Homemade metal detectors, or how to make a metal detector with your own hands. Do-it-yourself deep metal detector: diagram, instructions and reviews How to make more discrimination of the metal detector yourself
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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.
Operating principle
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.
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 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.
Varieties
Common parameters
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:
- Penetration, or penetrating power - the maximum depth to which the EMF of the coil extends in the ground. Deeper, the device will not detect anything at any size and properties of the object.
- The size and dimensions of the search area is an imaginary area in the ground in which the object will be found.
- 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 only beeps for precious metals.
- Noise immunity - the ability not to respond to EMF of extraneous sources: radio stations, lightning discharges, power lines, electric vehicles and other sources of interference.
- Mobility and efficiency are determined by power consumption (how many batteries are enough), the device’s weight and dimensions, and the size of the search area (how much you can “probe” in 1 pass).
- Discrimination, or resolution - gives the operator or control microcontroller the ability to judge the nature of the object found by the reaction of the device.
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:
- 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 material of the object.
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:- Super-low-frequency (VLF) - up to the first hundreds of Hz. Absolutely non-amateur devices: power consumption from tens of watts, without computer processing, it is impossible to judge anything from a signal, vehicles are needed to move.
- Low-frequency (LF) - from hundreds of Hz to several kHz. They are simple in circuitry and design, noise-resistant, but not very sensitive, discrimination is poor. Penetration - up to 4-5 m with power consumption from 10 W (the so-called deep metal detectors) or up to 1-1.5 m when powered by batteries. They react most sharply to ferromagnetic materials (ferrous metal) or large masses of diamagnetic materials (concrete and stone building structures), therefore they are sometimes called magnetic detectors. They are not very sensitive to the properties of the soil.
- Increased frequency (IF) - up to several tens of kHz. More difficult than bass, but the requirements for the coil are low. Penetration - up to 1-1.5 m, C grade noise immunity, good sensitivity, satisfactory discrimination. Can be universal when used in pulsed mode, see below. On flooded or mineralized soils (with fragments or particles of rock that shield EMF), they work poorly or do not smell anything at all.
- High, or radio frequency (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.) Power consumption - as before. n. The rest is on the verge of "failure". The efficiency of the device largely depends on the design and quality of the coil(s).
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.
Search method
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.
- Receiving-transmitting.
- With phase accumulation.
- On the beat.
Without receiver
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.
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 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 due to dispersion during reemission spreads, and part of the secondary pulse is in the gap between the primary ones, from where it is easy to distinguish.
Click to Click
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 the pulses during reradiation 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.
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
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?
Coil and more
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.
About coil dimensions
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".
Monoloop
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.
Inductance
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:
- We take the value of the inductance L from the description of the device and the dimensions of the loop D, l and t from there or at our choice; typical values: L = 10 mH, D = 20 cm, l = t = 1 cm.
- According to the nomogram, we determine the number of turns w.
- We set the laying coefficient k = 0.5, by the dimensions l (coil height) and t (its width) we determine the cross-sectional area of \u200b\u200bthe loop and find the area of pure copper in it as S = klt.
- Dividing S by w, we get the cross section of the winding wire, and along it - the diameter of the wire d.
- If it turned out d = (0.5 ... 0.8) mm, everything is OK. Otherwise, we increase l and t at d>0.8 mm or decrease at d<0,5 мм.
Noise immunity
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
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:
- 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 factor 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 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 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 (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.
How to attach a coil
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:
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 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.
transceiver
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.
With phase accumulation
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.
More about Pirate
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 overflows", 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:
Video: Pirate metal detector
and about the features of its settings - from the following video:
Video: setting the threshold of the Pirate metal detector
On the beat
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:
- Diameter 25 mm - 150 turns of PEV-1 0.1 mm.
- Diameter 75 mm - 80 turns of PEV-1 0.2 mm.
- Diameter 200 mm - 50 turns of PEV-1 0.3 mm.
It doesn't get easier
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.
- We unfold the box with a lid into a book.
- We stick strips of adhesive tape on the back sides of the calculator and the radio and fix both devices in the box, see fig. on right. The receiver - preferably in the lid, so that there is access to the controls.
- We turn on the receiver, we are looking for a section free from radio stations and as clean as possible from radio noise by setting it to maximum volume at the top of the AM band (bands). For MW this will be around 200 m or 1500 kHz (1.5 MHz).
- We turn on the calculator: the receiver should buzz, wheeze, growl; in general, give a tone. We do not remove 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 rotated about 90 degrees. Thus, we have found a position in which the magnetic vector of primary impulses is oriented perpendicular to the axis of the ferrite rod of the magnetic antenna and it does not receive them.
- We fix the cover in the found position with a foam insert and an elastic band or supports.
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.
Something strange in the end
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!
as an application
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:
- Clone. 8 ratings, average: 4,88 out of 5)
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.
Coming to the moment of choosing a metal detector, you need to clearly understand what exactly you want to get as a result of your searches. This time will be spent for the pleasure of being in nature and the possible discovery of any finds, or you want to purposefully look for certain artifacts that relate to military topics or the search for coins, jewelry and other items reflecting the stages of historical development other than militaristic.
The amount of future investment in the purchased metal detector depends on the choice made at this stage.
Eight-segment metal discrimination scale on the Bounty Hunter Platinum display
So, if you want to find objects made of iron or alloys containing a significant part of it, then it makes sense to choose a metal detector based on the detection depth, because. the inclusion of the discrimination function directly affects this indicator, and not for the better.
If the purpose of the search is non-ferrous metals (copper, bronze, silver, gold) or products made from them, then the discrimination mode is your good helper, since it will allow you to configure the device to exclude iron-containing objects from the detection area. And this, in turn, will allow you not to dig dozens of extra holes, which will save your strength, nerves and time at the next exit.
Exclusion from the search for targets that are undesirable for detection based on their electrical conductivity.
Depending on the level of professionalism of the metal detector, the options for setting the discrimination of metals change. The closer the device is to the initial level, the more simplified this process and, quite logically, the setting itself is less accurate.
If the metal detector is equipped with a display, then it must have a visual image of the discrimination scale, which usually displays an indication of the object from 0 to 99. What does this mean? The closer the indicator is to zero, the lower the electrical conductivity of the detected object.
If we consider the electrical conductivity of pure metals, then the discrimination scale of the metal detector should look like this:
iron/nickel/zinc/aluminum/gold/copper/silver
This gradation is observed almost everywhere, but with a caveat - as a rule, not only the names of metals, but also products made from them are indicated on the scale. Such an approach is absolutely fair considering that in most cases for the manufacture of a particular item, whether it be jewelry, coins or beer caps, not refined metal is used, but its alloy. And, depending on which metal was used as a ligature (additive), the electrical conductivity of the product itself also changes.
For example, zinc, nickel, platinum, and copper are added to gold jewelry, depending on the production technology. The first three metals have electrical conductivity closer to iron, i.e. even less than that of aluminum, therefore, when gold jewelry is detected, a value between aluminum and iron will be displayed on the discrimination scale, respectively, the sound signal of the metal detector will be low.
If more copper was used in the manufacturing process than other ligatures, then the signal will be higher (subject to polyphony) and the indicator will show the presence of metal in the segment from aluminum to copper.
Discriminator operating modes
As a rule, budget metal detectors are able to separate metals only into non-ferrous and ferrous ( variable discrimination), as well as provide the user with the ability to search for all metals (all metal mode).
Higher-level devices have the ability to customize the metal detector to exclude from the search a group of metals with a certain conductivity ( selective discrimination). For example, a detector can exclude signals from iron, nickel and aluminum, but respond to targets from silver and copper.
For the convenience of users, manufacturers include pre-set discrimination modes in the metal detector settings, for example, "coins", "coins and jewelry", "relics", "all metals", which makes it easier for a novice treasure hunter to work with the device. After all, you can use such a metal detector on the principle of "turned on and went", initially without delving into the subtleties of the settings. And having studied its work in practice, already consciously, based on the goals of the search, create your own discrimination masks (unless, of course, such an opportunity is built into the purchased metal detector).
Conclusion
Summing up the conversation about metal discrimination, make a conclusion for yourself which finds will be the target of your search and decide whether it is worth overpaying for this feature or not. If it is still difficult to make a final decision, do not forget that the metal detector can always be switched to the "all metals" mode by turning off the discrimination function.
Good luck in your search!
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Rash 22.11.2018
Great explanation, thank you very much!
Eee 03.10.2018
Thank you. Och is intelligible.
Guest 02.12.2017
Really good article. And I didn’t think about additives (ligatures) in jewelry at all ... it seems that the metal detector needs to be reconfigured ....
Today I want to bring to your attention the scheme of the metal detector, and everything related to it, what you see in the photo.
The metal detector can work in the mode of both searching for all metals and in background discrimination.
Technical characteristics of the metal detector.
The principle of operation is inductively balanced
-Working frequency, kHz 8-10kHz
-Dynamic operation mode
-Precise detection mode (Pin-Point) is in static
- Power, V 12
- There is a sensitivity level control
- There is a threshold tone control
- Detuning from the ground is (manual)
Depth of detection in the air with a sensor DD-250mm In the ground, the device sees targets almost the same as in the air.
- coins 25mm - about 30cm
- gold ring - 25cm
- helmet 100-120cm
-max depth 150cm
-Consumption current:
- Mute approximately 30mA
And the most important and intriguing is the scheme of the device itself
To assemble a metal detector, you need the following parts:
So that you do not have to strongly configure the device, do the assembly and soldering carefully, the board should not contain any clamps, and sticky.
For tinning boards, it is best to use rosin in alcohol, after tinning the tracks, do not forget to wipe the tracks with alcohol
Parts side fee
We start the assembly by soldering the jumpers, then the resistors, then the sockets for the microcircuits and everything else. Another small recommendation, now regarding the manufacture of the device board. It is highly desirable to have a tester that can measure the capacitance of capacitors. The fact is that the device has two identical amplification channels, so the amplification through them should be as identical as possible, and for this it is desirable to select those details that are repeated on each amplification stage so that they have the most identical parameters measured by the tester (that is, which readings in a particular stage on one channel - the same readings on the same stage and in another channel)
Making a coil for a metal detector
Today I would like to talk about the manufacture of the sensor in a finished case, so the photo is more than words.
We take the case, fix the pressure seal in the right place and install the cable, call the cable and mark the horses.
Next we wind the coils. The DD-sensor is made according to the same principle as for all balancers, so I will focus only on the required parameters.
TX - transmitting coil 100 turns 0.27 RX - receiving coil 106 turns 0.27 enameled winding wire.
Coils after winding are tightly wound with threads, impregnated with varnish.
After drying, tightly wrap with electrical tape around the entire circumference. It is shielded from above with foil, between the end and the beginning of the foil there must be a gap of 1 cm uncovered by it, in order to avoid a short-circuited turn.
It is possible to shield the coil with graphite, for this we mix graphite with nitro varnish 1: 1 and cover it with an even layer on top of 0.4 tinned copper wire wound on a coil (without gaps), we connect the wire to the cable screen.
We wet it into the case, connect and approximately bring the coils into balance, there should be a double beep on the ferit, a single beep on the coin, if on the contrary, we swap the conclusions of the receiving winding. Each of the coils is tuned in frequency separately, there should not be any metal objects nearby!!! The coils are tuned with a prefix for measuring resonance. We connect the prefix to the Eldorado board in parallel with the transmitting coil and measure the frequency, then with the RX coil and the selected conder we achieve a frequency 600 Hz higher than that in TX.
After selecting the resonance, we assemble the coil together and check whether the device sees the entire VDI scale from aluminum foil to copper, if the device does not see the entire scale, then we select the capacitance of the resonant capacitor in the RX circuit with a step of 0.5-1nf in one direction or another, so that the moment when the device will see foil and copper at the minimum of discrim, and when the disc is wound up, the entire scale will be cut out in turn.
We finally bring the coils to zero, fixing everything with hot glue. Next, to facilitate the coil, we glue the voids with pieces of foam, the foam sits on hot glue, otherwise it will pop up after filling the coil.
Pour the first layer of epoxy without adding 2-3mm to the top
We fill in the second layer of resin with color. As a color, aniline dye is well suited for dyeing fabrics, the powder comes in different colors and costs a penny. The dye must first be mixed with the hardener, then the hardener is added to the resin, the dye will not dissolve immediately in the resin.
Correct assembly of the board, start by checking the correct power supply to all nodes.
Take the circuit and the tester, turn on the power on the board, and referring to the circuit, go through the tester at all points of the nodes where power should be supplied.
When the discrim knob is set to the minimum, the device should vibrate all non-ferrous metals, when twisting the discrim, it should cut out
all metals in order to copper, less should not be cut, if the device works this way, then it is set up correctly. The discrim scale must be selected in such a way that it fits completely into the full turn of the discrim knob, this is done by selecting c10. When the capacity decreases, the scale stretches and vice versa.
In conclusion, I want to say about the cable, it has 4 cores in a common screen, two wires to the transmitting coil and two to the receiving coil, the screen to the body.
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