Recently, an activity such as searching for various ancient coins, household items, and just metal trinkets in the ground using a metal detector has become very popular. In fact, what could be better than taking a walk through the field in the morning, inhaling the smells of nature and enjoying the views. And if at the same time you manage to discover some worthwhile find in the ground, then it’s a fairy tale. Some people do this on purpose, spending days combing fields in search of valuable coins or other valuables. They have at their disposal expensive factory-made metal detectors, which not everyone can afford to buy. However, it is quite possible to assemble a full-fledged metal detector yourself.
This article will discuss the creation of the most popular, sought-after, time-tested, reliable pulse metal detector called “Pirate”. It allows you to find coins in the ground at a depth of 15-20 cm and large objects at a distance of up to 1.5 m. The diagram of the metal detector is presented below.
Metal detector circuit "Pirate"
The entire circuit can be divided into two parts - transmitter and receiver. The NE555 microcircuit generates rectangular pulses, which are fed to a coil through a powerful field-effect transistor. When the coil interacts with metal located next to it, complex physical phenomena occur, thanks to which the receiving part has the ability to “see” whether there is metal in the coil area or not. The receiver chip in the original Pirate circuit is the Soviet K157UD2, which is now becoming quite difficult to obtain. However, instead of it, you can use the modern TL072, the parameters of the metal detector will remain exactly the same. The printed circuit board proposed in this article is designed specifically for installing the TL072 chip (they have different pinouts).
Capacitors C1 and C2 are responsible for generating the frequency of rectangular pulses; their capacitance must be stable, so it is advisable to use film capacitors. Resistors R2 and R3 are responsible for the duration and frequency of the rectangular pulses that the microcircuit generates. From its output they are supplied to transistor T1, inverted and fed to the gate of the field-effect transistor. Here you can use any sufficiently powerful field-effect transistor with a drain-source voltage of at least 200 volts. For example, IRF630, IRF740. Diodes D1 and D2 are any low-power ones, for example, KD521 or 1N4148. Between pins 1 and 6 of the microcircuit, a variable resistor with a nominal value of 100 kOhm is connected, with which the sensitivity is set. It is most convenient to use two potentiometers, 100 kOhm for rough adjustment and 1-10 kOhm for fine adjustment. You can connect them according to the following scheme:
The speaker in the circuit is connected in series with a 10-47 Ohm resistor. The lower its resistance, the louder the sound and the greater the consumption of the metal detector. Transistor T3 can be replaced with any other low-power NPN transistor, for example, with the domestic KT3102. You can use any speaker you find. So, let's move from words to action.
Metal detector assembly
List of required parts
Chips:- NE555 – 1 pc.
- TL072 – 1 pc.
- BC547 – 1 pc.
- BC557 – 1 pc.
- 100 nF – 2 pcs.
- 1 nF – 1 pc.
- 10 µF – 2 pcs.
- 1 µF – 2 pcs.
- 220 uF – 1 pc.
- 100 kOhm – 1 pc.
- 1.6 kOhm – 1 pc.
- 1 kOhm – 1 pc.
- 10 Ohm – 2 pcs.
- 150 Ohm – 1 pc.
- 220 Ohm – 1 pc.
- 390 Ohm – 1 pc.
- 47 kOhm – 2 pcs.
- 62 kOhm – 1 pc.
- 2 MOhm – 1 pc.
- 120 kOhm – 1 pc.
- 470 kOhm – 1 pc.
- Speaker 1 – pcs.
- Diodes 1N4148 – 2 pcs.
- DIP8 sockets – 2 pcs.
- Potentiometer 100 kOhm – 1 pc.
- Potentiometer 10 kOhm – 1 pc.
Printed circuit board
The printed circuit board is made using the LUT method; there is no need to mirror it before printing.(downloads: 1646)
First of all, you need to solder resistors, diodes, then everything else on the board. It is advisable to install the microcircuits in sockets. The wires for connecting the coil, speaker, potentiometer and coil can be soldered directly into the board, but it is more convenient to use screw terminal blocks, then you can connect and disconnect the wires without using a soldering iron.
Making a coil
A few words about the search coil. The best option is to wind 20-25 turns of copper wire with a cross-section of 0.5 mm2 on a round frame with a diameter of about 20 cm. Sensitivity largely depends on the number of turns, so you should first wind more turns, about 30, and then gradually reducing the number of turns , choose a number at which the sensitivity will be maximum. The wires from the board to the coil should not be long, preferably copper and with a cross-section no smaller than the cross-section of the coil wire.
Setting up a metal detector
After assembling the board and winding the coil, the device can be turned on. In the first 5-10 seconds after switching on, various noises and crackles will be heard from the speaker, this is normal. Then, when the operational amplifier enters its operating mode, you need to use the potentiometer to find a mode when individual clicks will be heard from the speaker. When you bring a metal object to the coil, the frequency of clicks will increase significantly, and if you bring the metal into the very center of the coil, the sound will turn into a continuous hum. If the sensitivity is not enough, and changing the number of coil turns does not help, you should try to select the values of resistors R7, R11, changing them up or down. The board must be cleaned of flux; it often causes the metal detector to malfunction. Happy build!
Today, among a large number of people, such a hobby as searching for treasures in the ground, and even ordinary scrap metal, has become widespread. For some it is a way to have fun, for others it is a means of making money.
Metal detector "Pirate"
Now we will look at one of the modern metal detectors – the “Pirate” model. The operation of this device is based on such features as the conductivity of electricity, the inductive and magnetic properties of the metal. The “Pirate” metal detector (its photo is shown in this article) has a unified design. It consists of a generator that produces an alternating current passing through a coil with a magnetic field. If the metal that conducts the current is placed too close to the coil, then the vortex flows will be directed towards the metal. This will contribute to the emergence of an alternating magnetic field in the metal. To detect and measure the magnetic field, another coil is used.
The device can recognize coins in the ground at a depth of 20 cm. Large objects will actually be caught by the device at depths of one and a half meters. The disadvantage of this device is that it does not have such an option as a discriminator, that is, it does not recognize non-ferrous metals. Therefore, it will not be possible to work with it in areas that are contaminated with various types of metals.
Let's try to make the Pirate metal detector ourselves. Thanks to the detailed instructions posted here, even novice radio amateurs will do this without errors.
Step-by-step description of metal detector assembly
List of required materials and tools:microcircuit KR1006VI1 (or its foreign version NE555), on which the transmitting node is created;
transistor IRF740;
microcircuit K157UD2 and transistor VS547 (on which the receiving unit is built);
wire PEV 0.5 (to wind the coil);
NPN type transistors;
materials for making the body and so on;
insulating tape;
soldering iron, wires, other tools;
plastic box for mounting an electronic circuit;
plastic pipe from which the rod will be made.
The remaining radio components are shown in the diagram.
Metal detector Pirate circuit board
Since the electronics are the hardest part, that's where we'll start. First of all, let's make a printed circuit board. There are boards in different versions, which differ in the radioelements used. This could be a board for NE555, or a board with transistors. A list of all necessary files is attached to the article. In addition, you can find other types of boards on the Internet.
Installing electronic elements on the board
At this stage we begin soldering the board. When installing all electronic elements, you must be guided by the diagram. The image on the left shows capacitors. These are film type capacitors with high thermal stability. With them, the metal detector works more stably. It is especially convenient to use such a metal detector in the fall, during changeable weather, when cold weather sets in.
Power supply
The device is designed to operate from a 9...12 V power source. It must be remembered that the device consumes quite a lot of electricity - after all, it is a powerful device. One crown battery won’t help you for long here - take two, or better yet three, connected in parallel. An excellent option is to use one powerful battery.
Coil assembly
Since this is a pulse-type metal detector, the accuracy of the coil assembly is not so important. The optimal diameter is a mandrel of 1900-200 mm, a total of 25 turns are wound. After winding the coil, carefully wrap it on top with electrical tape. To increase the detection depth of the coil, we wind it on a mandrel with a diameter of about 260...270 mm, and reduce the number of turns to 21...22. We use wire with a thickness of 0.5 mm.When we have wound the coil, we install it on a rigid body, on which there should be no metal. Think about it here and see where you can find a case that fits the size. Maybe you can find it in the workshop. The housing will protect the coil from shock and damage while working with the metal detector.
The leads from the coil should be soldered to a stranded wire with a diameter of about 0.5...0.75 mm. The best option would be two wires that are twisted together.
Setting up the "Pirate" metal detector
During assembly, you do not need to configure the metal detector exactly according to the diagram; it already has maximum sensitivity. To fine-tune the device, twist the variable resistor R13. Rare clicks should be heard in the speaker. If they are heard only in the extreme positions of the resistor, change the value of resistor R12. The variable resistor should adjust the device to normal operation in the middle positions.
Working with a metal detector
We turn on the metal detector, wait 10...20 seconds - until the operation of the device stabilizes. Now you can twist resistor R13 to configure the device. Now that’s it, you can safely start looking for treasures, scrap metal, whatever you like.
You can buy it for about 100-300 dollars. The price of metal detectors is strongly related to their detection depth; not every metal detector can “see” coins at a depth of 15 cm. In addition, the cost of a metal detector is also greatly affected by the presence of a metal type recognizer and the type of interface; fashionable metal detectors are sometimes equipped with a display for convenient operation .
This article will look at an example of assembling a powerful metal detector called Pirat with your own hands. The device is capable of catching coins underground at a depth of 20 cm. As for large objects, it is quite possible to work at a depth of 150 cm.
Video of working with a metal detector:
This metal detector received this name due to the fact that it is pulsed, this is the designation of its first two letters (PI-pulse). Well, RA-T is consonant with the word radioskot - this is the name of the developers’ site, where the homemade product was posted. According to the author, the Pirate is assembled very simply and quickly; even basic skills in working with electronics are enough for this.
The disadvantage of such a device is that it does not have a discriminator, that is, it cannot recognize non-ferrous metals. So it will not be possible to work with it in areas contaminated with various types of metals.
Materials and tools for assembly:
- microcircuit KR1006VI1 (or its foreign analogue NE555) - the transmitting node is built on it;
- transistor IRF740;
- K157UD2 microcircuit and BC547 transistor (the receiving unit is assembled on them);
- wire PEV 0.5 (for winding the coil);
- NPN type transistors;
- materials for creating the body and so on;
- electrical tape;
- soldering iron, wires, other tools.
The remaining radio components can be seen in the diagram.
You also need to find a suitable plastic box for mounting the electronic circuit. You will also need a plastic pipe to create a rod on which the coil is attached.
Metal detector assembly process:
Step one. Creating a printed circuit board
The most complex part of the device is, of course, the electronics, so it makes sense to start there. First of all, you need to make a printed circuit board. There are several board options, depending on the radio elements used. There is a board for NE555, and there is a board with transistors. All the necessary files to create the board are included in the article. You can also find other board options on the Internet.
Step two. Installing electronic elements on the board
Now the board needs to be soldered, all electronic elements are installed exactly as in the diagram. In the picture on the left you can see the capacitors. These capacitors are film capacitors and have high thermal stability. Thanks to this, the metal detector will work more stably. This is especially true if you use a metal detector in the fall, when it is sometimes quite cold outside.
Step three. Power supply for metal detector
To power the device, you need a source from 9 to 12 V. It is important to note that the device is quite voracious in terms of energy consumption, and this is logical, because it is also powerful. One Krona battery will not last long here; it is recommended to use 2-3 batteries at once, which are connected in parallel. You can also use one powerful battery (best rechargeable).
Step four. Assembling a coil for a metal detector
Due to the fact that this is a pulse metal detector, the accuracy of the coil assembly is not so important here. The optimal diameter of the mandrel is 1900-200 mm; a total of 25 turns need to be wound. After the coil is wound, it needs to be thoroughly wrapped on top with electrical tape for insulation. To increase the detection depth of the coil, you need to wind it on a mandrel with a diameter of about 260-270 mm, and reduce the number of turns to 21-22. In this case, a wire with a diameter of 0.5 mm is used.
After the coil is wound, it must be installed on a rigid body; there should be no metal on it. Here you need to think a little and look for any suitable housing. It is needed to protect the coil from shock while working with the device.
The leads from the coil are soldered to a stranded wire with a diameter of about 0.5-0.75 mm. It is best if there are two wires twisted together.
Step five. Setting up a metal detector
When assembling exactly according to the diagram, you do not need to adjust the metal detector; it already has maximum sensitivity. To fine-tune the metal detector, you need to twist the variable resistor R13, you need to achieve rare clicks in the speaker. If this can only be achieved in the extreme positions of the resistor, then it is necessary to change the value of resistor R12. The variable resistor should set the device to normal operation in the middle positions.
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 personnel, geologists, builders to search for profiles under cladding, fittings, to verify plans and diagrams of underground communications, 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. This article is primarily intended for them, beginners; The devices described in it allow you to find a coin the size of a Soviet nickel at a depth of 20-30 cm or a piece of iron the size of a sewer manhole approximately 1-1.5 m below the surface. However, this homemade device can also be useful on the farm during repairs or at construction sites. Finally, having discovered a hundredweight or two of abandoned pipes or metal structures in the ground and selling the find for scrap metal, you can earn a decent amount. And there are definitely more such treasures in Russian land than pirate chests with doubloons or boyar-robber pods with efimkas.
Note: If you are not knowledgeable in electrical engineering and radio electronics, do not be intimidated by the diagrams, formulas and special terminology in the text. The essence is stated simply, and at the end there will be a description of the device, which can be made in 5 minutes on a table, without knowing how to solder or twist the wires. But it will allow you to “feel” the peculiarities of metal searching, and if interest arises, knowledge and skills will come.
A little more attention compared to the others will be paid to the “Pirate” metal detector, see fig. This device is simple enough for beginners to repeat, but its quality indicators are not inferior to many branded models costing up to $300-400. And most importantly, it showed excellent repeatability, i.e. full functionality when manufactured according to descriptions and specifications. The circuit design and operating principle of the “Pirate” are quite modern; There are enough manuals on how to set it up and how to use it.
Operating principle
The metal detector operates on the principle of electromagnetic induction. In general, the metal detector circuit consists of an electromagnetic vibration 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 operate on the same coil, the receiving part immediately releases the useful signal, etc.
The coil creates an electromagnetic field (EMF) of a certain structure in the medium. If there is an electrically conductive object in its area of 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 detects 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 soil is not. The main thing is that their electrical and/or magnetic properties are different.
Detector or scanner?
In commercial sources, expensive highly sensitive metal detectors, e.g. Terra-N are often called geoscanners. This is not true. Geoscanners operate on the principle of measuring the electrical conductivity of soil in different directions at different depths; this procedure is called lateral logging. Using logging data, the computer builds a picture on the display of everything in the ground, including geological layers of different properties.
Varieties
Common parameters
The operating principle of a metal detector can be implemented technically in different ways, depending on the purpose of the device. Metal detectors for beach gold prospecting and construction and repair prospecting may be similar in appearance, but differ significantly in design and technical data. To make a metal detector correctly, you need to clearly understand what requirements it must satisfy for this type of work. Based on this, The following parameters of search metal detectors can be distinguished:
- Penetration, or penetrating ability, is the maximum depth to which an EMF coil extends in the ground. The device will not detect anything deeper, regardless of the size and properties of the object.
- The size and dimensions of the search zone is an imaginary area in the ground in which the object will be detected.
- Sensitivity is the ability to detect more or less small objects.
- Selectivity is the ability to respond more strongly to desirable findings. The sweet dream of beach miners is a detector that beeps only for precious metals.
- Noise immunity is the ability not to respond to EMF from extraneous sources: radio stations, lightning discharges, power lines, electric vehicles and other sources of interference.
- Mobility and efficiency are determined by energy consumption (how many batteries will last), the weight and dimensions of the device and the size of the search zone (how much can be “probed” in 1 pass).
- Discrimination, or resolution, gives the operator or control microcontroller the opportunity to judge the nature of the found object by the device’s response.
Discrimination, in turn, is a composite parameter, because At the output of the metal detector there is 1, maximum 2 signals, and there are more quantities that determine the properties and location of the find. However, taking into account the change in the reaction of the device when approaching an object, 3 components are distinguished:
- Spatial – indicates the location of the object in the search area and the depth of its occurrence.
- Geometric – makes it possible to judge the shape and size of an object.
- Qualitative – allows you to make assumptions about the properties of the object’s material.
Operating frequency
All parameters of a metal detector are connected in a complex way and many relationships are mutually exclusive. So, for example, lowering the frequency of the generator makes it possible to achieve greater penetration and search area, but at the cost of increasing energy consumption, and worsens sensitivity and mobility due to an increase in the size of the coil. 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:- Ultra-low frequency (ELF) - up to the first hundred Hz. Absolutely not amateur devices: power consumption of tens of W, without computer processing it is impossible to judge anything from the signal, transportation requires vehicles.
- Low frequency (LF) - from hundreds of Hz to several kHz. They are simple in circuit design and design, noise-resistant, but not very sensitive, discrimination is poor. Penetration - up to 4-5 m with power consumption from 10 W (so-called deep metal detectors) or up to 1-1.5 m when powered by batteries. They react most acutely to ferromagnetic materials (ferrous metal) or large masses of diamagnetic materials (concrete and stone building structures), which is why they are sometimes called magnetic detectors. They are little sensitive to soil properties.
- High frequency (IF) – up to several tens of kHz. LF is more complex, but the requirements for the coil are low. Penetration - up to 1-1.5 m, noise immunity at C, good sensitivity, satisfactory discrimination. Can be universal when used in pulse mode, see below. On watered or mineralized soils (with fragments or particles of rock that shield EMF), they work poorly or do not sense anything at all.
- High, or radio frequencies (HF or RF) - typical metal detectors “for gold”: excellent discrimination to a depth of 50-80 cm in dry non-conductive and non-magnetic soils (beach sand, etc.) Energy consumption - as before. n. The rest is on the verge of failure. The effectiveness of the device largely depends on the design and quality of the coil(s).
Note: mobility of metal detectors according to paragraphs. 2-4 good: from one set of AA salt cells (“batteries”) you can work for up to 12 hours without overworking the operator.
Pulse metal detectors stand apart. In them, the primary current enters the coil in pulses. By setting the pulse repetition rate within the LF range, and their duration, which determines the spectral composition of the signal corresponding to the IF-HF ranges, you can obtain a metal detector that combines the positive properties of LF, IF and HF or is tunable.
Search method
There are at least 10 methods of searching for objects using EMFs. But such as, say, the method of direct digitization of the response signal with computer processing is for professional use.
A homemade metal detector is built in the following ways:
- Parametric.
- Transceiver.
- With phase accumulation.
- On the beats.
Without receiver
Parametric metal detectors in some way fall outside the definition of the operating principle: they have neither a receiver nor a receiving coil. For detection, the direct influence of the object on the parameters of the generator coil - inductance and quality factor - is used, 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 recorded in different ways: by measuring the frequency and amplitude, by changing the current consumption of the generator, by measuring the voltage in the PLL loop (a phase-locked loop system that “pulls” it to a given value), etc.
Parametric metal detectors are simple, cheap and noise-resistant, but using them requires certain skills, because... the frequency “floats” under the influence of external conditions. Their sensitivity is weak; Most of all they are used as magnetic detectors.
With receiver and transmitter
The device of the transceiver metal detector is shown in Fig. at the beginning, to an explanation of the principle of operation; The principle of operation is also described there. Such devices allow achieving the best efficiency in their frequency range, but are complex in circuit design and require a particularly high-quality coil system. Transceiver metal detectors with one coil are called induction detectors. Their repeatability is better, because the problem of the correct arrangement of the coils relative to each other disappears, but the circuit design is more complicated - you need to highlight the weak secondary signal against the background of the strong primary one.
Note: In pulsed transceiver metal detectors, the problem of isolation can also be eliminated. This is explained by the fact that the so-called “catch” is “caught” as a secondary signal. the “tail” of the pulse re-emitted by the object. Due to dispersion during re-emission, the primary pulse spreads out, and part of the secondary pulse ends up in the gap between the primary ones, from where it is easy to isolate.
Until it clicks
Metal detectors with phase accumulation, or phase-sensitive, are either single-coil pulsed or with 2 generators, each operating on its own coil. In the first case, the fact is used that the pulses not only spread out during re-emission, but are also delayed. The phase shift increases over time; when it reaches a certain value, the discriminator is triggered and a click is heard in the headphones. As you approach the object, the clicks become more frequent and merge into a sound of increasingly higher pitch. 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 oscillators operate, each with its own coil. In this case, due to the interaction of their EMFs, mutual synchronization occurs: the generators work in time. When the general EMF is distorted, synchronization disruptions begin, heard as the same clicks, and then a tone. Double-coil metal detectors with synchronization failure are simpler than pulse detectors, but less sensitive: their penetration is 1.5-2 times less. Discrimination in both cases is close to excellent.
Phase-sensitive metal detectors are the favorite tools of resort prospectors. Search aces adjust their instruments so that exactly above the object the sound disappears again: the frequency of 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 parametric ones.
By the squeak
Beats of 2 electrical signals - a signal with a frequency equal to the sum or difference of the fundamental frequencies of the original signals or their multiples - 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.
In a metal detector, there are 2 generators operating on beats: a reference and a working one. The coil of the reference oscillating circuit is small, protected from extraneous influences, or its frequency is stabilized by a quartz resonator (simply quartz). The circuit coil of the working (search) generator is a search generator, and its frequency depends on the presence of objects in the search area. Before searching, the working generator is set to zero beats, i.e. until the frequencies match. As a rule, a complete zero sound is not achieved, but is adjusted to a very low tone or wheezing, this is more convenient to search for. By changing the tone of the beats one judges the presence, size, properties and location of the object.
Note: Most often, the frequency of the search generator is taken several times lower than the reference one and operates on harmonics. This allows, firstly, to avoid the harmful mutual influence of generators in this case; secondly, adjust the device more accurately, and thirdly, search at the optimal frequency in this case.
Harmonic metal detectors are generally more complex than pulse detectors, but they work on any type of soil. Properly manufactured and tuned, they are not inferior to impulse ones. This can be judged at least by the fact that gold miners and beachgoers will not agree on what is better: an impulse or a beating one?
Reel and stuff
The most common misconception of novice radio amateurs is the absolutization of circuit design. Like, if the scheme is “cool”, then everything will be top-notch. Regarding metal detectors, this is doubly true, because... their operational advantages greatly depend on the design and quality of manufacture of the search coil. As one resort prospector put it: “Findability of the detector should be in the pocket, not the legs.”
When developing a device, its circuit and coil parameters are adjusted to each other until the optimum is obtained. Even if a certain circuit with a “foreign” coil works, it will not reach the declared parameters. Therefore, when choosing a prototype to replicate, look first of all at the description of the coil. If it is incomplete or inaccurate, it is better to build another device.
About coil sizes
A large (wide) coil emits EMF more effectively and will “illuminate” the soil more deeply. Its search area is wider, which allows it to reduce “being found with its feet.” However, if there is a large unnecessary object in the search area, its signal will “clog” the weak one from the small thing you are looking for. 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 searching for fittings and profiles, 130-150 mm for “beach gold” and 200-600 mm “for large iron”.
monoloop
The traditional type of metal detector coil is called. thin coil or Mono Loop (single loop): a ring of many turns of enameled copper wire with a width and thickness 15-20 times less than the average diameter of the ring. The advantages of a monoloop coil are a weak dependence of the parameters on the type of soil, a narrowing search zone, which allows, by moving the detector, to more accurately determine the depth and location of the find, and design simplicity. Disadvantages - low quality factor, which is why the setting “floats” during the search process, susceptibility to interference and vague response to the object: working with a monoloop requires considerable experience in using this particular instance of the device. It is recommended that beginners make homemade metal detectors with a monoloop in order to get a workable design without any problems and gain search experience with it.
Inductance
When choosing a circuit, in order to ensure the reliability of the author’s promises, and even more so when independently designing or modifying it, 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 calculations, so as not to rack your brains later: why, everything seems to be working properly, and not beeping.
Calculators for calculating the inductance of coils are available on the Internet, but a computer program cannot provide for all practical cases. Therefore, in Fig. an old, decades-tested nomogram for calculating multilayer coils is given; a thin coil is a special case of a multilayer coil.
To calculate the search monoloop, the nomogram is used as follows:
- We take the inductance value L from the description of the device and the dimensions of the loop D, l and t from the same place or according to our choice; typical values: L = 10 mH, D = 20 cm, l = t = 1 cm.
- Using the nomogram we determine the number of turns w.
- We set the laying coefficient k = 0.5, using the dimensions l (height of the coil) and t (its width) we determine the cross-sectional area of the loop and find the area of pure copper in it as S = klt.
- Dividing S by w, we obtain the cross-section of the winding wire, and from it the diameter of the wire d.
- If it turns out d = (0.5...0.8) mm, everything is OK. Otherwise, we increase l and t when d>0.8 mm or decrease when d<0,5 мм.
Noise immunity
The monoloop “catches” interference well, because is designed exactly the same 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 turn does not form, which will “eat up” all the EMF coils, see fig. on right. If on the original diagram there is a dotted line near the designation of the search coil (see diagrams below), this means that the coil of this device must be placed in the Faraday shield.
Also, the screen must be connected to the common wire of the circuit. There is a catch here for beginners: the grounding conductor must be connected to the screen strictly symmetrically to the cut (see the same figure) and brought to the circuit also symmetrically relative to the signal wires, otherwise noise will still “crawl” 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 pulse metal detectors; their coils cannot be shielded at all. In this case, increasing noise immunity can be achieved by balancing the winding. The point is that for a remote EMF source, the coil is a point object, and the emf. interference in its halves will suppress each other. A symmetrical coil may also be needed in the circuit if the generator is push-pull or inductive three-point.
However, in this case it is impossible to symmetry the coil using the bifilar method familiar to radio amateurs (see figure): when conductive and/or ferromagnetic objects are in the field of the bifilar coil, its symmetry is broken. That is, the noise immunity of the metal detector will disappear just when it is most needed. Therefore, you need to balance the monoloop coil 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 crosswise manner is hellish work, and then it is better to make a basket coil.
Basket
Basket reels have all the advantages of monoloops 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 emf. in crossing wires they cancel each other out. The only negative is that basket coils require a precisely made, rigid and durable mandrel: the total tension force of many turns reaches large values.
Basket coils are structurally flat and three-dimensional, but electrically a three-dimensional “basket” is equivalent to a flat one, i.e. creates the same EMF. The volumetric basket coil is even less sensitive to interference and, which is important for pulse metal detectors, the pulse dispersion in it is minimal, i.e. It's 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 figure), 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 2-coil transceiver, a good mount would be unusable computer disks. Their metallization will not harm: it is very thin and nickel. An indispensable condition: an odd, and no other, number of slots. A nomogram for calculating a flat basket is not required; the calculation is carried out as follows:
- They are set with a diameter D2 equal to the outer diameter of the mandrel minus 2-3 mm, and take D1 = 0.5D2, this is the optimal ratio for search coils.
- According to formula (2) in Fig. calculate the number of turns.
- From the difference D2 – D1, taking into account the flat laying coefficient of 0.85, the diameter of the wire in insulation is calculated.
How not to and how to wind baskets
Some amateurs take it upon themselves to wind large baskets using the method shown in Fig. below: make a mandrel from insulated nails (pos. 1) or self-tapping screws, wind them according to the diagram, pos. 2 (in this case, pos. 3, for a number of turns that is 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 to waste. That is, in order to wind it reliably, you need to glue pieces of durable plastic into the holes of the base, and only then wind it. And remember: independent calculation of a volumetric basket coil without appropriate computer programs is impossible; The technique for a flat basket is not applicable in this case.
DD coils
DD in this case does not mean long-range, but a double or differential detector; in the original – DD (Double Detector). This is a coil of 2 identical halves (arms), folded with some intersection. With an accurate electrical and geometric balance of the DD arms, the search EMF is contracted into the intersection zone, on the right in Fig. on the left is a monoloop coil and its field. The slightest heterogeneity of space in the search area causes an imbalance, and a sharp strong signal appears. An DD coil allows an inexperienced seeker to detect a small, deep, highly conductive object when a rusty can lies next to it and above it.
DD coils are clearly oriented “to gold”; All metal detectors marked GOLD are equipped with them. However, on shallow, heterogeneous and/or conductive soils, they either fail altogether 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 there is none at all. Therefore, metal detectors with DD coils are preferred by searchers who are only interested in “pocket-fitting”.
Note: More details about DD coils can be found further in the description of the corresponding metal detector. The DD shoulders are wound either in bulk, like a monoloop, on a special mandrel, see below, or with baskets.
How to attach the reel
Ready-made frames and mandrels for search coils are sold in a wide range, but sellers are not shy about mark-ups. Therefore, many hobbyists make the base of the coil from plywood, on the left in the figure:
Multiple designs
Parametric
The simplest metal detector for searching for fittings, wiring, profiles and communications in walls and ceilings can be assembled according to Fig. The ancient transistor MP40 can be replaced without any problems with the KT361 or its analogues; To use pnp transistors, you need to change the polarity of the battery.
This metal detector is a parametric type magnetic detector operating on LF. The sound tone in the headphones can be changed by selecting the capacitance C1. Under the influence of the object, the tone decreases, unlike all other types, so initially you need to achieve a “mosquito squeak”, and not wheezing or grumbling. The device distinguishes live wiring from “empty” wiring; a 50 Hz hum is superimposed on the tone.
The circuit is a pulse generator with inductive feedback and frequency stabilization by an LC circuit. A loop coil is an output transformer from an old transistor receiver or a low-power “bazaar-Chinese” low-voltage power one. A transformer from an unusable Polish antenna power source is very suitable; in its 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 coin cell battery. Winding II in Fig. – primary or network; I – secondary or step-down by 12 V. That’s right, the generator operates with transistor saturation, which ensures negligible power consumption and a wide range of pulses, making searching easier.
To turn a transformer into a sensor, its magnetic circuit must be opened: remove the frame with windings, remove the straight jumpers of the core - the yoke - and fold the W-shaped plates to one side, as on the right in the figure, then put the windings back on. If the parts are in working order, the device starts working immediately; if not, you need to swap the ends of any of the windings.
A more complex parametric scheme is shown in Fig. on right. L with capacitors C4, C5 and C6 is tuned to 5, 12.5 and 50 kHz, and the quartz passes the 10th, 4th harmonics and fundamental tone to the amplitude meter, respectively. The circuit is more for the amateur to solder on the table: there is a lot of fuss with the settings, but there is no “flair”, as they say. Provided as an example only.
Transceiver
Much more sensitive is a transceiver metal detector with an DD coil, which can be made at home without much difficulty, see Fig. On the left is the transmitter; on the right is the receiver. The properties of different types of DD are also described there.
This metal detector is LF; search frequency is about 2 kHz. Detection depth: Soviet nickel - 9 cm, tin can - 25 cm, sewer hatch - 0.6 m. The parameters are “three”, but you can master the technique of working with DD before moving on to more complex structures.
The coils contain 80 turns of PE wire 0.6-0.8 mm, 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. Overall, 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 low, the discrimination is very good despite the use of DD.
To set up the device, first turn on headphones instead of the L1 transmitter and check by the tone that the generator is working. Then L1 of the receiver is short-circuited and by selecting R1 and R3, a voltage equal to approximately half the supply voltage is set on the collectors VT1 and VT2, respectively. Next, R5 sets the collector current VT3 within 5..8 mA, opens L1 of the receiver and that’s it, you can search.
Cumulative phase
The designs in this section show all the advantages of the phase accumulation method. The first metal detector, primarily for construction purposes, will cost very little, because... its most labor-intensive parts are made... from cardboard, see fig.:
The device does not require adjustment; integrated timer 555 is an analogue of the domestic IC (integrated circuit) K1006VI1. All signal transformations occur in it; The search method is pulsed. The only condition is that the speaker needs to be piezoelectric (crystalline), a regular speaker or headphones will overload the IC and it will soon fail.
Coil inductance is 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 0.25 PE wire, and a 70 mm coil contains 290 turns.
Metal detector “Butterfly”, see fig. on the right, in its parameters it is already close to professional instruments: the Soviet nickel is found at a depth of 15-22 cm, depending on the soil; sewer hatch - at a depth of up to 1 m. Effective in case of synchronization failures; diagram, board and type of installation - in Fig. below. Please note that there are 2 separate coils with a diameter of 120-150 mm, not DD! They must not intersect! Both speakers are piezoelectric, as before. case. Capacitors - heat-stable, mica or high-frequency ceramic.
The properties of the “Butterfly” will improve, and it will be easier to configure it if, firstly, you wind the coils with flat baskets; 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 is 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 in the diagram) end. Secondly, if individual transistors are replaced with a 2-transistor assembly for K159NT1 amplifier circuits or its analogues; A pair of transistors grown on the same crystal has exactly the same parameters, which is important for circuits with synchronization failure.
To set up the Butterfly, you need to accurately adjust the inductance of the coils. The author of the design recommends moving the turns apart or moving them or adjusting the coils with ferrite, but from the point of view of electromagnetic and geometric symmetry, it would be better to connect 100-150 pF trimming capacitors in parallel with 10,000 pF capacitors and twist them in different directions when tuning.
The setup itself 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 to the trimmer, and in the opposite shoulder we remove it. In 3-4 cycles you can achieve complete silence in the speakers - the device is ready for searching.
More about "Pirate"
Let's return to the famous "Pirate"; It is a pulse transceiver with phase accumulation. The diagram (see figure) is very transparent and can be considered a classic for this case.
The transmitter consists of a master oscillator (MG) on the same 555 timer and a powerful switch on T1 and T2. On the left is the ZG version without an IC; in it you will have to set the pulse repetition rate on the oscilloscope to 120-150 Hz R1 and the pulse duration to 130-150 μs R2. Coil L is common. A 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 re-emitted pulses accumulate in container 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 speed of the “reservoir” and, consequently, the sensitivity of the device. You can learn more about “Pirate” from the video:
Video: “Pirate” metal detector
and about the features of its configuration - from the following video:
Video: setting the threshold of the “Pirate” metal detector
On the beats
Those who want to experience all the delights of the beating search process with replaceable coils can assemble a metal detector according to the diagram in Fig. Its peculiarity, firstly, is its efficiency: the entire circuit is assembled on CMOS logic and, in the absence of an object, consumes very little current. Secondly, the device operates 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” with a varicap VD1. Mixing of the working and reference signals occurs in DD1.4. Third, this metal detector is suitable for working with replaceable coils.
It is better to replace the 176 series IC with the same 561 series, the current consumption will decrease and the sensitivity of the device will increase. You can’t just replace old Soviet high-impedance headphones TON-1 (preferably TON-2) with low-impedance ones from the player: they will overload the DD1.4. You need to either install an amplifier like the “pirate” one (C7, R16, R17, T3 and a speaker on the “Pirate” circuit), or use a piezo speaker.
This metal detector does not require any adjustments after assembly. The coils are monoloops. Their data on a 10 mm thick mandrel:
- Diameter 25 mm – 150 turns PEV-1 0.1 mm.
- Diameter 75 mm – 80 turns PEV-1 0.2 mm.
- Diameter 200 mm – 50 turns PEV-1 0.3 mm.
It couldn't be simpler
Now let's fulfill the promise we made at the beginning: we'll tell you how to make a metal detector that searches without knowing anything about radio engineering. A metal detector “as simple as shelling pears” is 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, but to detect objects it is not dispersion or delay with phase accumulation that is used, but the rotation of the magnetic vector of the EMF 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 for it to be, if not “super,” but at least a fully functional device, its components—the receiver and the calculator—must meet certain requirements.
Calculator you need the most tattered and cheapest, “alternative”. They make these in offshore basements. They have no idea about the standards for electromagnetic compatibility of household appliances, and if they heard about something like that, they wanted to choke it from the bottom of their hearts and from above. Therefore, the products there are quite powerful sources of pulsed radio interference; they are provided by the calculator's clock generator. In this case, its strobe pulses on the air are used to probe space.
Receiver We also need a cheap one, from similar manufacturers, without any means of increasing noise immunity. It must have an AM band and, which is absolutely necessary, a magnetic antenna. Since receivers that receive short waves (HF, SW) with a magnetic antenna are rarely sold and are expensive, you will have to limit yourself to medium waves (SV, MW), but this will make setup easier.
- We unfold the box with the lid into a book.
- We paste strips of adhesive tape onto the back sides of the calculator and the radio and secure both devices in the box, see fig. on right. Receiver - preferably in a cover so that there is access to the controls.
- We turn on the receiver and look for an area at maximum volume at the top of the AM band(s) that is free from radio stations and as clean as possible from ethereal noise. For CB this will be around 200 m or 1500 kHz (1.5 MHz).
- We turn on the calculator: the receiver should hum, wheeze, growl; in general, give the tone. We don't turn down the volume!
- If there is no tone, carefully and smoothly adjust until it appears; We caught some of the harmonics of the calculator's strobe generator.
- We slowly fold the “book” until the tone weakens, becomes more musical, or disappears altogether. Most likely this will happen when the lid is turned about 90 degrees. Thus, we have found a position in which the magnetic vector of the primary pulses is oriented perpendicular to the axis of the ferrite rod of the magnetic antenna and it does not receive them.
- We fix the lid in the found position with a foam insert and an elastic band or supports.
Note: depending on the design of the receiver, the opposite option is possible - to tune to the harmonic, the receiver is placed on the turned on calculator, and then, by unfolding the “book,” the tone softens or disappears. In this case, the receiver will catch pulses reflected from the object.
What's next? If there is an electrically conductive or ferromagnetic object near the opening of the “book,” it will begin to re-emit probing pulses, but their magnetic vector will rotate. The magnetic antenna will “sense” them, and the receiver will again give a tone. That is, we have already found something.
Something weird at last
There are reports of another metal detector “for complete dummies” with a calculator, but instead of a radio, it supposedly requires 2 computer disks, a CD and a DVD. Also - piezo headphones (precisely piezo, according to the authors) and a Krona battery. Frankly speaking, this creation looks like a technomyth, like the ever-memorable mercury antenna. But - what the hell is not joking. Here's a video for you:
try it if you wish, maybe you’ll find something there, both in the subject matter and in the scientific and technical sense. Good luck!
As an application
There are hundreds, if not thousands, of metal detector designs and designs. Therefore, in the appendix to the material we also provide a list of models, in addition to those mentioned in the test, which, as they say, are in circulation in the Russian Federation, are not overly expensive and are available for repetition or self-assembly:
- Clone. 8 ratings, average: 4,88 out of 5)
The "PIRAT" metal detector (short for PI - pulse, RA-T - radioskot - developer site) is easy to manufacture and configure, does not contain programmable elements that many radio amateurs are so afraid of, it does not contain expensive and scarce elements, and in terms of its parameters it is not inferior some foreign copies cost 100-300 USD. The main advantages of this device over other simple metal detector designs are stability and range. Even people with basic knowledge in the field of electronics can assemble this MD. Have you decided? Then let's go.
Metal detector parameters:
Power - 9-12 volts
Current consumption – 30-40 mA
Sensitivity - 25 mm coin - 20 cm
- large metal objects - 150 cm
The device consists of two main units, transmitting and receiving. The transmitting unit consists of a pulse generator on the KR1006VI1 microcircuit (a foreign analogue of NE555) and a powerful switch on the IRF740 transistor. The receiving unit is assembled on a K157UD2 microcircuit and a VS547 transistor.
Schematic diagram of the PIRATE metal detector
The coil is wound on a 190mm mandrel and contains 25 turns of PEV 0.5 wire. We will replace transistor T2 with a bipolar transistor NPN structure with a K-E voltage of at least 200 volts. It can be taken from an energy-saving lamp or a mobile phone charger. In extreme cases, even KT817 works as a T2, so you can experiment. Almost any NPN structure transistor can be used as T3. A properly assembled device requires virtually no adjustment. You may have to select a resistor R12 so that clicks in the speaker appear when R13 is in the middle position. If you have an oscilloscope, you can check the duration of the control pulse and the frequency of the generator on the T2 gate. The optimal pulse option is 130-150 μs, frequency 120-150 Hz.
Working with the device. When turning on, wait 15-20 seconds, after which we use the SENSITIVITY regulator to find a position at which clicks are heard in the speaker - this will be the maximum sensitivity. The device is easy to use and the skills to work with it come in just a couple of starts.
Anyone who has a problem purchasing the KR1006VI1 microcircuit can assemble a generator using transistors. But here, due to the scatter of their parameters, it may be necessary to select the frequency and duration of the pulse. For this, it is advisable to have an oscilloscope. Oscillograms at various points in the circuit are shown in the pictures.
Metal detector circuitPIRATwith a transistor generator:
- R1 in the generator is responsible for the generation frequency.
- R2 - for the duration of the control pulse.
For those who like to measure something, here are the voltages at the op-amp terminals (without the presence of metal in the sensor area):
2-6.5v
3-6.5v
5-5.5v
6-3.5v
9-0.7v
13-6.2v
The metal detector circuit is assembled on a printed circuit board made of foil fiberglass. Drawings for both versions of the metal detector in LAY5.0 format, oscillograms on the legs of the op-amp and a short video of the MD in operation are available. The archive also contains an option from user troll. I would like to immediately warn those who like to solder with all kinds of fluxes and acids - solder only with pure rosin or an alcohol-rosin solution! After soldering, wash off any remaining rosin with alcohol. This will save you from many questions, like: “I did it, but it doesn’t work.” Special thanks to my friend Bars59, for the greatest contribution to the development of this device.
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