse generator with inductive feedback and frequency stabilization by an LC circuit. Loop coil - an output transformer from an old transistor receiver or a low-power "Bazaar-Chinese" low-voltage power transformer. A transformer from an unusable power source of a Polish antenna is very well suited, in its own case, by cutting off the mains plug, you can assemble the entire device, then it is better to power it from a 3 V lithium tablet battery. Winding II in fig. – primary or network; I - secondary or step-down at 12 V. That's right, the generator works with transistor saturation, which provides negligible power consumption and a wide range of pulses, making it easier to find.

To turn the transformer into a sensor, its magnetic circuit must be opened: remove the frame with the windings, remove the straight jumpers of the core - the yoke - and fold the W-shaped plates in one direction, as on the right in the figure, then put the windings back on. With serviceable parts, the device starts working immediately; if not, you need to swap the ends of any of the windings.

The parametric scheme is more complicated - in fig. on right. L with capacitors C4, C5 and C6 is tuned to 5, 12.5 and 50 kHz, and quartz passes the 10th, 4th harmonics and fundamental tone to the amplitude meter, respectively. The scheme is more for an amateur to get drunk on the table: there is a lot of fuss with the setting, but there is no "flair", as they say. Provided as an example only.

transceiver

Much more sensitive is a transceiver metal detector with a DD coil, which can be easily made at home, see fig. Left - transmitter; on the right is the receiver. It also describes the properties of different types of DD.

This metal detector is LF; search frequency is about 2 kHz. Depth of detection: Soviet penny - 9 cm, canning tin - 25 cm, sewer hatch - 0.6 m. The parameters are “triple”, but you can master the technique of working with DD before moving on to more complex structures.

The coils contain 80 turns of 0.6-0.8 mm PE wire, wound in bulk on a mandrel 12 mm thick, the drawing of which is shown in fig. left. In general, the device is not critical to the parameters of the coils, they would be exactly the same and located strictly symmetrically. In general, a good and cheap simulator for those who want to master any search technique, incl. "for gold". Although the sensitivity of this metal detector is not high, but the discrimination is very good despite the use of DD.

To set up the device, first, instead of the L1 transmitter, turn on the headphones and make sure that the generator is working by the tone. Then L1 of the receiver is short-circuited and, by selecting R1 and R3, a voltage is set on the collectors VT1 and VT2, respectively, equal to about half the supply voltage. Next, R5 set the collector current VT3 within 5..8 mA, open L1 of the receiver and that's it, you can search.

With phase accumulation

The designs in this section show all the advantages of the phase accumulation method. The first metal detector mainly for construction purposes will be very inexpensive, because. its most labor-intensive parts are made ... of cardboard, see fig.:

The device does not require adjustment; integrated timer 555 - an analogue of the domestic IC (integrated circuit) K1006VI1. All signal transformations take place in it; search method - impulse. The only condition is that the speaker needs a piezoelectric (crystalline), a regular speaker or headphones will overload the IC and it will soon fail.

Coil inductance - about 10 mH; operating frequency - within 100-200 kHz. With a mandrel thickness of 4 mm (1 layer of cardboard), a coil with a diameter of 90 mm contains 250 turns of PE 0.25 wire, and a 70 mm coil contains 290 turns.

Metal detector "Butterfly", see fig. on the right, in terms of its parameters it is already close to professional devices: the Soviet penny is found at a depth of 15-22 cm, depending on the soil; sewer manhole - at a depth of up to 1 m. Acts on disruption of synchronization; diagram, board and type of installation - in fig. below. Please note, there are 2 separate coils with a diameter of 120-150 mm, not DD! They must not overlap! Both speakers are piezoelectric, as in the previous. case. Capacitors - thermostable, mica or high-frequency ceramic.

The properties of the "Butterfly" will improve, and it will be easier to set it up if, firstly, wind the coils with flat baskets; the inductance is determined by the given operating frequency (up to 200 kHz) and the capacitances of the loop capacitors (10,000 pF each in the diagram). Wire diameter - from 0.1 to 1 mm, the larger the better. The tap in each coil is made from a third of the turns, counting from the cold (lower according to the diagram) end. Secondly, if individual transistors are replaced with a 2-transistor assembly for K159NT1 dif-amplifier circuits or its analogues; a pair of transistors grown on a single chip has exactly the same parameters, which is important for circuits with a synchronization failure.

To establish the "Butterfly" you need to accurately adjust the inductance of the coils. The author of the design recommends moving apart and shifting the turns or adjusting the coils with ferrite, but from the point of view of electromagnetic and geometric symmetry, it would be better to connect trimmer capacitors of 100-150 pF in parallel with 10,000 pF capacitances and twist them when tuning in different directions.

The actual adjustment is not difficult: the newly assembled device beeps. We alternately bring an aluminum saucepan or a beer can to the coils. To one - the squeak becomes higher and louder; to the other - lower and quieter or completely silent. Here we add a little capacity of the trimmer, and remove it in the opposite shoulder. For 3-4 cycles, you can achieve complete silence in the speakers - the device is ready to search.

More about Pirate

Let's return to the famous "Pirate"; it is a pulse transceiver with phase accumulation. The scheme (see fig.) is very transparent and can be considered a classic for this case.

The transmitter consists of a master oscillator (MG) on the same 555th timer and a powerful key on T1 and T2. On the left - a variant of the ZG without an IC; it will have to set the pulse repetition rate of 120-150 Hz R1 and the pulse duration of 130-150 μs R2 on the oscilloscope. Coil L - common. The limiter on diodes D1 and D2 for a current of 0.5 A saves the QP1 receiver amplifier from overload. The discriminator is assembled on QP2; together they make up the dual operational amplifier K157UD2. Actually, the "tails" of the reradiated pulses are accumulated in the capacitance C5; when the "reservoir overflows", a pulse jumps at the output of QP2, which is amplified by T3 and gives a click in the dynamics. Resistor R13 regulates the filling rate of the "reservoir" and, consequently, the sensitivity of the device. More about "Pirate" can be found in the video:

Video: Pirate metal detector

and about the features of its settings - from the following video:

Video: setting the threshold of the Pirate metal detector

On the beat

Those who want to experience all the delights of the search process on beats with replaceable coils can assemble a metal detector according to the diagram in fig. Its peculiarity, firstly, is efficiency: the entire circuit is assembled on CMOS logic and, in the absence of an object, consumes very little current. Secondly, the device works on harmonics. The reference oscillator on DD2.1-DD2.3 is stabilized by ZQ1 quartz at 1 MHz, and the search oscillator on DD1.1-DD1.3 operates at a frequency of about 200 kHz. When setting up the device before searching, the desired harmonic is “caught” by the VD1 varicap. The mixing of the working and reference signals occurs in DD1.4. Thirdly, this metal detector is suitable for work with replaceable coils.

It is better to replace the ICs of the 176th series with the same 561st ones, the current consumption will decrease, and the sensitivity of the device will increase. It is simply impossible to replace the old Soviet high-resistance headphones TON-1 (preferably TON-2) with low-resistance ones from the player: they will overload DD1.4. You need to either put an amplifier like a "pirate" one (C7, R16, R17, T3 and a speaker on the "Pirate" circuit), or use a piezo speaker.

This metal detector does not require settings after assembly. Coils are monoloops. Their data on a mandrel 10 mm thick:

Diameter 25 mm - 150 turns of PEV-1 0.1 mm.
Diameter 75 mm - 80 turns of PEV-1 0.2 mm.
Diameter 200 mm - 50 turns of PEV-1 0.3 mm.

It doesn't get easier

Now let's fulfill the promise given at the beginning: we will tell you how to make, without knowing anything about radio engineering, the metal detector that you are looking for. The metal detector is “easier than simple” assembled from a radio, a calculator, a cardboard or plastic box with a hinged lid, and pieces of double-sided tape.

The metal detector “from the radio” is pulsed, however, to detect objects, it is not dispersion and not delay with phase accumulation that are used, but the rotation of the EMF magnetic vector during re-emission. On the forums, they write different things about this device, from “super” to “sucks”, “wiring” and words that are not customary to use in writing. So, in order to get, if not “super”, but at least a fully functional device, its components - the receiver and the calculator - must meet certain requirements.

Calculator we need the tiniest and cheapest, "alternative". They make them in offshore cellars. They have no idea about the standards for electromagnetic compatibility of household appliances, and if they heard about something like that, then they wanted to spit from the bottom of their hearts. Therefore, local products are quite powerful sources of impulse radio interference; they are given by the clock generator of the calculator. In this case, its strobe pulses on the air are used to probe the space.

Receiver you also need a cheap one, from similar manufacturers, without any means of increasing noise immunity. It must have an AM band and, absolutely necessary, a magnetic antenna. Since receivers with short wave (HF, SW) reception on a magnetic antenna are rarely sold and are expensive, you will have to limit yourself to medium waves (MW, MW), but this will make tuning easier.

We unfold the box with a lid into a book.
We stick strips of adhesive tape on the back sides of the calculator and the radio and fix both devices in the box, see fig. on right. The receiver - preferably in the lid, so that there is access to the controls.
We turn on the receiver, we are looking for a section free from radio stations and as clean as possible from radio noise by setting it to maximum volume at the top of the AM band (bands). For MW this will be around 200 m or 1500 kHz (1.5 MHz).
We turn on the calculator: the receiver should buzz, wheeze, growl; in general, give a tone. We do not remove the volume!
If there is no tone, carefully and smoothly adjust until it appears; we caught some of the harmonics of the calculator's strobe generator.
We slowly fold the “book” until the tone weakens, becomes more musical, or disappears altogether. Most likely this will happen when the lid is rotated about 90 degrees. Thus, we have found a position in which the magnetic vector of primary impulses is oriented perpendicular to the axis of the ferrite rod of the magnetic antenna and it does not receive them.
We fix the cover in the found position with a foam insert and an elastic band or supports.

Note: depending on the design of the receiver, the reverse option is possible - to tune in to the harmonica, the receiver is placed on the included calculator, and then, laying out the “book”, the tone is softened or disappears. In this case, the receiver will catch the pulses reflected from the object.

And what's next? If there is an electrically conductive or ferromagnetic object near the opening of the "book", it will re-emit probing pulses, but their magnetic vector will turn. The magnetic antenna will “smell” them, the receiver will again give a tone. That is, we have already found something.

Something strange in the end

There are reports of another metal detector "for complete dummies" with a calculator, but instead of a radio, supposedly 2 computer disks, a CD and a DVD, are needed. Also - piezo headphones (precisely piezo, according to the authors) and a Krona battery. Frankly speaking, this creation looks like a techno-myth, like a memorable mercury antenna. But - what the hell is not joking. Here's a video for you:

try it, if you wish, maybe something will be found there, both in the subject and in the scientific and technical sense. Good luck!

as an application

There are hundreds, if not thousands, of schemes and designs of metal detectors. Therefore, in the appendix to the material, we also give a list of models, in addition to those mentioned in the test, which, as they say, are in circulation in the Russian Federation, are not overly expensive and are available for repetition or self-assembly:

Clone.

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