I can say without a doubt that this is the simplest metal detector I have ever seen. Which is based on only one chip TDA0161. You will not need to program anything - just assemble and that's it. Also, its great difference is that it does not make any sounds during operation, in contrast to the metal detector on the NE555 chip, which initially squeaks unpleasantly and you need to guess about the metal found by the tone.

In this scheme, the buzzer starts beeping only when it detects metal. The TDA0161 chip is a specialized industrial version for inductive sensors. And metal detectors for production are mainly built on it, giving a signal when metal approaches the induction sensor.
You can buy such a microchip at -
It is not expensive and is quite accessible to everyone.

Here is a diagram of a simple metal detector

Metal detector characteristics

• Chip supply voltage: from 3.5 to 15V
• Oscillator frequency: 8-10KHz
• Current consumption: 8-12 mA in alarm mode. In the search state, approximately 1 mA.
• Operating temperature: -55 to +100 degrees Celsius

Consider a simple metal detector on a K561LA7 chip and a sound amplifier. Power is supplied by a voltage of 9 volts. Since the current consumption is small, the krone battery lasts for a long time. According to the characteristics, the device has an average detection depth, worthy for such a simple scheme. There are similar metal detectors based on K561LA9 microcircuits, but they do not give a significant increase in performance, so we prefer to assemble this simplified circuit.

In metal detection, the main role is played by a sensor consisting of a round coil, a housing and a connecting wire to the control circuit (Fig. 1).

The appearance of a metal sensor in the coverage area is reflected in the inductance of the coil, which, in turn, affects the frequency of the search circuit on the microcontroller. The final logical element of the microcircuit compares the reference frequency value and the frequency of the search circuit and outputs the difference through the amplifier in the form of a tonal sound in the speaker.

Sensor manufacturing

The schemes of metal detectors for different devices are completely different from each other. However, a well-assembled sensor can be used as a universal one for various metal detectors operating on the same principle of operation.

For the winding of the sensor, we use a varnished PEV or PEL wire with a diameter of 0.5 - 0.7 mm, which can be easily found in a store or old kinescope TVs and monitors (Fig. 2).

With a coil diameter of 20 cm, we wind 100 turns of wire. For other diameters, we change the number of turns, counting that at 25 and 15 cm of diameter, 80 and 120 turns are wound, respectively. After completing the winding, we wrap it tightly with electrical tape, leaving the beginning and end of the wire with a margin.

We make a Faraday screen to eliminate various interference in the coil and microcontrollers. It is necessary to wrap the coil over the electrical tape with food foil. At the end of the winding, we do not connect the foil and leave a gap of 2-3 cm. On top of the foil, we wind a little uninsulated wire of small cross section (Fig. 3).

In several places, you can solder the wire and foil. All this is again wrapped with electrical tape.

After the actions taken, we should get an isolated coil with two winding outputs and a screen output. We connect them with a shielded cable from video or audio equipment. We connect the cable screen with the wire from the foil, and the cable cores with the wires from the coil. We solder all this and securely isolate it with electrical tape. At the end of the cable we attach a plug with high-quality contacts. The best option is if they are gold plated or silver. The plug can be found in cables for various equipment, we also take the connector there.

It remains to make a housing for the coil. You can use two round discs made of dielectric material - plywood, thick cardboard or plastic. We place a winding between the disks. Then, with plastic fasteners, which can be purchased at a plumbing store, we tightly fasten these two discs. To search in the aquatic environment, you can seal the sensor with epoxy resin or special sealants.

On the upper disk, we fasten or glue ears made of plastic or other dielectric material. They will be needed for attaching to the bar (Fig. 4).

Schematic accessories

The following describes the main details and requirements for them necessary for high-quality assembly of the circuit:

1. Capacitors are recommended to be purchased at the radio store, but if you want to get them for free from old circuits, then measure the capacitance before use. The main requirement for them is temperature stability, this will save you from constant metal detector failures. Ceramic or mica are perfect. When assembling, do not forget to take into account the polarity of electrolytic capacitors - one or more stripes are drawn on the barrel in the minus side (Fig. 5). You will need the following capacitors: electrolytic 100 microfarad x 16 V - 1 pc.; 1000 pF - 3 pcs.; 22 nF - 2 pcs.; 300 pF - 1 pc.

1. Fixed resistors can be used old, as they do not lose their characteristics over time. Variables are best bought new to allow precise frequency tuning on the ICs. Particular attention should be paid to the contacts of the variable resistor, since according to the scheme, two contacts must be connected to each other, and experience shows that many beginners do not notice this. It is also necessary to ground their case to eliminate interference during adjustment. You will need 5 fixed resistors of 22 ohm, 1 kOhm, 4.7 kOhm, 10 kOhm, 470 kOhm and 3 variable resistors of 1, 5 and 20 kOhm.
2. Chip K561LA7 in a DIP package. The counting of the legs on the microcircuits starts from the top counterclockwise from the key - a special recess on the case. As an analogue, you can make a metal detector on the K561LE5 or CD4011 chip.
3. The KT315 transistor is very common in old radio equipment. But it can be replaced by many other transistors: KT3102, BC546, 2SC639 and similar low-power low-frequency transistors. We carefully study the conclusions of the transistor before soldering, for KT315 they are located from left to right from the front part - emitter, collector, base (Fig. 6):

1. We choose any low-power diode from domestic or imported manufacturers - kd522B, kd105, kd106, in4148, in4001 and others. Before soldering, call it a multimeter, so as not to confuse the anode and cathode.
2. Standard headphones from a phone or mp3 player, or a miniature speaker from old technology. In the case of using headphones, you can use a connector or direct soldering.
3. Battery krone 9 V and contacts for it (Fig. 7):

1. We select the connector for the sensor cable plug in advance, during the manufacture of the sensor.

After assembling all the necessary parts, you can safely proceed with their installation according to the scheme described below.

Mounting the control circuit

The electrical circuit consists of a K561LA7 microcircuit, its strapping for adjustment, an amplifier, power supply and a speaker. The microcircuit has 4 logical elements. Two of them create the desired frequency, the third plays the role of the search part. The final logic element compares both frequencies and, at different values, gives a positive signal to the amplifier, which supplies an amplified signal to the speaker.

The circuit of the metal detector on the microcircuit described above is shown in Figure 8.

It is very convenient to assemble electrical circuit diagrams on a breadboard with holes (Fig. 9). Or we make a home-made printed circuit board, shown in Figure 10. You can make a board using the laser ironing method or ordinary drawing. We produce persecution by any known method.

We solder parts and solder all remote parts with wires - regulators, headphone jack, sensors and batteries.

After assembling the circuit, we fix it in the case. We put the battery in there. Plastic, mounting, homemade wooden and other boxes of your choice are suitable as a case (Fig. 11).

For the three regulators and the sensor connector, appropriate holes must be made. You can add a switch in series to the battery and also bring it to the case. It is necessary to provide small holes for the speaker, or, in the case of headphones, firmly fix the connector.

The main condition for assembling the case is accessibility, for example, to change the battery, and, at the same time, tightness - from sudden rain. You can attach beautiful caps to the regulators, decorate the box and sign the regulators with a switch.

Assembling and configuring the device

When the sensor and control unit are ready, it is necessary to link them into a finished metal detector. For this you need a bar. It can be made from PVC pipes and adapters, which, by heating, are bent to the desired size and shape. You can also use an ordinary wooden pole, crutch or telescopic fishing rod. What materials to choose depends on your preferences - consider weight, flexibility and length. For convenience, you can build a handle and armrest, as well as make the bar collapsible (Fig. 12).

Next, we fix the sensor with ready-made ears to the bar. Use plastic fasteners, reliable glue or plumbing adapters. In the same way we fix the control unit.

To make adjustments, connect the battery and the sensor. Since metal detectors are sensitive devices, for proper adjustment it is necessary to remove all metal objects around. We turn it on and observe one of two options:

If after switching on there is perfect silence or a barely audible squeak, then there are two options:

a) Generators operate at the same frequency. Such cases are rare, but they do happen. Try turning the smooth R7 and coarse R8 controls. If the silence changes to a loud tonal sound, then the circuit is working. We return the controls to their initial position and try to achieve the best results with the R7 smooth control, for example, the complete absence of sound;

b) Circuit failure. We carefully recheck the entire circuit and radio components.

If there is a hum or high tone after switching on, then we try to reduce it by rotating the coarse adjustment knob R8, and having achieved a better result, we adjust R7. If the metal detector does not respond to the rotation of the controls, then the frequency of the reference oscillator is too different from the frequency of the search circuit. In this case, we try to catch the desired frequency by changing the capacitor C6 and the resistor R6.

The whole setup can be greatly simplified by an oscilloscope. The essence of the setting is to achieve the same or close frequency of pins 5 and 6 on the microcontroller. Frequency adjustment can be done as described above.

If you mastered the assembly of this device, you can safely try to assemble a more complex metal detector on three microcircuits or a microcontroller.

Even a novice radio amateur can make this design. At the same time, the metal detector has a fairly high sensitivity. Using the proposed device, it is possible to detect a copper coin with a diameter of 20 mm and a thickness of 1.5 mm at a depth of up to 9 cm.

The principle of operation of the metal detector is simple, it is based on comparing two frequencies. One of them is reference (from the reference generator), and the other is changing (from the search generator). Moreover, its deviations depend on the appearance of metal objects in the field of a highly sensitive search coil.

In modern metal detectors, to which the design under consideration can be quite reasonably attributed, the reference oscillator operates at a frequency that differs by an order of magnitude from that which occurs in the field of the search coil.

circuit diagram

The schematic diagram of the metal detector is shown in Figure 1, a. The reference oscillator is implemented on two logic elements ZI-NOT of the DD2 chip. Its frequency is stabilized and is determined by a ZQ1 quartz resonator (1 MHz).

Rice. 1. A simple metal detector on microcircuits: a - a circuit diagram; b - printed circuit board.

The search generator is made on the first two elements of the DD1 chip. The oscillatory circuit here is formed by the search coil L1, capacitors C2 and C3, as well as the varicap VD1. To tune to a frequency of 100 kHz, the potentiometer R2 is used, which sets the required voltage to the varicap VD1.

As buffer signal amplifiers, logic elements DD1.3 and DD2.3 are used, working on a mixer DD1.4. The indicator is a high-resistance telephone capsule BF1, capacitor C10 is used as a shunt for the high-frequency component coming from the mixer.

Details and design

The metal detector is powered by a 9 V direct current source, a Krona battery is used. Capacitors C8 and C9 successfully work as a filter.

The search coil requires special precision and attention in manufacturing. It is desirable to wind the coil on a vinyl tube with an outer diameter of 15 mm and an inner diameter of 10 mm, bent in the shape of a circle with a diameter of 200 mm.

The coil contains 100 turns of GTEV-0.27 wire. When the winding is done, the coil is wrapped with aluminum foil to create an electrostatic shield (reducing the effect of capacitance between the coil and ground).

When winding and wrapping with foil, it is important to prevent electrical contact between the winding wire and the sharp edges of the foil. In particular, “twisting obliquely” will help here.

To protect the aluminum coating itself from mechanical damage, the coil should additionally be wrapped with an insulating bandage tape. The diameter of the coil may be different. But the following rule applies.

The smaller the diameter of the search coil, the higher the sensitivity of the entire device, but the search area for hidden metal objects narrows. With an increase in the diameter of the coil, the effect is reversed.

Working with a metal detector

You need to work with a metal detector as follows. Having placed the search coil in close proximity to the ground, adjust the generator with potentiometer R2, and so that there is no sound in the telephone capsule. bugged.

When the coil moves above the surface of the earth (almost close to the latter), a metal object is found - by the appearance of sound in the telephone capsule.

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

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

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

The design and principle of operation of the device

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

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

Devices of this class have the following advantages:

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

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

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

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

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

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

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

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

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

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

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

Typical design

The structure of the metal detector includes the following components:

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

All devices included in the metal detector are waterproof.

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

Varieties of metal detectors

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

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

Operating frequency

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

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

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

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

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

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

How to assemble a homemade metal detector with your own hands

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

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

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

The simplest do-it-yourself metal detector

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

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

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

Metal detector without chips

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

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

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

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

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

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

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

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

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

A little about smartphones

There is an opinion that it is quite possible to make a metal detector from a smartphone. This is wrong! Yes, there are applications that install under the Android OS.

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

Radio constructor: A simple metal detector on a K561LA7 chip. (021)

This metal detector circuit from all simple circuits showed the best results. Using this device, you can detect both ferrous metals (fittings in the walls of rooms) and metal objects in the ground (both ferrous and non-ferrous). The detection depth depends on the size of the metal object (small objects are detected at a depth of up to 12 cm). The operation of the circuit is based on the beating of the frequencies of two generators assembled on the basis of the domestic K561LA7 microcircuit, consisting of four 2I-NOT logic elements (K561LA7 can be replaced with K561LE5 or an imported analogue of CD4011). It can be seen from the diagram that an exemplary generator is assembled on the elements DD1.3 and DD1.4, with the frequency of which the frequency of the search generator assembled on the elements DD1.1 and DD1.2 will be compared. Let's consider how the circuit elements work: The frequency of the exemplary generator is determined by the parameters of the capacitor C1 and the total resistance of the variable resistors R1 and R2 and lies in the range of 200 - 300 kHz. The frequency of the search generator is set by the parameters of the C2, L1 circuit (it is within 100 kHz), that is, it depends on the capacitance of the capacitor and the inductance of the coil and is constant (conditionally, because the frequency stability depends largely on changes in temperature, supply voltage, humidity). During the operation of the search generator, not only the fundamental frequency of 100 kHz is generated, but also multiple harmonics of 200 kHz, 300 kHz, 400 kHz, and so on. The higher the harmonic, the lower its level. When the exemplary oscillator (OG) is operating at a frequency of 300 kHz, the "necessary" harmonic of the search oscillator (PG) is the third, that is, also 300 kHz. If we set the OG frequency to 305KHz with resistors R2 and R3, and the PG frequency is 100KHz, then the third harmonic of the PG, equal to 300KHz (frequencies above 20KHz are no longer determined by ear), from the output of capacitor C4 is mixed with the frequency of the OG at the output of capacitor C3. Further, these frequencies are fed to the diode mixer VD1, VD2, assembled according to the voltage doubling scheme (in one half-cycle, the signals from the outputs of the generators pass through the diode VD1 and charge the capacitors C3 and C4, in the second half-cycle, the voltages from the outputs of the generators are added to the voltages of the charged capacitors C3 and C4 and come through the VD2 diode to the headphones T. The diode mixer, acting as a detector, emits a difference frequency of 305 kHz - 300 kHz = 5 kHz, which is heard in the headphones in the form of a tone signal. Higher harmonics are significantly inferior in signal strength and are no longer heard in headphones, and lower harmonics do not give such a difference in frequency change - when a metal object enters the zone of the receiving coil, its inductance changes slightly, which affects the frequency of the PG. For example, the frequency is not 100.000Hz, but 100.003Hz. A difference of 3 hertz is hardly audible, but at the third harmonic 100.003 Hz will be equal to 300.009 Hz, and the difference with the OG frequency will be 9 Hz, which is more noticeable by ear and increases the sensitivity of the device. Diodes VD1, VD2 can be any, but always germanium. C6 serves to bypass the high-frequency components of the signal at the output of the mixer. The headphones of the head phones must be connected in series (the photo shows the terminals of the telephone jacks for serial connection of standard stereo headphones). All these rules allow the most efficient use of the output signal without resorting to additional amplifiers that complicate our design. In our case, the volume of the signal does not affect the sensitivity of the device. The main thing in tuning is to set the beat frequency correctly and focus on its change. Now to the main element of our circuit - the search coil. The ability of the device to detect metal objects will depend on the quality of its manufacture.

The search coil (PC) consists of 50 turns of copper wire type PEV, PEL, PELSHO with a diameter of 0.2 - 0.6 mm, wound on a mandrel with a diameter of 12 - 18 cm. There are several ways to make a PC. You can draw a circle with a diameter of 12 - 18 cm on plywood, board, plywood, etc., hammer nails around the circumference, then wind a coil around the nails, tie it firmly in a circle with threads, then pull out the nails. You can wind the coil on any round plastic structure of the appropriate diameter (for example, a piece of plastic sewer pipe, the lower part of a plastic bucket, which are thrown away by stores after selling herring, pickles. The excess part is cut off. It is advisable to impregnate the coil wound in this way with varnish or paint (but not nitro! The solvent will damage the lacquer insulation of the coil wire) to fill the cavities between the turns, which can later get water in. After drying, the coil must be tightly wrapped with electrical tape over the entire surface. To improve the protective properties of the PC and reduce the effect of external electric fields on it, it must be shielded. You can immediately wind the coil on a copper or aluminum tube bent into a circle and sawn on the outside with a hacksaw or a “grinder” with a thin disk, but it’s easier to take aluminum foil for baking, cut it into strips and wrap the coil from the initial to the final taps with these strips, leaving a gap of about 1 - 2 cm not wound. Otherwise, a short-circuited turn will result, which will not allow the coil to work. Considering that not everyone has the opportunity to solder the "ground" wire to the aluminum screen, you can strip 3 - 8 cm of insulation from the wire by wrapping the aluminum screen with the bare end and wrapping it tightly with electrical tape. It is desirable to shield the insulated connecting wires from the coil to the board with aluminum foil, connecting it to the same ground wire in the same way as in the coil. You can start setting up the device after winding the PC before its impregnation and shielding. Everything else is an improvement of the device. If everything is assembled correctly, then after connecting the PC to the circuit and applying power (observe the polarity of the power supply connection and the correct installation of the microcircuit in the socket) in the headphones, when the variable resistor R2 “Roughly” is rotated, the beating frequencies of the generators will be heard. In the absence of special instruments (oscilloscope, frequency meter), the operation of the generators can be determined by any voltmeter connected instead of headphones. Having soldered capacitor C4 from the diode mixer, the voltmeter will show the work of the exhaust gas in the form of a voltage approximately equal to the supply voltage of the circuit. And vice versa, having unsoldered C3, we will see the operation of the SG according to similar readings of the voltmeter. The work of both is manifested in listening to the beat tone in the headphones. Resistor R2 allows you to tune the frequency of the OG in a wide range, which is manifested in repeatedly appearing beats in the headphones. Now we need to carefully check these beats, choose the most “powerful” (resistor R3 should be in the middle position). When checking each of the harmonics, the resistor R2 must be set in such a position that the “voiced” tone of the signal goes down. Further tuning must be carried out with the resistor R3 “Exactly” and ensure that the beat tone turns into wheezing and clicks. This position is the working one with maximum sensitivity. Next, we take an object made of ferrous metal and bring it to the coil - the signal tone should increase. When a non-ferrous metal object (aluminum, copper, brass) is brought to the coil, the signal tone should, on the contrary, decrease or completely break. If this does not happen or happens vice versa, it is necessary to rebuild the exhaust gas to a different harmonic and do it all over again. As soon as you have found the “necessary” harmonic, you need to remember the position of R2 and in the future work only with R3, tuning as much as possible to the working section of the beats. The more accurately you tune in to it, the higher the search results will be. After you understand the principle of operation, you can begin to improve the search coil. When assembling the circuit, the metal parts of the variable resistors R2, R3 must be connected to a common (negative) wire, otherwise the approach of the hand to the handle will affect the beat frequency. It is desirable, to reduce the influence of external factors, to place the device circuit in a metal case connected to a common