Small-sized sensitive electronic metal detector. Homemade metal detectors: simple and more complex - for gold, ferrous metal, for construction sites Setting the operating frequency

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 in it as a result of irradiation with alternating magnetic field Eddy currents arise, 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 the indication device, which indicates the presence of the 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 of the signal, which in the first case is summed directly with high voltage on the transmitting coil, in the second - with a commensurately 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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Fundamental electrical diagram 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 Russian Federation and abroad and meets the requirements of the “novelty” category.

The claimed technical solution is original, significantly simplifying and improving the known technical solutions, optimizes 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 distinguishing 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.

This metal detector is an improved version of a metal detector based on comparing the frequencies of two generators, one of which is a reference generator, and the second a search one - changes the frequency of its oscillations when approaching metal objects. The device can “distinguish” between non-ferrous and ferrous metals.

Schematic diagram

The reference oscillator is assembled on element DD1.1, and the search oscillator is assembled on elements DD2.1 and DD2.2. The oscillation frequency of the reference oscillator, determined by the data of its loop coil L1 and capacitors C1 and C2, and at the indicated ratings is 100 kHz (Fig. 1).

The frequency of the search oscillator, the oscillatory circuit of which is formed by the remote coil L2 and capacitors SZ-C5, is close to the frequency of the reference oscillator. It is smoothly changed by a variable capacitor SZ within one to two kilohertz.

Element DD1.2 performs the function of a cascade, serving to isolate alternating voltage between generators. Microcircuits DD1 and DD2 of the metal detector are powered from a source direct current GB1 through decoupling filters R6C8 and R7C9.

Element DD3.1 is a generator signal mixer. At its output, oscillations are formed with the sum and difference frequencies of the generators and their harmonics. A filter is designed to isolate difference signals, i.e., audio frequency. low frequencies(LF) R3C6.

This circuit design of the metal detector makes it possible to obtain generator beats with a frequency of several hertz.

To ensure that signals of such low frequencies can be heard on headphones, a sinusoidal, or more precisely, a triangular signal is converted into short pulses with a double repetition rate. This is achieved using a voltage comparator assembled on elements DD3.2 - DD3.4.

Rice. 1. Schematic diagram high-sensitivity metal detector based on three K561LE5 microcircuits.

During one period of the beat frequency, the comparator switches twice from one logical state to another. Formed by him square pulses differentiation is controlled by the C7R8 chain.

Therefore, phones connected to connector X2 receive short voltage pulses and the volume of the sound signal depends little on its frequency.

In phones, which can be either high-impedance or low-impedance, “clicks” are heard. Their volume is regulated by variable resistor R8 (it is combined with the power switch SA1).

All parts, except the connectors and the search generator loop coil, must be placed on a printed circuit board made of double-sided foil material (Fig. 2).

Installation is one-sided - from the side of the printed conductors. The foil on the other side, which is connected to the common power wire at the edges of the board, acts as a screen.

Details and design of the metal detector

It is better to place the circuit board and power source (Corundum battery) in a metal case of suitable dimensions, for example, soldered from foil PCB plates.

If the housing is a plastic box, then along the edges of the board, as well as in the places indicated in Fig. 1 with dashed lines, you need to solder vertically strips of copper foil 7-10 mm wide.

Chips K561ЛE5 can be replaced with K176ЛE5, K176ЛА7, K561ЛА7. Capacitor SZ - KP-180 or another, with a maximum capacity of 180-240 pF. Capacitors C8-C10 are oxide K50-6 or series K52, K53, the rest are KM, KLS.

Figure 2. Printed circuit board for a high-sensitivity metal detector based on K561LE5 microcircuits.

Resistor R8 is SPZ-Zv, the rest are BC, MLT. Connectors X1 and X2 - any small-sized ones. To increase thermal stability, capacitors C1, C2, C4 and C5 must be used with TKE no worse than MI500.

Coil L1, containing 300 turns of PEV-2 0.08 wire, must be wound on the frame of the IF circuit of the Alpinist-407 radio receiver.

It is recommended to perform the remote coil L2 of the search generator (Fig. 3) in the following sequence:

1. on a mandrel with a diameter of 240-250 mm, wind 30 turns of PEV-2 0.6 mm wire;
2. fasten the resulting tourniquet in 10-12 places with a thin, strong thread;
3. heating the coil over the flame gas stove to a temperature of 50-60 °C, impregnate with epoxy resin;
4. after the resin has cured, wrap the coil with varnish or (as a last resort) insulating tape;
5. screen the finished coil by wrapping it with thin copper foil so that a small, 5-10 mm long, open section of the coil screen is formed in the front part (you can, of course, also use aluminum foil);
6. Connect the finished remote coil and its screen (via connector X1) to the metal detector structure using a two-core shielded wire.

Rice. 3. Remote coil of the metal detector search generator.

Setting up the device

Setting up a metal detector should begin with setting up the reference oscillator and checking the functionality of the voltage comparator. To do this, set the rotor of capacitor C3 to the middle capacitance position and use the trimmer of coil L1 to change the frequency of the reference oscillator until a sound signal appears in the phone.

Then, using the same trimmer, you should achieve “zero beats” - “clicks” in the phone, following with a frequency of several hertz. It happens that this cannot be achieved. The reason for this may be problems in the comparator.

In this case, you need to check the functionality of the rest of the device - connect a high-impedance telephone (for example, TON-2) to the output of the DD3.1 element and use the same trimmer of the L1 coil to achieve an audio signal.

Otherwise, you will have to look for an error in the installation of generators or faulty parts.

Setting up the comparator consists of selecting resistor R9, shown in Fig. 1 by dashed lines. Its resistance can be in the range of 300 kOhm...1 MOhm.

If the output of the comparator (pins 10, 11 of the DD3 microcircuit) has a high voltage level, then this resistor is connected between pins 5 and 6 of the DD3.2 element and the common wire.

After adjusting the reference generator, the L1 coil trimmer must be fixed in the frame with a drop of glue. For the convenience of working with a metal detector, it is best to equip its remote coil with a wooden or plastic handle. In addition, you can make several remote coils of different diameters.

Looking for the best metal detector

Today we will introduce you to the most popular metal detectors in Russia in 2015. In this article, you will learn about the best ground metal detectors, which are designed specifically for searching for coins, ancient relics, pieces of iron, non-ferrous metals, etc. in the ground.

Rating of the most popular metal detectors in Russia

Metal detector models are arranged in the table as the average price increases, starting with the most inexpensive. All models are widely available at the time of writing (metal detectors that are difficult or impossible to find in stores now were not considered).

Model	Price	Description	Characteristic
JJ-Connect Adventure V500		Depth - from 12 cm. Reel 205 mm. 1 search mode. Mechanical adjustment. Dial indicator	The best inexpensive metal detector	8 out of 10
Garrett Ace 150		3 search modes. 1 operating frequency. Digital LCD display. Oval reel. Weight 1.2 kg	Metal detector for searchers of any age	8 out of 10
Bounty Hunter Discovery 2200 RU		1 operating frequency. Detection depth: coins up to 30 cm, larger objects up to 91 cm. Digital LCD display. Round concentric coil. Weight 1.09 kg	The best metal detector for finding coins	10 of 10
FISHER F-2		1 operating frequency. Detection depth: coins up to 21 cm, larger objects up to 90 cm. 8-inch round coil. Weight 1.2 kg	The most ergonomic among budget metal detectors	10 of 10
Garrett Ace 250		5 search modes. Detection depth up to 120 cm. Weight 1.2 kg	The most reliable metal detector for beginners	9 out of 10
Garrett Ace 350		5 search modes. Coin detection depth up to 35 cm. Maximum depth more than 1 m. Weight 1.2 kg	The best metal detector for advanced users	10 of 10
Minelab X-Terra 305		2 search modes. LCD display. Weight 1.3 kg	The best semi-professional metal detector	10 of 10
Minelab X-Terra 705		4 search modes. 3 operating frequencies. LCD display. Weight 1.3 kg	The best professional metal detector	10 of 10
Minelab Eureka Gold		2 search modes. 3 operating frequencies. Weight 2.2 kg	Best metal detector for gold	10 of 10

And now more about the best and most popular metal detectors:

1. The best inexpensive metal detector
JJ-Connect Adventure V500

Photo: pro-spec.ru

One of the best among the most inexpensive metal detectors on our market and the first in popularity among budget models. Its capabilities are sufficient to detect coins and jewelry (silver, gold) at shallow depths. The metal detector is suitable for beginners, for those who just want to try and are not ready to spend money on a serious device.

From reviews:

“No matter how many times the metal detector has been dropped and had to work in water, it doesn’t break. In winter it could withstand up to -8 °C, if it drops below, it just starts to glitch and go astray. Finds everything: iron in my practice - at a depth of up to 50 cm, non-ferrous metals are smaller. Convenient twisters. Very easy to use."

Pros:

• affordable price;
• ease of control (dial indicator, auto-adjustment);
• convenient multi-tone sound indication;
• flexible system of settings, adjustment to the type of soil;
• Suitable for a wide range of weather conditions;
• waterproof reel;
• built-in speaker;
• you can connect headphones;
• insensitive to falling.

Minuses:

• when the air temperature drops, they begin to malfunction;
• weakness- coil;
• small search depth;
• not suitable for serious searches.

2. Metal detector for searchers of any age
Garrett ACE-150

Photo: www.geodom.ru

Another metal detector suitable for a beginner. But the range of its capabilities and ease of use are already better. Has 3 search modes. Detects any metals, coins, jewelry. It can be customized for a specific target type. Able to work in littered areas. Suitable for adults and teenagers.

From reviews:

“I would like to note that it is quite expensive to go somewhere specially far to search. And this metal detector can be quickly disassembled and taken with you on vacation. And then, in a free minute, walk through unfamiliar places.”

“For me, this is the best metal detector. It can do almost everything that more respectable models can do. It distinguishes metals and reacts to them with 3 different sounds. The screen is also simple and intuitive.”

Pros:

• affordable price;
• ease of management and use;
• compactness;
• light weight;
• S-shaped 3-section bar with a comfortable armrest;
• multi-tone sound identification system;
• built-in speaker, you can connect headphones;
• oval coil in a sealed housing;
• indication of burial depth;
• 5 discrimination segments;
• battery charge indication.

Minuses:

• the control unit and screen are sensitive to moisture;
• small search depth;
• no volume control;
• no ground balance;
• does not determine the exact location;
• low selectivity.

3. The best metal detector for finding coins
Bounty Hunter Discovery 2200 RU

Photo: www.optivisions.ru

One of the best metal detectors if searching for coins is a hobby for you.

From reviews:

“For a first metal detector, the model is simply superb! Lightweight, easy to set up.”

“I bought it about six months ago and have already collected a small collection of relics. I sold them at a local online auction and almost completely paid for the device.”

“The metal detector is worth the money. It works without false alarms in both dew and rain. Good discriminator."

Pros:

• sensitivity adjustment;
• 4-tone sound indication;
• 2 search modes;
• discrimination of 9 segments with adjustment;
• ground balance;
• function to ignore unwanted objects.

Minuses: not detected.

4. The most ergonomic among budget metal detectors
FISHER F-2

Photo: spec-teh.com

If you are an amateur searcher and value not only the result, but also the search process itself, Fisher metal detector F-2 will suit you. Its design is thought out to make the search not only exciting, but also convenient. The ergonomic handle fits comfortably in the hand and protects against fatigue. Thanks to the coating, it prevents your hand from freezing in cold weather. Adjustable to suit any height.

From reviews:

“When I went out with this device for the first time (before that I went with a detector from another company), its manner of making sounds on everything it wanted surprised and disappointed me. But after some time I adapted. Although I will still say that the discrimination is unusual, you need to get used to it.”

“The rods hold well, without play, nothing hangs out anywhere, does not creak. The reel is very mobile, allows you to rummage through bushes, through puddles and in shallow water, no case is needed, it is very durable: it can withstand glancing impacts on stones, branches, and roots. There are no false alarms during impacts. Overall, the best metal detector for the hobbyist.”

Pros:

• easy to operate;
• reliable;
• easy;
• ergonomic;
• target type indication;
• 7 discrimination programs;
• sealed coil.

Minuses:

• no ground balance (balancing is automatic only);
• small detection depth;
• demanding on batteries.

5. The most reliable metal detector for beginners
Garrett Ace 250

Photo: www.doskaurala.ru

One of the leaders in reliability. It works almost flawlessly. For the first seasons of searching, this is a good solution.

From reviews:

“There was nothing difficult to learn. Turned it on. Set it up and off you go. The discriminator is average, but enough for a beginner.”

“Best suited for those who take a metal detector with them to a picnic or the beach in addition to their main outdoor activities. But you won’t be able to drag kids away from searching with this device by the ears))”

Pros:

• reliability;
• ease of use;
• easy;
• determination of burial depth in Pin-point mode;
• quick response to the target (you can quickly cover large areas).

Minuses:

• sensitive to cold;
• the reel quickly gets scratched on stones;
• sometimes an unnecessary background appears;
• small search depth;
• no sound control.

6. The best metal detector for advanced users
Garrett Ace 350

Photo: marvel-construction.com

A semi-professional model with a reinforced coil has improved (compared to other metal detectors in this price category) characteristics and allows you to find objects at greater depths.

From reviews:

“I’ve been walking with him since spring, both across the field and along the beach. False positives are few. In terms of depth, it’s very acceptable.”

"Very satisfied. There are no complaints about the reel. On the Oka I lowered it by 0.5 m - it works.”

Pros:

• comfortable;
• easy;
• reliable;
• powerful coil;
• allows immersion in water;
• improved detection of small objects;
• effectively screens out debris;
• long battery life.

Minuses:

• The weak point is the spring in the battery compartment.

7. The best semi-professional metal detector
Minelab X-Terra 305

Photo: detectorsource.tripod.com

Another semi-professional metal detector from Minelab is distinguished by reliable operation and minimal sensitivity to bad weather conditions, temperature changes, and electromagnetic interference. Many users call the X-Terra 305 the best in its price category.

From reviews:

“In 3 years of work I have never been disappointed.”

“The best metal detector, reliable, unpretentious. It takes both small objects (“scales”) and various caches and treasures - at a decent depth. Identifies metals accurately. There were never any crashes. The weight is perfectly balanced."

Pros:

• two identification systems (sound, graphics);
• manual ground balance;
• interference rejection;
• PinPoint mode;
• backlight;
• long battery life.

Minuses: not detected.

8. Best professional metal detector
Minelab X-Terra 705

Photo: tz-9fd.kxcdn.com

The metal detector is a legend. Great capabilities plus intuitive controls, light weight and efficiency.

From reviews:

“I took the 705, started working “in the field” and couldn’t help but think about how much time was lost with “beginner” devices! I tried to walk with it to already proven places - and a coin or two would come across. In one place it was as if I hadn’t walked at all.”

Pros:

• Ease of use;
• good depth;
• high level of discrimination;
• clear menu;
• backlight;
• uses batteries economically.

Minuses:

• high price;
• There is no waterproof reel included.

9. Best metal detector for gold
Minelab Eureka Gold

Photo: www.minelab.com

Professional, very sensitive metal detector. Suitable for detecting gold, even the smallest particles, in any environment. Hence both its advantages and disadvantages.

From reviews:

“The device is very good, but... it is very difficult to master it on your own. And I advise you to use it only away from garbage, cars and electrical interference; it is useless in the city and suburbs.”

“The device is specific. Suitable for a person with very sensitive hearing and nerves of steel. It is highly advisable to understand the physics of the search process, otherwise your hands will fall off the shovel. I advise you to immediately buy a voice recorder and record all sounds for comparison with the standard. Immediately push the control unit under your elbow, you will get less tired.”

Pros:

• high sensitivity;
• ground balance;
• metal recognition by sound.

Minuses:

• high price;
• decent weight (more than 2 kg);
• very high susceptibility.

Let's sum it up

Among the ground metal detectors there are three: large groups devices: for beginners, semi-professional and high-level, for professionals. In our article we looked at best models from all three groups. However, professional equipment is presented here only to help readers navigate. After all, experienced search engines have probably already chosen the most suitable model. But for beginners, we hope our rating of the best metal detectors will be useful. Good luck with your search! And remember: after all, it is not the detector that is searching, but the person.

Radio circuits for domestic use

Somov M.P.
Radioconstructor, 2000, No. 10, pp. 34-35

First of all, I would like to dispel the preconceived opinion of others towards a person with a metal detector in his hand. In the eyes of many, such people may look ridiculous (here, they say, a person is trying to find a treasure!), and for some it is even worse - the person is associated with “black diggers” or “grave desecrators”...

In fact, most “treasure hunters” have much more mundane tasks - they are either collectors trying to find some kind of artifact in the ruins of an ancient building or, even more often, just ordinary scrap metal in order to earn a little extra money in our difficult times...

As a rule, most metal detectors are built on the same principle - two generators: one with a reference frequency and a search one. The frequency of the search generator changes from external influences - when the external search coil approaches a metal object.
The frequencies of the generators are set approximately the same (usually about 100 kHz). A slight detuning of the search generator, which is necessary for the operation of the device, is carried out using a variable capacitor or varicap.
The device triggers when the frequencies of these two generators do not match; that is, at the same frequency the device will be silent. If the difference in frequencies is at least 10 Hz, beats with this frequency appear on the comparison device, which trigger the detector’s sound alarm. Thus, the greater the difference between the two generators, the stronger the signal.

You can increase the sensitivity of a metal detector with this circuit design if you increase the frequency of the reference oscillator by 10 times compared to the search oscillator.
Thus, beats arise between the oscillations of the reference oscillator and the 10th harmonic of the oscillations of the search oscillator. As a result, changing the frequency of the search generator by at least 10 Hz will increase the frequency of difference pulsations by 100 Hz, and this is already more noticeable.

By applying this principle, it was possible to create a metal detector capable of detecting penny coin at a depth of 100 mm, and a larger object at a depth of 0.6-0.7 meters. The diagram of this metal detector is shown in the figure.

As you can see, the reference oscillator (and it is assembled on elements D2.1, D2.2 and D2.3) here operates at a fairly high frequency: even a quartz resonator had to be used in its frequency-setting circuit.
The search generator is assembled on elements D1.1-D1.3. Its frequency-setting circuit includes search coil L1. The frequency detuning of the search generator is performed using a varicap VD1; changes in parameters occur by adjusting the voltage with a variable resistor R3.

The search coil is made on a frame with an outer diameter of 200 mm. The thickness of the coil fin is 15 mm. It is wound with PELSHO-0.27 wire in the amount of 50 turns. If this wire is not available, you can use PEL-0.27...0.35.
Shielding foil is wound over the winding, but in such a way that the turns of the foil do not touch each other - that is, the winding is carried out in increments of 1...5 mm.
After which a layer of electrical tape is wound over the top.
It is advisable to place the finished coil on a frame to give it rigidity. For this, you can use any dielectric ring with an outer diameter of 200-250 mm.

If you are interested in the details and description of the device, you can read this in the source magazine by downloading it from our free library.

The circuit of this metal detector is simple, the active elements are one microcircuit, a transistor and several diodes. Despite the simplicity of the circuit, the metal detector is capable of responding to the approach of a copper coin (2.5 cm in diameter) to the coil at a distance of about 10 cm, and large objects made of non-ferrous metals at a distance of more than 1 meter!

Due to the small number of parts, the device has very low energy consumption (about 5 mA from a 9V Krona battery), ease of setup and no problems with any interference.

Schematic diagram of a simple metal detector

Its sensitive element is the oscillatory circuit of the generator, assembled according to the classical circuit on the transistor VT1. In this case, with the help of resistor R1, on which the depth of feedback depends, the generator is set to a special mode, very sensitive to the quality factor of the oscillatory circuit. The latter, in turn, depends on the environment in which the circuit is located.

The generator excitation depth determines constant pressure at point "A".

Since this voltage does not depend on the frequency, but only on the depth of excitation of the generator, this, unfortunately, does not make it possible to differentiate the detected metals by their magnetic properties, but thanks to this, high demands are not placed on the coil in terms of rigidity and other parameters to achieve the required sensitivity.

The constant voltage removed from point “A” is supplied through a shielded wire (of any brand) to a two-stage amplifier assembled on two op-amps that are part of the DA1 microcircuit.

It is advisable to connect capacitor C4 not to the common wire, but exactly as shown in the diagram - to the power supply positive to eliminate positive feedback.

Diodes VD1 and VD2 are silicon, with low reverse current. They are necessary for quick recovery amplifier modes when detecting large metal objects.

An audio frequency generator is assembled on the DA1.3 op-amp, the excitation of which occurs when the potential difference at the inverting and non-inverting inputs decreases.

Using diodes VD3 and VD4, the voltage at the inputs is limited, and a frequency control effect is achieved. This is very useful property, because with some skill, changing the frequency helps not only to determine the location of an object, but also to estimate its size. Diodes VD3 and VD4 must have a minimum voltage drop in direct connection (for example, you can use KD419).

An inverter is assembled on the DA1.4 element, which serves to increase the sound volume of the piezo emitter.

Generator setup

The generator is configured as follows. Instead of a constant resistor R1, a variable resistor with a resistance of 10 kOhm is installed, and its slider is moved to the position corresponding to the maximum resistance.

As its resistance decreases, the voltage at point “A” will also decrease, as shown in Fig. left. At some point it will stop decreasing and begin to increase. It is necessary to record the moment when the voltage at point “A” becomes minimal, measure the corresponding resistance variable resistor and be sure to replace it with a constant one with the same resistance.

The generator is located on a separate small board in close proximity to the coil. All generator parts must be precision.

The transistor can be of almost any p–n–p structure, even germanium with low gain.

It would be advisable to select capacitor C1 with a capacitance in the range of 5–20 nF (502 - 203) according to the maximum sensitivity of the circuit. Sometimes a good result occurs when connecting C1 not to winding II, which is the base one, but to the common wire. Capacitors C1 and C2 are preferably film capacitors with low TKE.

The circuit coil has a diameter of 14–16 cm, 260 turns of wire with a diameter of 0.2–0.5 mm in varnish insulation are wound on it with a tap from the one hundred and sixtieth turn. If the coil is assembled in good faith, then the sensitivity of the device will be noticeably higher (up to 15–20 cm for a coin).

The coil frame made of three circles of corrugated cardboard is very simple and quite rigid. The middle circle should be slightly smaller in diameter than the outer circles. In addition to rigidity, corrugated cardboard has good thermal insulation properties, which can be used to increase the stability of the device.

So, if the generator is assembled on chip elements (SMD), it can be easily placed between layers of cardboard, which will sharply reduce the impact on it of temperature drops and changes. It is not necessary to shield or insulate the rest of the device.

The device must be powered from a stabilized source. One of the stabilizer options is shown in Fig. higher. As a stabilizer, you can use an imported L7808 microcircuit with 8 V stabilization or a domestic analogue.

Option of the printed circuit board from the tracks.

P O P U L A R N O E:

A set of radio components and a printed circuit board for self-assembly of the “PIRAT” pulse metal detector

Metal detector PIRATE- one of the popular simple pulse metal detectors with good sensitivity. Even a novice radio amateur can assemble it.

If everything is soldered correctly, the coil is made without errors, all the parts are in good working order, then the circuit starts working immediately. Of the main settings, there is only one variable resistor.

SMALL RADIATION HAZARD DETECTOR WITH LIGHT INDICATION.

The device responds to changes in ionizing radiation in the environment around a person

space by increasing or decreasing the number of light flashes per unit of time, for example per minute. It is sensitive to the cosmic radiation background, warns a person about changes in the radiation situation in a given area and can be used, for example, as an indicator of the level of concentration of radioactive elements contained in materials. It may be useful to geologists, commanders of civil defense headquarters, and maintenance personnel in radiological institutions when carrying out work related to non-destructive quality control of materials using sources of ionizing radiation in conditions of increased noise, when sound alarms are ineffective. It is also useful for those who study and protect nature.

Operating principle of the air conditioner

A split system (air conditioner) is now available in almost every home. Let's figure it out - how does a split system (air conditioner) work?