High sensitivity metal detector. A simple sensitive metal detector. Is there any harm

I propose a circuit for a beat metal detector. The essence of the technical solution is that the search generator operates at a low frequency F p (of the order of tens of kilohertz), and the reference generator operates at a high frequency F 0 (of the order of megahertz) and is stabilized by quartz. The power circuits of all circuit nodes are decoupled by RC filters. The beat frequency is isolated by a phase detector on a D-trigger.

What does this give?

1. The low frequency of the search coil reduces the influence of weakly conducting media (damp earth, cement). The influence of conductive media increases sharply with increasing search frequency, which limits the sensitivity of the metal detector.

2. A high reference frequency allows the metal detector to achieve high sensitivity, since small relative changes in the search frequency cause large changes in the beat frequency. The reference frequency is stabilized, and this allows the sensitivity to be approximately doubled.

3. Good isolation of power circuits sufficiently weakens the mutual synchronization of the generators, both directly and through the phase detector. Moreover, for the same reason, it is not recommended to use free logic elements of microcircuits in other parts of the circuit, so they are connected to each other (DD1.3 and DD1.4, DD2.3 and DD2.4, DD3.2), but the outputs are unused.

4. The use of a D-trigger as a detector makes it possible to isolate beats at any integer ratio of the reference and search frequencies, and the amplitude of the selected signal is determined only by logical levels.

The formula for beat frequency F b is simple:

F b = F 0 -NF n, F p

where F 0 - reference frequency; F p - search frequency; N is an integer part of the frequency deviation, i.e. N = int(F 0 /F p).

In the diagram I used
F0 = 1000 kHz;
Fп = 50 kHz+11 kHz;

Any other frequencies can be used, depending on what sensitivity and stability are required, and what quartz is at hand.

A search LC generator is assembled on DD1.1. In it L1 is the search coil.

A reference crystal oscillator is assembled on DD2.1.

DD3.1 - phase detector. The reference signal is gated by the search signal at the C-input. The selected low-frequency signal is fed through the C6-R4 filter to headphones or a piezo speaker. Capacitor C1 sets the initial beat frequency (above or below zero beats - as is more convenient for searching). Capacitors C2, C4 are selected when setting up the search frequency generator, and C3, C5 are selected to match the frequency of the quartz resonator used. Microcircuits DD1, DD2 - type K561LA7 (LE5). DD3 - K561TM2 (or similar).

The search coil can have an arbitrary diameter, depending on the size of the search objects. It must be shielded with a non-magnetic material, and the screen should not form a short-circuited turn in the plane of the coil. I used a coil with a diameter of 55 mm and a height of 10 mm, wound on an open cylinder of copper foil. Its inductance is 4.5 mH. Approximately, the sensitivity of the metal detector at the indicated frequencies is such that it allows you to detect a diamagnetic object with a diameter equal to a quarter of the diameter of the coil at a distance of one and a half...two diameters from the coil.

A metal detector is a relatively simple device, the electronic circuit of which provides good sensitivity and stability.

A distinctive feature of such a device is its low operating frequency. The metal detector's inductors operate at a frequency of 3 kHz. This provides:

• on the one hand, a weak response to unwanted signals (for example, signals arising from the presence of wet sand, small pieces of metal, etc.);
• on the other hand, good sensitivity when searching for hidden water pipes and central heating routes, coins and other metal objects.

The metal detector generator excites oscillations in the transmitting coil at a frequency of about 3 kHz, creating an alternating magnetic field in it. The receiving coil is located perpendicular to the transmitting coil in such a way that the magnetic lines of force passing through it will create a small EMF. At the output of the receiving coil, the signal is either absent or very small.

A metal object entering the field of the coil changes the inductance value. In this case, an electrical signal appears at the output, which is then amplified, rectified and filtered.

Thus, at the output of the system there is a constant voltage signal, the value of which increases slightly as the coil approaches the metal object.

This signal is supplied to one of the inputs of the comparison circuit, where it is compared with the reference voltage that is applied to its second input. The reference voltage level is adjusted in such a way that even a small increase in the signal voltage leads to a change in state at the output of the comparison circuit.

This in turn operates the electronic switch. As a result of this process, an audio signal is sent to the output amplifier stages, alerting the operator to the presence of a metal object.

The electrical circuit diagram of the metal detector is shown in Fig. 3.38.

A transmitter, consisting of transistor VT1 and associated elements, excites oscillations in coil L1. The signals entering the L2 coil are then amplified by the D1 chip and rectified by the D2 chip, connected according to the amplitude detector circuit.

The signal from the detector goes to capacitor C9 and is smoothed by a low-pass filter, which consists of resistors R14, R15 and capacitors C10 and C11.

The signal is then fed to the input of the comparison circuit D3, where it is compared with the reference voltage set by variable resistors RP3 and RP4. Variable resistor RP4 provides quick and coarse adjustments, while RP3 provides fine adjustment of the reference voltage.

Rice. 3.38. Schematic diagram of a metal detector with a low operating frequency.

The generator, assembled on a transistor with one junction VT2, operates in continuous mode. However, the signal produced by it arrives at the base of transistor VT4 only when transistor VT3 closes. After all, being in the open state, this transistor shunts the output of the generator.

When a signal arrives at the input of microcircuit D3, the voltage at its output decreases, transistor VT3 closes, and the signal from transistor VT2 through transistor VT4 and the volume control RP5 goes to the output stage and loudspeaker.

The circuit uses two power supplies, eliminating the possibility of any feedback from the circuit's output to its sensitive input.

The main circuit is powered by a battery with a voltage of 18 V, which is reduced to a stable voltage of 12 V using the D4 chip. In this case, a decrease in battery voltage during operation of the circuit does not change the device settings.

The output stages are powered by a separate 9V power supply. Power requirements are quite low, so three batteries can be used to power the device. The output stage battery does not require a special switch, since in the absence of a signal the output stage consumes virtually no current.

First of all, you need to make 64 cuts in the strips and drill three installation holes.

Then on the back of the board you need to install:

• 20 jumpers;
• pins for external connections;
• two pins for capacitor C5.

Then you can install capacitors C16, SL7 and microcircuit D4. These elements form a 12 V power supply.

This cascade is checked by temporarily connecting a battery with a voltage of 18 V. In this case, the voltage on capacitor C16 should be 12 ± 0.5 V.

After this, you can proceed to installing the elements of the output stage: - resistors R23-R26;

• capacitors C14 and C15;
• transistors VT4—VT6.

The housing of the VT6 transistor is connected to its collector, so contact of the housing with adjacent elements and jumpers is unacceptable.

Since the output stage does not consume current in the absence of a signal, it is enough to check it by temporarily connecting a loudspeaker, a variable resistor RP5 and a 9 V battery.

Then you need to install resistors R20-R22 and transistor VT2, forming an audio signal generator.

Rice. 3.39. Printed circuit board and arrangement of elements.

When two power sources are connected, a sound background is heard in the speaker, changing with the position of the volume control knob.

After this, it is necessary to mount resistors R16-R19, capacitor C12, transistor VTZ and microcircuit D3 on the board.

The operation of the comparison circuit is checked as follows. Variable resistors RP3 and RP4 must be connected to the measuring input D3. This input is formed using two 10 kOhm resistors, one of which is connected to the positive +12 V supply rail, and the other to the zero rail.

Connect the second terminals of the resistors to pin 2 of the D3 chip. The jumper from this pin serves as a temporary connection point.

With rough tuning (both batteries are included), which is carried out by variable resistor RP4, the sound signal is interrupted at a certain position, while with fine tuning with variable resistor RP3 the signal should change smoothly near this position.

If these conditions are met, you can begin installing resistors R6-R15, capacitors C6-C11, diode VD3 and microcircuits D1 and D2.

Having turned on the power supply, you first need to check the presence of a signal at the output of the D1 chip (pin 6). It should not exceed half the power supply value (approximately 6 V).

The voltage on capacitor C9 should not differ from the output signal voltage of this microcircuit, although interference from the AC mains may cause a slight increase in this voltage.

Touching the input of the microcircuit (the base of capacitor C6) with your finger causes an increase in voltage due to an increase in noise level.

If the adjustment knobs are in a position in which there is no sound signal, touching the capacitor Sb with your finger causes the sound signal to appear and disappear.

This concludes the preliminary check of the performance of the cascades.

The final check and adjustment of the metal detector is carried out after the manufacture of the inductors. After preliminary checking the cascades of the circuit, you can install the remaining elements on the board, with the exception of capacitor C5.

Temporarily set variable resistor RP2 to the middle position. Attach the board to the L-shaped aluminum chassis through plastic washers (to eliminate the possibility of a short circuit) using three screws.

The chassis is secured in the control panel body with two bolts holding two clamps, which are designed to secure the control panel body to the finder rod.

The side of the chassis secures the power supplies in the chassis.

When assembling the remote control, make sure that the switch terminals on the reverse side of the variable resistor RP5 do not touch the board elements.

After drilling a rectangular hole, glue the speaker. The rod and connecting parts that form the finder head holder can be made from plastic tubes with a diameter of 19 mm.

The finder head itself is a plate with a diameter of 25 cm, made of durable plastic. Its internal part must be thoroughly sanded with sandpaper, which ensures good adhesion to the epoxy resin.

Making a transmission coil. The main characteristics of a metal detector largely depend on the coils used, so their manufacture requires special treatment.

Coils of the same shape and size should be wound on a D-shaped circuit, which is created from pins secured to a suitable piece of board. Each coil should consist of 180 turns of 0.27 mm enameled copper wire with a tap at the 90th turn.

Fig, 3.40. Metal detector coils: a - method of winding coils; 6—assembly diagram of finished coils.

Before removing the coils from the pins, they need to be bandaged in several places, as shown in Fig. 3.40, a.

Then each coil must be wrapped with strong thread so that the turns fit snugly against each other. This completes the production of the transmitting coil.

Making a receiving coil. The receiving reel must be equipped with a screen. The coil is shielded as follows. First you need to wrap it with wire, and then wrap it with a layer of aluminum foil, which needs to be wrapped with wire again.

This double winding guarantees good contact with the aluminum foil. There should be a small gap or gap in the wire windings and in the foil, as shown in Fig. 3.40, 6, preventing the formation of a closed turn around the circumference of the coil.

The coils made in this way must be secured using clamps along the edges of the plastic plate and connected to the control unit using a four-core shielded cable.

Connect the two central taps and the screen of the receiving coil to the neutral bus through shielding wires.

If you turn on a metal detector and a radio receiver located near the coil, you can hear a high-pitched whistle (at the frequency of the metal detector), caused by the pickup of an audio signal in the radio receiver. This indicates that the metal detector generator is working properly.

In this case, it does not matter what band the radio is tuned to, so you can use any cassette recorder instead to check it.

The working position of the coils is determined:

• or by the output signal of the metal detector, which should be minimal;
• or based on the readings of a search device (voltmeter) connected directly to capacitor C9.

The second option for adjusting the coils is much simpler.

The voltage across the capacitor should be approximately 6V. After this, the outer parts of the coils can be glued with epoxy resin, but the inner ones, passing through the center, should be left loose, allowing for final adjustment.

The final adjustment consists of setting the loose parts of the coils in such a position that non-ferrous objects, such as coins, cause a rapid increase in the output signal, and other objects cause a slight decrease.

If the required result is not achieved, it is necessary to swap the ends of one of the coils.

It should be remembered that the final adjustment or adjustment of the coils should be carried out in the absence of metal objects.

After installing and firmly securing the coils, you need to cover them with a layer of epoxy resin, then put fiberglass on them and seal the whole thing with epoxy resin.

After manufacturing the finder head, the following steps should be taken:

• integrate capacitor C5 into the circuit;
• set variable resistor RP1 to the middle position;
• adjust the variable resistor RP2 to the minimum output signal.

In this case, on one side of the middle position, the variable resistor RP1 ensures the recognition of steel objects, and on the other side - objects made of non-ferrous metal.

Each time the nominal value of the resistance of the variable resistor RP1 changes, it is necessary to re-configure the device.

In practice, a metal detector is a lightweight, well-balanced, sensitive device. During the first few minutes after turning on the device, there may be an imbalance of the zero level, but after some time it disappears or becomes insignificant.

Most metal detectors are built according to a zero-beat circuit, this is when there are 2 generators with a constant and a search frequency, while the frequency of the search generator depends on the inductance of the search coil. You can increase the sensitivity of a metal detector built using this method by raising the reference frequency to approximately 10 times the frequency of the search generator. Using this method, it was possible to obtain a metal detector capable of detecting a penny coin at a depth of up to 1 m.

The diagram of such a metal detector is shown in the figure. It uses 2 K561LA7 microcircuits, D1 has a search generator and an output amplifier, D2 has a reference generator (with a square resonator).
The search generator uses 2 elements D1.1 D1.2. The generation frequency is set by a circuit consisting of a search coil L1 and capacitors C1 C2 VD1. Varicap VD1 is used to adjust the frequency within small limits during operation. The adjustment itself is carried out by resistor R3, which changes the voltage on the varicap.
The generator mode (POS) is set by elements R2 C4 C5 C6. From the outputs of both generators, pulses arrive at D2.4 on which the mixer is made, and a frequency beat signal appears at its output. This signal goes to the D1.4 power amplifier and then goes to the sound emitter (headphones or Chinese headphones). Volume control R6.

The electronic part of the circuit is mounted in a foil PCB housing. In this case, the case must have a shielded partition between the microcircuits (between the generators). The search coil is wound on a ring made of cambric with an outer diameter of 15 mm. You can use a plastic tube or a soft plastic pin of the same diameter. In any case, the diameter of the ring should be 200mm. 50 turns of PELSHO 0.27 or PEL 0.27-0.35 are wound on the ring. Afterwards, the winding is wrapped with electrical tape and then the coil is shielded using aluminum foil.
The setting comes down to setting the L1C1 circuit to 100 kHz with the middle position of R3 (frequency control at pin 10 of D1.3).

Literature – RK

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We bought a metal detector and became a treasure hunter and a lover of instrumental searching. Congratulations, our regiment has arrived. However, a novice searcher immediately begins to have questions about setting up his metal detector, because it is difficult to immediately understand what and how to set up and what determines the correctness of a particular setting. In this article we will understand what the sensitivity of a metal detector is and how to adjust it depending on the search locations and other conditions.

Many people think that if I set the sensitivity of the metal detector to maximum, then the depth of detection of coins and treasures will increase. This is the very first misconception. The depth does not always increase; very often, with maximum sensitivity, the device begins to glitch and lag, show incorrect values ​​and generally behave inappropriately. At the slightest blow to the stems of plants, phantom signals appear, misleading the novice digger. And he digs and finds nothing. If you do this 10-20 times, your nerves will begin to wear off and a novice digger may give up such an exciting activity. Therefore, before setting the sensitivity to the maximum, understand what determines the correct sensitivity setting. By the way, this applies not only to cheap metal detectors, but also to more expensive ones.

General rules for adjusting sensitivity:

If you are looking for coins and other objects in a field, whether it is plowed or not, if there is very little metal debris in the field, then the sensitivity of the metal detector can be set to a value close to the maximum. In general, I don’t like to set the “feeling” to maximum, because when hitting rocks, strange signals may appear. Therefore, on my Minelab T34 I set the sensitivity to 7-8, even on the field. Although, if you don’t come across any trash metal in the field at all, and everything is in order with mineralization, then you can set it to maximum, and if you also add a powerful coil, then the search depth will increase noticeably.

The second factor on which sensitivity adjustment depends is the degree of soil mineralization. The higher the mineralization, the less you need to set the “feel”. We had one case when we went to dig in an ordinary-looking field, but both of our metal detectors began to “glitch” and show the devil. Having reduced the sensitivity, everything seemed to return to normal. The same can be said about search locations along electrical wires. Although entry-level models have no protection at all and it is generally impossible to walk along power lines.

By the way, when searching on the beach, it is also better to reduce the sensitivity to 70 percent of the maximum, since there is also mineralization there, and there is also a huge amount of garbage.

Next up is trash metal. We have all dug in places where we want to get out of there as quickly as possible, because the device rings incessantly, there is a huge amount of metal debris (corks, wire and other metal remains), but the place is promising; in the spaces between the corks and wire, some rather interesting finds emerge. That's why we dig in these trash cans. Also set the sensitivity of your detector to 60-70 percent, otherwise you will go crazy digging for garbage. Of course, first of all, the effectiveness of searching in littered areas depends on the correct choice of search coil, standard Mono coils on inexpensive detectors are excellent, you can also recommend the so-called “snipers” - small diameter coils (6 inches), with them you can search in a littered area - just beauty.

Field of activity (technology) to which the described invention relates

The development know-how, namely this invention of the author, relates to the field of physics and geophysics in terms of detecting conductive and ferromagnetic objects using induction coils that create an alternating magnetic field.

DETAILED DESCRIPTION OF THE INVENTION

The metal detector (8) contains a reference and tunable generators, a feedback amplifier, a transmitting coil connected to the output of the amplifier, and an indication device.

The metal detector (8) works as follows. When the transmitting coil of a tunable generator approaches a metal object, as a result of irradiation with an alternating magnetic field, eddy currents arise in it, creating a secondary magnetic field of the object, which reaches the transmitting coil of the tunable generator and induces an EMF signal in it, causing a change in the frequency of the generator and the activation of an indication device, which indicates the presence of a search object in the range of the transmitting coil.

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However, due to the sharp weakening of low-frequency magnetic fields with distance from the field source (according to the laws of physics) and the lack of their amplification, the depth of detection of objects in this metal detector is relatively small.

The objective of the present invention is to increase the sensitivity of the metal detector.

The problem is solved due to the fact that in a metal detector containing a reference and tunable generators and an indication device, the tunable generator contains a feedback amplifier and a transmitting coil and feedback circuit connected to its output, the tunable generator is additionally equipped with a receiving coil and an adder, the inputs of which connected to the output of the receiving coil and the output of the feedback circuit, and the output is connected to the input of the feedback amplifier.

Thus, due to the combination of features, the creation of a metal detector has been achieved, the sensitivity of which, and therefore the depth of detection of objects, is significantly higher than that of the prototype.

The claimed technical solution is illustrated in the drawing.

The block shown in Fig. 1, consists of the following elements:
1. Tunable generator
2. Reference oscillator
3. Feedback amplifier
4. Transfer coil
5. Take-up reel
6. Display device
7. Adder
In addition, to explain the operation of the metal detector, the block diagram of FIG. 1 additionally shown:
9. Subject of search
10. Alternating magnetic field of the transmitting coil.

11. Secondary field from the search subject
12. Residual field of the transmitting coil, inducing a residual EMF in the receiving coil.

works like this

In the metal detector (see block diagram in Fig. 1), the transmitting 4 and receiving 5 coils are mutually oriented and secured so that the residual voltage induced in the receiving coil 5 from the field of the transmitting coil 4 is close to zero, and the transmission coefficient of the feedback circuit 7 is chosen so small that the total voltage at the input of amplifier 3 is minimal, but sufficient for self-excitation of tunable generator 1.

When a metal object 9 appears in the alternating magnetic field 10 of the transmitting coil 4, the resulting secondary field 11 reaches both coils 4 and 5 simultaneously and induces in them a small EMF signal, which in the first case is summed directly with the high voltage on the transmitting coil, in the second - with commensurate low voltage at the input of amplifier 3. The phase of the EMF signal induced by the secondary field in the coil depends on the type of metal (magnetic, non-magnetic), the configuration of the object, its position, etc. and almost never coincides with the phase of the feedback voltage at the amplifier input. As is known, when summing alternating voltages of different phases, the phase shift of the total signal will be greater, the smaller the difference in amplitude of the signals. Since the amplifier has the property of keeping the phase relationship between the input and output unchanged, we find that a small EMF of the signal, summed at the input of the amplifier with a commensurately small feedback voltage, will lead to a significantly larger phase shift in the voltage on the transmitting coil and will also cause a significantly larger frequency shift tunable generator than the direct influence of the EMF signal on the transmitting coil 4, i.e. will lead to a significant increase in the sensitivity of the metal detector. In this case, the direct influence of the secondary field 11 of the object 9 on the transmitting coil 4 is relatively so small that it can be neglected.

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The circuit diagram of a sensitive metal detector is shown in Fig. 2. In it, the tunable generator 1 is made on the DD1 microcircuit and contains a transmitting coil L1 and a capacitor C1 for manual tuning. The reference oscillator 2 is made of a DD2 chip, a quartz resonator ZQ1 and a resistor R3. Feedback amplifier 3 is made on a DA1 chip, consisting of two elements DA1.1 and DA1.2. The receiving coil L2 is connected to the input of the first element DA1.1. The feedback circuit consists of resistors R5 and R6; the summation of the signals at the amplifier input is carried out using a adder made on resistors R4 and R5. The display device is made on DD3...DD8 microcircuits.

The claimed technical solution completely solves the problem facing the invention.

At present, a technical solution characterized by a set of claimed distinctive features is not known in the Russian Federation and abroad and meets the requirements of the “novelty” category.

The claimed technical solution is original, significantly simplifying and improving known technical solutions, optimizing the design, does not follow in an obvious way from the existing level of technology and meets the requirements of the “inventive step” criterion.

Currently, two prototypes have been manufactured and tested and the products are being prepared for launch into production.

The inventive metal detector can be implemented industrially using known technical means, technologies, materials and components and meets the requirements of the “industrial applicability” criterion.

The proposed solution significantly improves the main parameter of a metal detector - its sensitivity, and therefore the depth of detection of objects while maintaining other important properties, such as the discrimination of ferrous and non-ferrous metals, the effectiveness of a combination of optical and sound indication, etc.

Information sources

1. Measuring device, A.S. USSR N 393713, G 01 V 3/10, 1973

2. Metal detector, A.S. USSR N 1327033, G 01 V 3/11, 1987

3. Metal detector, A.S. USSR N 1422200, G 01 V 3/11, 1986

4. Radio Amator (Kyiv), 5 - 7, 1993 p. 30 V. Petrushenko, Metal detector with increased sensitivity.

5. Radio, N 10, 1994, p. 26, I. Aleksandrov, Metal detector with increased sensitivity.

6. Radio, N 8, 1990, p. 33, P.Sketeris, Three metal detectors on microcircuits.

7. Modeler-designer, N 4, 1996, p. 15, With electronics behind treasures (based on materials from the Bulgarian "Mlad Constructor").

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8. Beat metal detector, in the book. A.I. Shchedrin, Metal detectors for searching treasures and relics, M., "Arbat-Inform", 1998, p. 82.

Claim

A sensitive metal detector containing a reference and tunable generators and an indication device, wherein the tunable generator contains a feedback amplifier and a transmitting coil and a feedback circuit connected to its output, characterized in that the tunable generator is additionally equipped with a receiving coil and an adder, the inputs of which are connected to the receiving output coil and the output of the feedback circuit, and the output is connected to the input of the feedback amplifier.