Homemade metal detectors: simple and more complicated - for gold, ferrous metal, for construction. Homemade metal detector on a microcircuit Do-it-yourself metal detector on beats
Not so often, but still, losses happen in our lives. For example, they went into the forest to pick mushrooms for berries and dropped the keys. In the grass under the leaves, they will not be so easy to find. Do not despair: a home-made metal detector, which we will make with our own hands, will help us. So I decided to collect my first metal detector. Nowadays, few people decide to manufacture a metal detector. Home-made devices were popular twenty or twenty-five years ago, when there was simply nowhere to buy them.
Modern metal detectors from manufacturers such as Garrett, Fisher and many others have high sensitivity, metal discrimination, and some even a hodograph. They are able to adjust the ground balance, rebuild from electrical interference. Thanks to this, the detection depth of a modern metal detector per coin reaches 40 cm.
I chose a scheme that was not very complicated so that it could be repeated at home. The principle of operation is based on the difference between the beats of two frequencies, which we will pick up by ear. The device is assembled on two microcircuits, contains a minimum of parts, at the same time it has quartz frequency stabilization, thanks to which the device works stably.
Scheme of a metal detector on microcircuits
The circuit is very simple. It can be easily repeated at home. It is built on two microcircuits of the 176 series. The reference oscillator is made on la9 and stabilized by quartz at 1 MHz. Unfortunately, I didn’t have this, I had to set it to 1.6 MHz.
The tunable generator is assembled on a k176la7 chip. Varicap D1 will help to achieve zero beats, the capacitance of which varies depending on the position of the variable resistor R2 engine. The basis of the oscillatory circuit is the search coil L1, when it approaches a metal object, the inductance changes, as a result of which the frequency of the tunable generator changes, which we hear in the headphones.
I use the usual headphones from the player, the emitters of which are connected in series in order to load the output stage of the microcircuit less:
If the volume turns out to be too much, you can enter a volume control into the circuit:
Details of a homemade metal detector:
- Microcircuits; K176LA7, K176LA9
- Quartz resonator; 1 MHz
- Varicap; D901E
- Resistors; 150k-3pcs, 30k-1pc.
- Variable resistance resistor; 10k-1pc.
- Electrolytic capacitor; 50Mkf / 15 volts
- Capacitors; 0.047-2pcs, 100-4pcs, 0.022, 4700, 390
Most of the details are located on the printed circuit board:
I placed the entire device in an ordinary soap dish, shielding it from interference with aluminum foil, which I connected to a common wire:
Since there is no place on the printed circuit board for quartz, it is located separately. For convenience, I removed the headphone jack and frequency control from the end of the soap dish:
With the help of two clamps, I placed the entire metal detector unit on a segment of a ski pole:
The most important part remains: to make a search coil.
Coil for metal detector
The sensitivity of the device, resistance to false positives, the so-called phonons, will depend on the quality of the coil manufacturing. I would like to immediately note that the depth of object detection directly depends on the size of the coil. So, the larger the diameter, the deeper the device will be able to detect the target, but the size of this target should also be larger, for example, a sewer manhole (the metal detector simply will not see a small object with a large coil). Conversely, a small diameter coil is able to detect a small object, but not very deep (for example, a small coin or ring).
Therefore, I first wound a medium-sized coil, so to speak, universal. Looking ahead, I want to say that the metal detector was conceived for all occasions, that is, the coils must be of different diameters and they can be changed. To quickly change the coil, I put a connector on the rod, which I pulled out of an old tube TV:
I fixed the mating part of the connector on the coil:
As a frame for the future coil, I used a plastic bucket, which was purchased at a hardware store. The bucket diameter should be selected approximately equal to 200 mm. A part of the handle and bottom should be cut off from the bucket so that a plastic rim remains, on which 50 turns of PELSHO wire with a diameter of 0.27 millimeters should be wound. Attach the connector to the part of the remaining handle. We isolate the resulting coil with electrical tape in one layer. Then we need to shield this coil from interference. To do this, we need aluminum foil in the form of a strip, which we wrap on top so that the ends of the resulting screen do not close and the distance between them is approximately 20 millimeters. The resulting screen should be connected to a common wire. I also wrapped the top with duct tape. Of course, you can soak all this with epoxy glue, but I left it like that.
After testing the large coil, I realized that I needed to make a small one, the so-called sniper rifle, so that it would be easier to detect small objects.
The finished coils look like this:
Setting up the finished metal detector
Before you start setting up the metal detector, you need to make sure that there are no metal objects near the search coil. The setting consists in selecting the capacitance of the capacitor C2 in order to get the maximum level of beats that we hear in the headphones, since there are many harmonics in the signal (you need to select the strongest one). In this case, the engine of the variable resistor R2 should be as close to the middle as possible:
The rod I got out of two parts, the tubes were selected in such a way that they fit into each other very tightly, so I didn’t have to come up with a special mount for these tubes. An armrest and a handle were also made to make it convenient to carry out wiring above the ground. As practice has shown, this is very convenient: the hand does not get tired at all. When disassembled, the metal detector turned out to be very compact and literally fits in a package:
The appearance of the finished device looks like this:
In conclusion, I would like to say that this metal detector is not suitable for people who are going to work in the old way. Since it does not discriminate by metal, you will have to dig everything in sight. You will most likely be very disappointed. But for those who like to collect scrap metal, this device will help. Yes, and just as entertainment for children.
A metal detector is an electronic device for searching and distinguishing metals, metal objects that can be hidden at different depths under a layer of sand, earth, in the walls of rooms and various structures.
Schematic diagrams of metal detectors made on transistors, microcircuits and microcontrollers are given. A factory-made metal detector is a fairly expensive device, so making a homemade metal detector on your own can save a lot of money.
Schemes of modern metal detectors can be built according to different principles of operation, we list the most popular of them:
- Beat method (measuring the change in the reference frequency);
- Induction balance at low frequencies;
- Induction balance on spaced coils;
- impulse method.
Many novice radio amateurs and treasure hunters are wondering: how to make a metal detector yourself? It is advisable to start your acquaintance with assembling a simple metal detector circuit, this will allow you to understand the operation of such a device, get the first skills in searching for treasures and products made of multi-colored metals.
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The circuit shown in the figure is a classic metal detector. The operation of the circuit is based on the principle of superheterodyne frequency conversion, which is usually used in a superheterodyne receiver. A schematic diagram of a metal detector with an integrated ULF, it uses two radio frequency generators, the frequencies of which are 5.5 MHz. The first radio frequency generator is assembled on a BF494 type T1 transistor, frequency ...
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This metal detector, despite the small number of parts and ease of manufacture, has a fairly high sensitivity. It can detect large metal objects, such as a heating battery, at a distance of up to 60 cm, while small ones, for example, a coin with a diameter of 25 mm, at a distance of 15 cm. The principle of operation of the device is based on a change in frequency in the measuring generator under the influence of nearby metals and. ..
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The metal detector circuit does not have any features, it is simple and accessible for repetition even by beginner radio amateurs. As is often written in books and magazines, with proper installation and serviceable parts, it starts working right away. The printed circuit board of the device is shown in the figure, it is made for SMD components, all parts are installed from the foil side, and no drilling is required. Search coil manufacturing requires high...
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:
- 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.
- 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.
- 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.
- The top bar, as a rule, has a curved appearance. 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.
- 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.
I propose a scheme for a metal detector on beats. The essence of the technical solution lies in the fact that the search generator operates at a low frequency F p (of the order of tens of kilohertz), and the reference generator - at a high frequency F 0 (of the order of megahertz) and is stabilized by quartz. The power circuits of all nodes of the circuit are decoupled by RC filters. The selection of the beat frequency is performed by a phase detector on a D-trigger.
What does it give?
1. The low frequency of the search coil reduces the influence of weakly conductive media (wet 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 can achieve a high sensitivity of the metal detector, since small relative changes in the search frequency cause large changes in the beat frequency. The reference frequency is stabilized, and this allows you to approximately double the sensitivity.
3. Good decoupling of the supply 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, therefore they are interconnected (DD1.3 and DD1.4, DD2.3 and DD2.4, DD3.2), but the outputs are not used.
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 logic levels.
The formula for the beat frequency F b is simple:
F b \u003d F 0 -NF n, F p where F 0 - reference frequency; F p - search frequency; N is the integer part of the frequency deviation value, i.e. N \u003d int (F 0 / F p). In the diagram I used Any other frequencies can be applied, based on what sensitivity and stability are required, and what kind of quartz is at hand. A search LC generator was assembled on DD1.1. In it, L1 is a 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 it is more convenient for the search). Capacitors C2, C4 are selected when setting the search frequency generator, and C3, C5 are selected to match the frequency of the applied quartz resonator. Chips 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 non-magnetic material, and the shield must not form a short-circuited coil 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 detecting 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 or metal detector is designed to detect objects that differ in their electrical and / or magnetic properties from the environment in which they are located. Simply put, it allows you to find metal in the ground. But not only metal, and not only in the ground. Metal detectors are used by inspection services, criminologists, military, geologists, builders to search for profiles under the skin, fittings, reconciliation of underground utility plans, and people of many other specialties. Do-it-yourself metal detectors are most often made by amateurs: treasure hunters, local historians, members of military historical associations. They, beginners, are primarily intended for this article; the devices described in it make it possible to find a coin with a Soviet penny at a depth of up to 20-30 cm or a piece of iron with a sewer manhole about 1-1.5 m below the surface. However, this home-made device can also be useful on the farm during repairs or at a construction site. Finally, having found a centner or two of an abandoned pipe or metal structures in the ground and handing over the find for scrap, you can get a decent amount. And there are definitely more such treasures in the Russian land than pirate chests with doubloons or boyar-robber egg-pods with efimki. Note:
if you are not well versed in electrical engineering with radio electronics, do not be afraid of diagrams, formulas and special terminology in the text. The very essence is stated simply, and at the end there will be a description of the device, which can be made in 5 minutes on the table, not knowing how to not only solder, but twist the wires. But it will allow you to "feel" the features of the search for metals, and if interest arises, knowledge and skills will come. A little more attention compared to the rest will be given to the Pirate metal detector, see fig. This device is quite simple for beginners to repeat, but in terms of its quality indicators it is not inferior to many branded models priced up to $300-400. And most importantly, it showed excellent repeatability, i.e. full performance when manufactured according to descriptions and specifications. Circuitry and the principle of operation of the "Pirate" are quite modern; There are plenty of guides on how to set it up and how to use it. The metal detector operates on the principle of electromagnetic induction. In general, the metal detector circuit consists of an electromagnetic oscillation transmitter, a transmitting coil, a receiving coil, a receiver, a useful signal extraction circuit (discriminator) and an indication device. Separate functional units are often combined in circuitry and design, for example, the receiver and transmitter can work on one coil, the receiving part immediately highlights the useful signal, etc. The coil creates an electromagnetic field (EMF) of a certain structure in the medium. If an electrically conductive object is in the area of \u200b\u200bits action, pos. And in the figure, eddy currents or Foucault currents are induced in it, which create its own EMF. As a result, the structure of the coil field is distorted, pos. B. If the object is not electrically conductive, but has ferromagnetic properties, then it distorts the original field due to shielding. In both cases, the receiver captures the difference between the EMF and the original one and converts it into an acoustic and/or optical signal. Note:
in principle, for a metal detector it is not necessary that the object be electrically conductive, the ground is not. The main thing is that their electrical and / or magnetic properties are different. In commercial sources, expensive highly sensitive metal detectors, e.g. Terra-N are often called geoscanners. This is not true. Geoscanners operate on the principle of measuring the electrical conductivity of the soil in different directions at different depths, this procedure is called lateral logging. According to the logging data, the computer builds on the display a picture of everything in the earth, including geological layers of various properties. The principle of operation of a metal detector can be implemented in technically different ways, according to the purpose of the device. Metal detectors for beach gold digging and construction and repair searches may look similar in appearance, but differ significantly in design and technical data. To properly make a metal detector, you need to clearly understand what requirements it must meet for this type of work. Based on this, the following parameters of search metal detectors can be distinguished: Discrimination, in turn, is a composite parameter, since there are 1, maximum 2 signals at the output of the metal detector, and there are more values that determine the properties and location of the find. However, taking into account the change in the reaction of the device while approaching the object, 3 components are distinguished in it: Note:
mobility of metal detectors according to paragraphs. 2-4 is good: from one set of salt cells ("batteries") AA and without overworking the operator, you can work up to 12 hours. Pulse metal detectors stand apart. Their primary current flows into the coil in pulses. By setting the pulse repetition rate within the LF, and their duration, which determines the spectral composition of the signal corresponding to the IF-HF ranges, you can get a metal detector that combines the positive properties of LF, IF and HF or is tunable. There are at least 10 EMF search methods. But such as, say, the method of direct digitization of the response signal with computer processing is the lot of professional use. A home-made metal detector is schematically built most of all in the following ways: Parametric metal detectors in some way fall outside the definition of the principle of operation: they have neither a receiver nor a receiving coil. For detection, the direct influence of the object on the parameters of the generator coil is used - inductance and quality factor, and the structure of the EMF does not matter. Changing the parameters of the coil leads to a change in the frequency and amplitude of the generated oscillations, which is fixed in various ways: by measuring the frequency and amplitude, by changing the current consumption of the generator, by measuring the voltage in the PLL loop (phase locked loop, "pulling" it to a given value), etc. Parametric metal detectors are simple, cheap and noise-resistant, but their use requires certain skills, because. the frequency "floats" under the influence of external conditions. Their sensitivity is weak; most of all are used as magnetic detectors. The device of the transceiver metal detector is shown in fig. at the beginning, to an explanation of the principle of operation; the principle of operation is also described there. Such devices allow to achieve the best efficiency in their frequency range, but are complex in circuitry, require a particularly high-quality coil system. Transceiver metal detectors with a single coil are called induction. Their repeatability is better, because the problem of the correct arrangement of the coils relative to each other disappears, but the circuitry is more complicated - you need to highlight a weak secondary signal against the background of a strong primary. Note:
in pulsed transceiver metal detectors, the emission problem can also be eliminated. This is explained by the fact that as a secondary signal they “catch” the so-called. "tail" of the pulse re-radiated by the object. The primary pulse spreads due to dispersion during reemission, and a part of the secondary pulse is in the gap between the primary ones, from where it can be easily distinguished. Metal detectors with phase accumulation, or phase-sensitive, are either single-coil pulsed, or with 2 generators, each working on its own coil. In the first case, the fact is used that during re-emission pulses not only spread out, but are also delayed. In time, the phase shift increases; when it reaches a certain value, the discriminator is triggered and a click is heard in the headphones. As you get closer to the object, the clicks become more frequent and merge into a higher pitched sound. It is on this principle that Pirate is built. In the second case, the search technique is the same, but 2 strictly symmetrical electrically and geometrically generators work, each on its own coil. At the same time, due to the interaction of their EMF, mutual synchronization occurs: the generators work in time. When the overall EMF is distorted, synchronization breaks begin, audible as the same clicks, and then a tone. Two-coil metal detectors with a synchronization breakdown are simpler than impulse ones, but less sensitive: their penetration is 1.5-2 times less. Discrimination in both cases is close to excellent.
Beats of 2 electrical signals - a signal with a frequency equal to the sum or difference of the main frequencies of the original signals or multiples of them - harmonics. So, for example, if signals with frequencies of 1 MHz and 1,000,500 Hz or 1.0005 MHz are applied to the inputs of a special device - a mixer, and headphones or a speaker are connected to the output of the mixer, then we will hear a pure tone of 500 Hz. And if the 2nd signal is 200 100 Hz or 200.1 kHz, the same thing will happen, because 200 100 x 5 = 1,000,500; we "caught" the 5th harmonic. There are 2 generators in the beat detector: reference and working. The reference oscillatory circuit coil is small, protected from extraneous influences, or its frequency is stabilized by a quartz resonator (simply, quartz). The contour coil of the working (search) generator is a search coil, and its frequency depends on the presence of objects in the search area. Before searching, the working generator is tuned to zero beats, i.e. until the frequencies match. As a rule, they don’t achieve a complete zero sound, but tune it to a very low tone or wheezing, so it’s more convenient to search. By changing the tone of the beats, the presence, size, properties and location of the object are judged. Note:
most often, the frequency of the search generator is taken several times lower than the reference one and works on harmonics. This allows, firstly, to avoid the mutual influence of generators, which is harmful in this case; secondly, to tune the device more precisely, and thirdly, to search at the optimal frequency in this case. In general, metal detectors based on harmonics are more complicated than impulse ones, but they work on any ground. Properly made and tuned, they are not inferior to impulse ones. This can be judged at least by the fact that beach gold diggers do not agree in any way on what is better: impulse or beat? The most common misconception of novice radio amateurs is the absolutization of circuitry. Like, if the scheme is "cool", then everything will be tip-top. With regard to metal detectors, this is doubly untrue, because. their operational advantages strongly depend on the design and workmanship of the search coil. As a resort prospector put it: "The findability of a detector should pull the pocket, not the legs." When developing a device, its circuit and coil parameters are adjusted to each other until an optimum is obtained. A certain scheme with a “foreign” coil, if it works, will not reach the declared parameters. Therefore, when choosing a prototype for repetition, see first of all the description of the coil. If it is incomplete or inaccurate, it is better to build another device. A large (wide) coil radiates EMF more efficiently and “enlightens” the ground deeper. Its search area is wider, which allows you to reduce the "finding by feet". However, if there is a large unnecessary object in the search area, its signal will be “hammered” by a weak one from the desired trifle. Therefore, it is advisable to take or make a metal detector designed to work with coils of different sizes. Note:
typical coil diameters are 20-90 mm for finding rebar and profiles, 130-150 mm "for beach gold" and 200-600 mm "for big iron". The traditional type of metal detector coil is the so-called. thin coil or Mono Loop (single loop): a ring of many turns of enameled copper wire with a width and thickness of 15-20 times less than the average diameter of the ring. The advantages of a monoloop coil are the weak dependence of parameters on the type of soil, the search area narrowing downwards, which allows, by moving the detector, to more accurately determine the depth and location of the find, and structural simplicity. Disadvantages - low quality factor, which is why the tuning “floats” during the search, susceptibility to interference and a vague reaction to the object: working with a monoloop requires considerable experience in using this particular instance of the device. It is recommended for beginners to make homemade metal detectors with a mono-loop in order to get a workable design without any problems and gain search experience with it. When choosing a circuit, in order to verify the authenticity of the author's promises, and even more so when designing or refining it yourself, you need to know the inductance of the coil and be able to calculate it. Even if you are making a metal detector from a purchased kit, you still need to check the inductance by measurements or by calculation, so as not to rack your brains later: why, everything seems to be in order, and not beeping. Calculators for calculating the inductance of coils are available on the Internet, but a computer program cannot foresee all cases of practice. Therefore, in fig. given an old, decades-tested nomogram for calculating multilayer coils; a thin coil is a special case of a multilayer coil. To calculate the search monoloop, the nomogram is used as follows: The monoloop "catches" interference well, because arranged in exactly the same way as a loop antenna. You can increase its noise immunity, firstly, by placing the winding in the so-called. Faraday shield: a metal tube, braid or foil winding with a break so that a short-circuited coil does not form, which will “eat” all the EMI of the coil, see fig. on right. If there is a dotted line near the designation of the search coil on the original diagram (see the diagrams below), this means that the coil of this device must be placed in the Faraday screen. Also, the screen must be connected to the common wire of the circuit. There is a catch for beginners here: the grounding conductor must be connected to the screen strictly symmetrically to the section (see the same figure) and connected to the circuit also symmetrically with respect to the signal wires, otherwise the interference will still “penetrate” into the coil. The screen also absorbs some of the search EMF, which reduces the sensitivity of the device. This effect is especially noticeable in pulsed metal detectors; their coils cannot be shielded at all. In this case, an increase in noise immunity can be achieved by balancing the winding. The bottom line is that for a remote source of EMF, the coil is a point object, and emf. interference in its halves will overwhelm each other. A symmetrical coil may also be needed in circuitry if the generator is a push-pull or inductive three-point. However, in this case, it is impossible to symmetricalize the coil with the usual bifilar method (see Fig.): when conducting and / or ferromagnetic objects are in the field of the bifilar coil, its symmetry is violated. That is, the noise immunity of the metal detector will disappear just when it is most needed. Therefore, the monoloop coil must be symmetrical by cross winding, see the same fig. Its symmetry is not broken under any circumstances, but winding a thin coil with a large number of turns in a cross way is hellish work, and then it is better to make a basket coil. Basket coils have all the advantages of mono-loops to an even greater extent. In addition, basket coils are more stable, their quality factor is higher, and the fact that the coil is flat is a double plus: sensitivity and discrimination will increase. Basket coils are less susceptible to interference: harmful emfs. in crossing wires they cancel each other out. The only negative is that basket coils need a precisely made rigid and durable mandrel: the total tension force of many turns reaches large values. Basket coils are structurally flat and voluminous, but electrically voluminous "basket" is equivalent to flat, i.e. creates the same EMF. The volumetric basket coil is even less sensitive to interference and, which is important for pulsed metal detectors, the pulse dispersion in it is minimal, i.e. easier to catch the variance caused by the object. The advantages of the original "Pirate" metal detector are largely due to the fact that its "native" coil is a voluminous basket (see Fig.), but its winding is complex and time-consuming. It is better for a beginner to wind a flat basket on his own, see fig. below. For metal detectors "for gold" or, say, for the "butterfly" metal detector described below and a simple transceiver 2-coil, unusable computer disks will be a good mandrel. Their plating will not hurt: it is very thin and nickel. An indispensable condition: an odd, and nothing else, the number of slots. A nomogram is not required for calculating a flat basket; calculation is carried out in this way: Some amateurs take it upon themselves to wind bulky baskets in the manner shown in fig. below: make a mandrel from insulated nails (pos. 1) or self-tapping screws, wind according to the scheme, pos. 2 (in this case, pos. 3, for the number of turns, a multiple of 8; every 8 turns the “pattern” is repeated), then foam, pos. 4, the mandrel is pulled out, and the excess foam is cut off. But it soon turns out that the stretched coils cut the foam and all the work went soft. That is, in order to wind securely, you need to glue pieces of durable plastic into the holes of the base, and only then wind it. And remember: an independent calculation of a volumetric basket coil without appropriate computer programs is impossible; the flat basket technique is not applicable in this case. DD in this case does not mean long-range, but a double or differential detector; in the original - DD (Double Detector). This is a coil of 2 identical halves (shoulders), folded with some intersection. With an accurate electrical and geometric balance of the DD arms, the search EMF is pulled into the intersection zone, on the right in Fig. on the left - a monoloop coil and its field. The slightest inhomogeneity of space in the search area causes an imbalance, and a sharp strong signal appears. The DD-coil allows an inexperienced searcher to detect a shallow, deep, well-conducting object when a rusty can lies next to it and above. Coils DD are clearly oriented "on gold"; all metal detectors with the GOLD marking are equipped with them. However, on finely heterogeneous and / or conductive soils, they either fail completely, or often give false signals. The sensitivity of the DD coil is very high, but the discrimination is close to zero: the signal is either marginal or not at all. Therefore, metal detectors with DD coils are preferred by seekers who are only interested in "being in the pocket." Note:
more details about DD coils can be found later in the description of the corresponding metal detector. They wind their shoulders DD or in bulk, like a monoloop, on a special mandrel, see below, or with baskets. Ready-made frames and mandrels for search coils are sold in a wide range, but sellers are not shy about cheating. Therefore, many amateurs make the base of the plywood coil, on the left in the figure: The simplest metal detector for searching for fittings, wiring, profiles and communications in walls and ceilings can be assembled according to fig. The ancient transistor MP40 changes without any change to KT361 or its analogues; to use pnp transistors, you need to reverse the polarity of the battery. This metal detector is a parametric type magnetic detector operating at low frequencies. The tone of the sound in the headphones can be changed by selecting the capacitance C1. Under the influence of the object, the tone drops, unlike all other types, so initially you need to achieve a “mosquito squeak”, and not wheezing or grumbling. The device distinguishes wiring under current from “empty”, a hum of 50 Hz is superimposed on the tone. The circuit is a pulse generator with inductive feedback and frequency stabilization by an LC circuit. Loop coil - an output transformer from an old transistor receiver or a low-power "Bazaar-Chinese" low-voltage power transformer. A transformer from an unusable power source of a Polish antenna is very well suited, in its own case, by cutting off the mains plug, you can assemble the entire device, then it is better to power it from a 3 V lithium tablet battery. Winding II in fig. – primary or network; I - secondary or step-down at 12 V. That's right, the generator works with transistor saturation, which provides negligible power consumption and a wide range of pulses, making it easier to find. To turn the transformer into a sensor, its magnetic circuit must be opened: remove the frame with the windings, remove the straight jumpers of the core - the yoke - and fold the W-shaped plates in one direction, as on the right in the figure, then put the windings back on. With serviceable parts, the device starts working immediately; if not, you need to swap the ends of any of the windings. The parametric scheme is more complicated - in fig. on right. L with capacitors C4, C5 and C6 is tuned to 5, 12.5 and 50 kHz, and quartz passes the 10th, 4th harmonics and fundamental tone to the amplitude meter, respectively. The scheme is more for an amateur to get drunk on the table: there is a lot of fuss with the setting, but there is no "flair", as they say. Provided as an example only. Much more sensitive is a transceiver metal detector with a DD coil, which can be easily made at home, see fig. Left - transmitter; on the right is the receiver. It also describes the properties of different types of DD. This metal detector is LF; search frequency is about 2 kHz. Depth of detection: Soviet penny - 9 cm, canning tin - 25 cm, sewer hatch - 0.6 m. The parameters are “triple”, but you can master the technique of working with DD before moving on to more complex structures. The coils contain 80 turns of 0.6-0.8 mm PE wire, wound in bulk on a mandrel 12 mm thick, the drawing of which is shown in fig. left. In general, the device is not critical to the parameters of the coils, they would be exactly the same and located strictly symmetrically. In general, a good and cheap simulator for those who want to master any search technique, incl. "for gold". Although the sensitivity of this metal detector is not high, but the discrimination is very good despite the use of DD. To set up the device, first, instead of the L1 transmitter, turn on the headphones and make sure that the generator is working by the tone. Then L1 of the receiver is short-circuited and, by selecting R1 and R3, a voltage is set on the collectors VT1 and VT2, respectively, equal to about half the supply voltage. Next, R5 set the collector current VT3 within 5..8 mA, open L1 of the receiver and that's it, you can search. The designs in this section show all the advantages of the phase accumulation method. The first metal detector mainly for construction purposes will be very inexpensive, because. its most labor-intensive parts are made ... of cardboard, see fig.: The device does not require adjustment; integrated timer 555 - an analogue of the domestic IC (integrated circuit) K1006VI1. All signal transformations take place in it; search method - impulse. The only condition is that the speaker needs a piezoelectric (crystalline), a regular speaker or headphones will overload the IC and it will soon fail. Coil inductance - about 10 mH; operating frequency - within 100-200 kHz. With a mandrel thickness of 4 mm (1 layer of cardboard), a coil with a diameter of 90 mm contains 250 turns of PE 0.25 wire, and a 70 mm coil contains 290 turns. Metal detector "Butterfly", see fig. on the right, in terms of its parameters it is already close to professional devices: the Soviet penny is found at a depth of 15-22 cm, depending on the soil; sewer manhole - at a depth of up to 1 m. Acts on disruption of synchronization; diagram, board and type of installation - in fig. below. Please note, there are 2 separate coils with a diameter of 120-150 mm, not DD! They must not overlap! Both speakers are piezoelectric, as in the previous. case. Capacitors - thermostable, mica or high-frequency ceramic. The properties of the Butterfly will improve, and it will be easier to set it up if, firstly, wind the coils with flat baskets; the inductance is determined by the given operating frequency (up to 200 kHz) and the capacitances of the loop capacitors (10,000 pF each in the diagram). Wire diameter - from 0.1 to 1 mm, the larger the better. The tap in each coil is made from a third of the turns, counting from the cold (lower according to the diagram) end. Secondly, if individual transistors are replaced with a 2-transistor assembly for K159NT1 dif-amplifier circuits or its analogues; a pair of transistors grown on a single chip has exactly the same parameters, which is important for circuits with a synchronization failure. To establish the "Butterfly" you need to accurately adjust the inductance of the coils. The author of the design recommends moving apart and shifting the turns or adjusting the coils with ferrite, but from the point of view of electromagnetic and geometric symmetry, it would be better to connect trimmer capacitors of 100-150 pF in parallel with 10,000 pF capacitances and twist them when tuning in different directions. The actual adjustment is not difficult: the newly assembled device beeps. We alternately bring an aluminum saucepan or a beer can to the coils. To one - the squeak becomes higher and louder; to the other - lower and quieter or completely silent. Here we add a little capacity of the trimmer, and remove it in the opposite shoulder. For 3-4 cycles, you can achieve complete silence in the speakers - the device is ready to search. Let's return to the famous "Pirate"; it is a pulse transceiver with phase accumulation. The scheme (see fig.) is very transparent and can be considered a classic for this case. The transmitter consists of a master oscillator (MG) on the same 555th timer and a powerful key on T1 and T2. On the left - a variant of the ZG without an IC; it will have to set the pulse repetition rate of 120-150 Hz R1 and the pulse duration of 130-150 μs R2 on the oscilloscope. Coil L - common. The limiter on diodes D1 and D2 for a current of 0.5 A saves the QP1 receiver amplifier from overload. The discriminator is assembled on QP2; together they make up the dual operational amplifier K157UD2. Actually, the "tails" of the reradiated pulses are accumulated in the capacitance C5; when the “reservoir is full”, a pulse jumps at the output of QP2, which is amplified by T3 and gives a click in the dynamics. Resistor R13 regulates the filling rate of the "reservoir" and, consequently, the sensitivity of the device. More about "Pirate" can be found in the video:
and about the features of its settings - from the following video:
Those who want to experience all the delights of the search process on beats with replaceable coils can assemble a metal detector according to the diagram in fig. Its peculiarity, firstly, is efficiency: the entire circuit is assembled on CMOS logic and, in the absence of an object, consumes very little current. Secondly, the device works on harmonics. The reference oscillator on DD2.1-DD2.3 is stabilized by ZQ1 quartz at 1 MHz, and the search oscillator on DD1.1-DD1.3 operates at a frequency of about 200 kHz. When setting up the device before searching, the desired harmonic is “caught” by the VD1 varicap. The mixing of the working and reference signals occurs in DD1.4. Thirdly, this metal detector is suitable for work with replaceable coils. It is better to replace the ICs of the 176th series with the same 561st ones, the current consumption will decrease, and the sensitivity of the device will increase. It is simply impossible to replace the old Soviet high-resistance headphones TON-1 (preferably TON-2) with low-resistance ones from the player: they will overload DD1.4. You need to either put an amplifier like a "pirate" one (C7, R16, R17, T3 and a speaker on the "Pirate" circuit), or use a piezo speaker. This metal detector does not require settings after assembly. Coils are monoloops. Their data on a mandrel 10 mm thick: Now let's fulfill the promise given at the beginning: we will tell you how to make, without knowing anything about radio engineering, the metal detector that you are looking for. The metal detector is “easier than simple” assembled from a radio, a calculator, a cardboard or plastic box with a hinged lid, and pieces of double-sided tape. The metal detector “from the radio” is pulsed, however, to detect objects, it is not dispersion and not delay with phase accumulation that are used, but the rotation of the EMF magnetic vector during re-emission. On the forums, they write different things about this device, from “super” to “sucks”, “wiring” and words that are not customary to use in writing. So, in order to get, if not “super”, but at least a fully functional device, its components - the receiver and the calculator - must meet certain requirements. Calculator we need the tiniest and cheapest, "alternative". They make them in offshore cellars. They have no idea about the standards for electromagnetic compatibility of household appliances, and if they heard about something like that, then they wanted to spit from the bottom of their hearts. Therefore, local products are quite powerful sources of impulse radio interference; they are given by the clock generator of the calculator. In this case, its strobe pulses on the air are used to probe the space. Receiver you also need a cheap one, from similar manufacturers, without any means of increasing noise immunity. It must have an AM band and, absolutely necessary, a magnetic antenna. Since receivers with short wave (HF, SW) reception on a magnetic antenna are rarely sold and are expensive, you will have to limit yourself to medium waves (MW, MW), but this will make tuning easier. Note:
depending on the design of the receiver, the reverse option is possible - to tune in to the harmonica, the receiver is placed on the included calculator, and then, laying out the “book”, the tone is softened or disappears. In this case, the receiver will catch the pulses reflected from the object. And what's next? If there is an electrically conductive or ferromagnetic object near the opening of the "book", it will re-emit probing pulses, but their magnetic vector will turn. The magnetic antenna will “smell” them, the receiver will again give a tone. That is, we have already found something. There are reports of another metal detector "for complete dummies" with a calculator, but instead of a radio, supposedly 2 computer disks, a CD and a DVD, are needed. Also - piezo headphones (precisely piezo, according to the authors) and a Krona battery. Frankly speaking, this creation looks like a techno-myth, like a memorable mercury antenna. But - what the hell is not joking. Here's a video for you:
try it, if you wish, maybe something will be found there, both in the subject and in the scientific and technical sense. Good luck! There are hundreds, if not thousands, of schemes and designs of metal detectors. Therefore, in the appendix to the material, we also give a list of models, in addition to those mentioned in the test, which, as they say, are in circulation in the Russian Federation, are not overly expensive and are available for repetition or self-assembly:
F0 = 1000 kHz;
Fp = 50 kHz + 11 kHz; 
Operating principle
Detector or scanner?
Varieties
Common parameters
Operating frequency
All parameters of the metal detector are connected in a complex way and many relationships are mutually exclusive. So, for example, lowering the oscillator frequency makes it possible to achieve greater penetration and search area, but at the cost of increased power consumption, and worsens sensitivity and mobility due to an increase in coil size. In general, each parameter and their complexes are somehow tied to the frequency of the generator. That's why The initial classification of metal detectors is based on the operating frequency range:
Search method
Without receiver
With receiver and transmitter
Click to Click
Phase-sensitive metal detectors are the favorite tools of resort miners. Aces of the search adjust their devices so that exactly above the object the sound disappears again: the frequency of the clicks goes into the ultrasonic region. In this way, on a shell beach, it is possible to find gold earrings the size of a fingernail at a depth of up to 40 cm. However, on soil with small inhomogeneities, watered and mineralized, metal detectors with phase accumulation are inferior to others, except for parametric ones.By squeak
Coil and more
About coil dimensions
Monoloop
Inductance
Noise immunity
Basket
How not to and how to wind baskets
DD coils
How to attach a coil
Multiple designs
Parametric
transceiver
With phase accumulation
More about Pirate
Video: Pirate metal detector
Video: setting the threshold of the Pirate metal detector
On the beat
It doesn't get easier
Something strange in the end
as an application
