Even the most serious and respectable citizens, at the word "treasure" experience a slight excitement. We literally walk through treasures, of which there are immeasurably many in our land.
But how do you look under the soil layer to know exactly where to dig?
Professional treasure hunters use expensive equipment, the purchase of which can pay off after one successful find. Archaeologists, builders, geologists, members of search societies - use the equipment provided by the organization in which they work.
But what about novice treasure hunters on a limited budget? You can make a metal detector at home with your own hands.
Popular metal detectors work using the properties of electromagnetic induction. Main components:
The generator, with the help of a transmitting coil, creates an electromagnetic field (EMF) around it with specified characteristics. The receiver scans the environment and compares the field performance with the reference. If there are no changes, nothing happens in the scheme.
Important! There is an erroneous opinion that the soil in which searches are made should not be electrically conductive.
This is wrong. The main thing is that the electromagnetic or ferromagnetic properties of the medium and search objects should be different from each other.
That is, against the background of certain characteristics of the EMF formed by the search environment, the field of individual objects will stand out.
A metal detector or metal detector is designed to detect objects that differ in their electrical and / or magnetic properties from the environment in which they are located. Simply put, it allows you to find metal in the ground. But not only metal, and not only in the ground. Metal detectors are used by inspection services, criminologists, military, geologists, builders to search for profiles under the skin, fittings, reconciliation of underground utility plans, and people of many other specialties.
Do-it-yourself metal detectors are most often made by amateurs: treasure hunters, local historians, members of military historical associations. They, beginners, are primarily intended for this article; the devices described in it make it possible to find a coin with a Soviet penny at a depth of up to 20-30 cm or a piece of iron with a sewer manhole about 1-1.5 m below the surface. However, this home-made device can also be useful on the farm during repairs or at a construction site. Finally, having found a centner or two of an abandoned pipe or metal structures in the ground and handing over the find for scrap, you can get a decent amount. And there are definitely more such treasures in the Russian land than pirate chests with doubloons or boyar-robber egg-pods with efimki.
Note: if you are not well versed in electrical engineering with radio electronics, do not be afraid of diagrams, formulas and special terminology in the text. The very essence is stated simply, and at the end there will be a description of the device, which can be made in 5 minutes on the table, not knowing how to not only solder, but twist the wires. But it will allow you to "feel" the features of the search for metals, and if interest arises, knowledge and skills will come.
A little more attention compared to the rest will be given to the Pirate metal detector, see fig. This device is quite simple for beginners to repeat, but in terms of its quality indicators it is not inferior to many branded models priced up to $300-400. And most importantly, it showed excellent repeatability, i.e. full performance when manufactured according to descriptions and specifications. Circuitry and the principle of operation of the "Pirate" are quite modern; There are plenty of guides on how to set it up and how to use it.
The metal detector operates on the principle of electromagnetic induction. In general, the metal detector circuit consists of an electromagnetic oscillation transmitter, a transmitting coil, a receiving coil, a receiver, a useful signal extraction circuit (discriminator) and an indication device. Separate functional units are often combined in circuitry and design, for example, the receiver and transmitter can work on one coil, the receiving part immediately highlights the useful signal, etc.
The coil creates an electromagnetic field (EMF) of a certain structure in the medium. If an electrically conductive object is in the area of \u200b\u200bits action, pos. And in the figure, eddy currents or Foucault currents are induced in it, which create its own EMF. As a result, the structure of the coil field is distorted, pos. B. If the object is not electrically conductive, but has ferromagnetic properties, then it distorts the original field due to shielding. In both cases, the receiver captures the difference between the EMF and the original one and converts it into an acoustic and/or optical signal.
Note: in principle, for a metal detector it is not necessary that the object be electrically conductive, the ground is not. The main thing is that their electrical and / or magnetic properties are different.
In commercial sources, expensive highly sensitive metal detectors, e.g. Terra-N are often called geoscanners. This is not true. Geoscanners operate on the principle of measuring the electrical conductivity of the soil in different directions at different depths, this procedure is called lateral logging. According to the logging data, the computer builds on the display a picture of everything in the earth, including geological layers of various properties.
The principle of operation of a metal detector can be implemented in technically different ways, according to the purpose of the device. Metal detectors for beach gold digging and construction and repair searches may look similar in appearance, but differ significantly in design and technical data. To properly make a metal detector, you need to clearly understand what requirements it must meet for this type of work. Based on this, the following parameters of search metal detectors can be distinguished:
Discrimination, in turn, is a composite parameter, since there are 1, maximum 2 signals at the output of the metal detector, and there are more values that determine the properties and location of the find. However, taking into account the change in the reaction of the device while approaching the object, 3 components are distinguished in it:
Note: mobility of metal detectors according to paragraphs. 2-4 is good: from one set of salt cells ("batteries") AA and without overworking the operator, you can work up to 12 hours.
Pulse metal detectors stand apart. Their primary current flows into the coil in pulses. By setting the pulse repetition rate within the LF, and their duration, which determines the spectral composition of the signal corresponding to the IF-HF ranges, you can get a metal detector that combines the positive properties of LF, IF and HF or is tunable.
There are at least 10 EMF search methods. But such as, say, the method of direct digitization of the response signal with computer processing is the lot of professional use.
A home-made metal detector is schematically built most of all in the following ways:
Parametric metal detectors in some way fall outside the definition of the principle of operation: they have neither a receiver nor a receiving coil. For detection, the direct influence of the object on the parameters of the generator coil is used - inductance and quality factor, and the structure of the EMF does not matter. Changing the parameters of the coil leads to a change in the frequency and amplitude of the generated oscillations, which is fixed in various ways: by measuring the frequency and amplitude, by changing the current consumption of the generator, by measuring the voltage in the PLL loop (phase locked loop, "pulling" it to a given value), etc.
Parametric metal detectors are simple, cheap and noise-resistant, but their use requires certain skills, because. the frequency "floats" under the influence of external conditions. Their sensitivity is weak; most of all are used as magnetic detectors.
The device of the transceiver metal detector is shown in fig. at the beginning, to an explanation of the principle of operation; the principle of operation is also described there. Such devices allow to achieve the best efficiency in their frequency range, but are complex in circuitry, require a particularly high-quality coil system. Transceiver metal detectors with a single coil are called induction. Their repeatability is better, because the problem of the correct arrangement of the coils relative to each other disappears, but the circuitry is more complicated - you need to highlight a weak secondary signal against the background of a strong primary.
Note: in pulsed transceiver metal detectors, the emission problem can also be eliminated. This is explained by the fact that as a secondary signal they “catch” the so-called. "tail" of the pulse re-radiated by the object. The primary pulse spreads due to dispersion during reemission, and a part of the secondary pulse is in the gap between the primary ones, from where it can be easily distinguished.
Metal detectors with phase accumulation, or phase-sensitive, are either single-coil pulsed, or with 2 generators, each working on its own coil. In the first case, the fact is used that during re-emission pulses not only spread out, but are also delayed. In time, the phase shift increases; when it reaches a certain value, the discriminator is triggered and a click is heard in the headphones. As you get closer to the object, the clicks become more frequent and merge into a higher pitched sound. It is on this principle that Pirate is built.
In the second case, the search technique is the same, but 2 strictly symmetrical electrically and geometrically generators work, each on its own coil. At the same time, due to the interaction of their EMF, mutual synchronization occurs: the generators work in time. When the overall EMF is distorted, synchronization breaks begin, audible as the same clicks, and then a tone. Two-coil metal detectors with a synchronization breakdown are simpler than impulse ones, but less sensitive: their penetration is 1.5-2 times less. Discrimination in both cases is close to excellent.
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.
Beats of 2 electrical signals - a signal with a frequency equal to the sum or difference of the main frequencies of the original signals or multiples of them - harmonics. So, for example, if signals with frequencies of 1 MHz and 1,000,500 Hz or 1.0005 MHz are applied to the inputs of a special device - a mixer, and headphones or a speaker are connected to the output of the mixer, then we will hear a pure tone of 500 Hz. And if the 2nd signal is 200 100 Hz or 200.1 kHz, the same thing will happen, because 200 100 x 5 = 1,000,500; we "caught" the 5th harmonic.
There are 2 generators in the beat detector: reference and working. The reference oscillatory circuit coil is small, protected from extraneous influences, or its frequency is stabilized by a quartz resonator (simply, quartz). The contour coil of the working (search) generator is a search coil, and its frequency depends on the presence of objects in the search area. Before searching, the working generator is tuned to zero beats, i.e. until the frequencies match. As a rule, they don’t achieve a complete zero sound, but tune it to a very low tone or wheezing, so it’s more convenient to search. By changing the tone of the beats, the presence, size, properties and location of the object are judged.
Note: most often, the frequency of the search generator is taken several times lower than the reference one and works on harmonics. This allows, firstly, to avoid the mutual influence of generators, which is harmful in this case; secondly, to tune the device more precisely, and thirdly, to search at the optimal frequency in this case.
In general, metal detectors based on harmonics are more complicated than impulse ones, but they work on any ground. Properly made and tuned, they are not inferior to impulse ones. This can be judged at least by the fact that beach gold diggers do not agree in any way on what is better: impulse or beat?
The most common misconception of novice radio amateurs is the absolutization of circuitry. Like, if the scheme is "cool", then everything will be tip-top. With regard to metal detectors, this is doubly untrue, because. their operational advantages strongly depend on the design and workmanship of the search coil. As a resort prospector put it: "The findability of a detector should pull the pocket, not the legs."
When developing a device, its circuit and coil parameters are adjusted to each other until an optimum is obtained. A certain scheme with a “foreign” coil, if it works, will not reach the declared parameters. Therefore, when choosing a prototype for repetition, see first of all the description of the coil. If it is incomplete or inaccurate, it is better to build another device.
A large (wide) coil radiates EMF more efficiently and “enlightens” the ground deeper. Its search area is wider, which allows you to reduce the "finding by feet". However, if there is a large unwanted object in the search area, its signal will be "hammered" by a weak one from the desired trifle. Therefore, it is advisable to take or make a metal detector designed to work with coils of different sizes.
Note: typical coil diameters are 20-90 mm for finding rebar and profiles, 130-150 mm "for beach gold" and 200-600 mm "for big iron".
The traditional type of metal detector coil is the so-called. thin coil or Mono Loop (single loop): a ring of many turns of enameled copper wire with a width and thickness of 15-20 times less than the average diameter of the ring. The advantages of a monoloop coil are the weak dependence of parameters on the type of soil, the search area narrowing downwards, which allows, by moving the detector, to more accurately determine the depth and location of the find, and structural simplicity. Disadvantages - low quality factor, which is why the tuning “floats” during the search, susceptibility to interference and a vague reaction to the object: working with a monoloop requires considerable experience in using this particular instance of the device. It is recommended for beginners to make homemade metal detectors with a mono-loop in order to get a workable design without any problems and gain search experience with it.
When choosing a circuit, in order to verify the authenticity of the author's promises, and even more so when designing or refining it yourself, you need to know the inductance of the coil and be able to calculate it. Even if you are making a metal detector from a purchased kit, you still need to check the inductance by measurements or by calculation, so as not to rack your brains later: why, everything seems to be in order, and not beeping.
Calculators for calculating the inductance of coils are available on the Internet, but a computer program cannot foresee all cases of practice. Therefore, in fig. given an old, decades-tested nomogram for calculating multilayer coils; a thin coil is a special case of a multilayer coil.
To calculate the search monoloop, the nomogram is used as follows:
The monoloop "catches" interference well, because arranged in exactly the same way as a loop antenna. You can increase its noise immunity, firstly, by placing the winding in the so-called. Faraday shield: a metal tube, braid or foil winding with a break so that a short-circuited coil does not form, which will “eat” all the EMI of the coil, see fig. on right. If there is a dotted line near the designation of the search coil on the original diagram (see the diagrams below), this means that the coil of this device must be placed in the Faraday shield.
Also, the screen must be connected to the common wire of the circuit. There is a catch for beginners here: the grounding conductor must be connected to the screen strictly symmetrically to the section (see the same figure) and connected to the circuit also symmetrically with respect to the signal wires, otherwise the interference will still “penetrate” into the coil.
The screen also absorbs some of the search EMF, which reduces the sensitivity of the device. This effect is especially noticeable in pulsed metal detectors; their coils cannot be shielded at all. In this case, an increase in noise immunity can be achieved by balancing the winding. The bottom line is that for a remote source of EMF, the coil is a point object, and emf. interference in its halves will overwhelm each other. A symmetrical coil may also be needed in circuitry if the generator is a push-pull or inductive three-point.
However, in this case, it is impossible to symmetricalize the coil with the usual bifilar method (see Fig.): when conducting and / or ferromagnetic objects are in the field of the bifilar coil, its symmetry is violated. That is, the noise immunity of the metal detector will disappear just when it is most needed. Therefore, the monoloop coil must be symmetrical by cross winding, see the same fig. Its symmetry is not broken under any circumstances, but winding a thin coil with a large number of turns in a cross way is hellish work, and then it is better to make a basket coil.
Basket coils have all the advantages of mono-loops to an even greater extent. In addition, basket coils are more stable, their quality factor is higher, and the fact that the coil is flat is a double plus: sensitivity and discrimination will increase. Basket coils are less susceptible to interference: harmful emfs. in crossing wires they cancel each other out. The only negative is that basket coils need a precisely made rigid and durable mandrel: the total tension force of many turns reaches large values.
Basket coils are structurally flat and voluminous, but electrically voluminous "basket" is equivalent to flat, i.e. creates the same EMF. The volumetric basket coil is even less sensitive to interference and, which is important for pulsed metal detectors, the pulse dispersion in it is minimal, i.e. easier to catch the variance caused by the object. The advantages of the original "Pirate" metal detector are largely due to the fact that its "native" coil is a voluminous basket (see Fig.), but its winding is complex and time-consuming.
It is better for a beginner to wind a flat basket on his own, see fig. below. For metal detectors "for gold" or, say, for the "butterfly" metal detector described below and a simple transceiver 2-coil, unusable computer disks will be a good mandrel. Their plating will not hurt: it is very thin and nickel. An indispensable condition: an odd, and nothing else, the number of slots. A nomogram is not required for calculating a flat basket; calculation is carried out in this way:
Some amateurs take it upon themselves to wind bulky baskets in the manner shown in fig. below: make a mandrel from insulated nails (pos. 1) or self-tapping screws, wind according to the scheme, pos. 2 (in this case, pos. 3, for the number of turns, a multiple of 8; every 8 turns the “pattern” is repeated), then foam, pos. 4, the mandrel is pulled out, and the excess foam is cut off. But it soon turns out that the stretched coils cut the foam and all the work went soft. That is, in order to wind securely, you need to glue pieces of durable plastic into the holes of the base, and only then wind it. And remember: an independent calculation of a volumetric basket coil without appropriate computer programs is impossible; the flat basket technique is not applicable in this case.
DD in this case does not mean long-range, but a double or differential detector; in the original - DD (Double Detector). This is a coil of 2 identical halves (shoulders), folded with some intersection. With an accurate electrical and geometric balance of the DD arms, the search EMF is pulled into the intersection zone, on the right in Fig. on the left - a monoloop coil and its field. The slightest inhomogeneity of space in the search area causes an imbalance, and a sharp strong signal appears. The DD-coil allows an inexperienced searcher to detect a shallow, deep, well-conducting object when a rusty can lies next to it and above.
Coils DD are clearly oriented "on gold"; all metal detectors with the GOLD marking are equipped with them. However, on finely heterogeneous and / or conductive soils, they either fail completely, or often give false signals. The sensitivity of the DD coil is very high, but the discrimination is close to zero: the signal is either marginal or not at all. Therefore, metal detectors with DD coils are preferred by seekers who are only interested in "being in the pocket."
Note: more details about DD coils can be found later in the description of the corresponding metal detector. They wind their shoulders DD or in bulk, like a monoloop, on a special mandrel, see below, or with baskets.
Ready-made frames and mandrels for search coils are sold in a wide range, but sellers are not shy about cheating. Therefore, many amateurs make the base of the plywood coil, on the left in the figure:
The simplest metal detector for searching for fittings, wiring, profiles and communications in walls and ceilings can be assembled according to fig. The ancient transistor MP40 changes without any change to KT361 or its analogues; to use pnp transistors, you need to reverse the polarity of the battery.
This metal detector is a parametric type magnetic detector operating at low frequencies. The tone of the sound in the headphones can be changed by selecting the capacitance C1. Under the influence of the object, the tone drops, unlike all other types, so initially you need to achieve a “mosquito squeak”, and not wheezing or grumbling. The device distinguishes wiring under current from “empty”, a hum of 50 Hz is superimposed on the tone.
The circuit is a pulse generator with inductive feedback and frequency stabilization by an LC circuit. Loop coil - an output transformer from an old transistor receiver or a low-power "Bazaar-Chinese" low-voltage power transformer. A transformer from an unusable power source of a Polish antenna is very well suited, in its own case, by cutting off the mains plug, you can assemble the entire device, then it is better to power it from a 3 V lithium tablet battery. Winding II in fig. – primary or network; I - secondary or step-down at 12 V. That's right, the generator works with transistor saturation, which provides negligible power consumption and a wide range of pulses, making it easier to find.
To turn the transformer into a sensor, its magnetic circuit must be opened: remove the frame with the windings, remove the straight jumpers of the core - the yoke - and fold the W-shaped plates in one direction, as on the right in the figure, then put the windings back on. With serviceable parts, the device starts working immediately; if not, you need to swap the ends of any of the windings.
The parametric scheme is more complicated - in fig. on right. L with capacitors C4, C5 and C6 is tuned to 5, 12.5 and 50 kHz, and quartz passes the 10th, 4th harmonics and fundamental tone to the amplitude meter, respectively. The scheme is more for an amateur to get drunk on the table: there is a lot of fuss with the setting, but there is no "flair", as they say. Provided as an example only.
Much more sensitive is a transceiver metal detector with a DD coil, which can be easily made at home, see fig. Left - transmitter; on the right is the receiver. It also describes the properties of different types of DD.
This metal detector is LF; search frequency is about 2 kHz. Depth of detection: Soviet penny - 9 cm, canning tin - 25 cm, sewer hatch - 0.6 m. The parameters are “triple”, but you can master the technique of working with DD before moving on to more complex structures.
The coils contain 80 turns of 0.6-0.8 mm PE wire, wound in bulk on a mandrel 12 mm thick, the drawing of which is shown in fig. left. In general, the device is not critical to the parameters of the coils, they would be exactly the same and arranged strictly symmetrically. In general, a good and cheap simulator for those who want to master any search technique, incl. "for gold". Although the sensitivity of this metal detector is not high, but the discrimination is very good despite the use of DD.
To set up the device, first, instead of the L1 transmitter, turn on the headphones and make sure that the generator is working by the tone. Then L1 of the receiver is short-circuited and, by selecting R1 and R3, a voltage is set on the collectors VT1 and VT2, respectively, equal to about half the supply voltage. Next, R5 set the collector current VT3 within 5..8 mA, open L1 of the receiver and that's it, you can search.
The designs in this section show all the advantages of the phase accumulation method. The first metal detector mainly for construction purposes will be very inexpensive, because. its most labor-intensive parts are made ... of cardboard, see fig.:
The device does not require adjustment; integrated timer 555 - an analogue of the domestic IC (integrated circuit) K1006VI1. All signal transformations take place in it; search method - impulse. The only condition is that the speaker needs a piezoelectric (crystalline), a regular speaker or headphones will overload the IC and it will soon fail.
Coil inductance - about 10 mH; operating frequency - within 100-200 kHz. With a mandrel thickness of 4 mm (1 layer of cardboard), a coil with a diameter of 90 mm contains 250 turns of PE 0.25 wire, and a 70 mm coil contains 290 turns.
Metal detector "Butterfly", see fig. on the right, in terms of its parameters it is already close to professional devices: the Soviet penny is found at a depth of 15-22 cm, depending on the soil; sewer manhole - at a depth of up to 1 m. Acts on disruption of synchronization; diagram, board and type of installation - in fig. below. Please note, there are 2 separate coils with a diameter of 120-150 mm, not DD! They must not overlap! Both speakers are piezoelectric, as in the previous. case. Capacitors - thermostable, mica or high-frequency ceramic.
The properties of the "Butterfly" will improve, and it will be easier to set it up if, firstly, wind the coils with flat baskets; the inductance is determined by the given operating frequency (up to 200 kHz) and the capacitances of the loop capacitors (10,000 pF each in the diagram). Wire diameter - from 0.1 to 1 mm, the larger the better. The tap in each coil is made from a third of the turns, counting from the cold (lower according to the diagram) end. Secondly, if individual transistors are replaced with a 2-transistor assembly for K159NT1 dif-amplifier circuits or its analogues; a pair of transistors grown on a single chip has exactly the same parameters, which is important for circuits with a synchronization failure.
To establish the "Butterfly" you need to accurately adjust the inductance of the coils. The author of the design recommends moving apart and shifting the turns or adjusting the coils with ferrite, but from the point of view of electromagnetic and geometric symmetry, it would be better to connect trimmer capacitors of 100-150 pF in parallel with 10,000 pF capacitances and twist them when tuning in different directions.
The actual adjustment is not difficult: the newly assembled device beeps. We alternately bring an aluminum saucepan or a beer can to the coils. To one - the squeak becomes higher and louder; to the other - lower and quieter or completely silent. Here we add a little capacity of the trimmer, and remove it in the opposite shoulder. For 3-4 cycles, you can achieve complete silence in the speakers - the device is ready to search.
Let's return to the famous "Pirate"; it is a pulse transceiver with phase accumulation. The scheme (see fig.) is very transparent and can be considered a classic for this case.
The transmitter consists of a master oscillator (MG) on the same 555th timer and a powerful key on T1 and T2. On the left - a variant of the ZG without an IC; it will have to set the pulse repetition rate of 120-150 Hz R1 and the pulse duration of 130-150 μs R2 on the oscilloscope. Coil L - common. The limiter on diodes D1 and D2 for a current of 0.5 A saves the QP1 receiver amplifier from overload. The discriminator is assembled on QP2; together they make up the dual operational amplifier K157UD2. Actually, the "tails" of the reradiated pulses are accumulated in the capacitance C5; when the “reservoir is full”, a pulse jumps at the output of QP2, which is amplified by T3 and gives a click in the dynamics. Resistor R13 regulates the filling rate of the "reservoir" and, consequently, the sensitivity of the device. More about "Pirate" can be found in the video:
and about the features of its settings - from the following video:
Those who wish to experience all the delights of the process of searching on beats with replaceable coils can assemble a metal detector according to the scheme in fig. Its peculiarity, firstly, is efficiency: the entire circuit is assembled on CMOS logic and, in the absence of an object, consumes very little current. Secondly, the device works on harmonics. The reference oscillator on DD2.1-DD2.3 is stabilized by ZQ1 quartz at 1 MHz, and the search oscillator on DD1.1-DD1.3 operates at a frequency of about 200 kHz. When setting up the device before searching, the desired harmonic is “caught” by the VD1 varicap. The mixing of the working and reference signals occurs in DD1.4. Thirdly, this metal detector is suitable for work with replaceable coils.
It is better to replace the ICs of the 176th series with the same 561st ones, the current consumption will decrease, and the sensitivity of the device will increase. It is simply impossible to replace the old Soviet high-resistance headphones TON-1 (preferably TON-2) with low-resistance ones from the player: they will overload DD1.4. You need to either put an amplifier like a "pirate" one (C7, R16, R17, T3 and a speaker on the "Pirate" circuit), or use a piezo speaker.
This metal detector does not require settings after assembly. Coils are monoloops. Their data on a mandrel 10 mm thick:
Now let's fulfill the promise given at the beginning: we will tell you how to make, without knowing anything about radio engineering, the metal detector that you are looking for. The metal detector is “easier than simple” assembled from a radio, a calculator, a cardboard or plastic box with a hinged lid, and pieces of double-sided tape.
The metal detector “from the radio” is pulsed, however, to detect objects, it is not dispersion and not delay with phase accumulation that are used, but the rotation of the EMF magnetic vector during re-emission. On the forums, they write different things about this device, from “super” to “sucks”, “wiring” and words that are not customary to use in writing. So, in order to get, if not “super”, but at least a fully functional device, its components - the receiver and the calculator - must meet certain requirements.
Calculator we need the tiniest and cheapest, "alternative". They make them in offshore cellars. They have no idea about the standards for electromagnetic compatibility of household appliances, and if they heard about something like that, then they wanted to spit from the bottom of their hearts. Therefore, local products are quite powerful sources of impulse radio interference; they are given by the clock generator of the calculator. In this case, its strobe pulses on the air are used to probe the space.
Receiver you also need a cheap one, from similar manufacturers, without any means of increasing noise immunity. It must have an AM band and, absolutely necessary, a magnetic antenna. Since receivers with short wave (HF, SW) reception on a magnetic antenna are rarely sold and are expensive, you will have to limit yourself to medium waves (MW, MW), but this will make tuning easier.
Note: depending on the design of the receiver, the reverse option is possible - to tune in to the harmonica, the receiver is placed on the included calculator, and then, laying out the “book”, the tone is softened or disappears. In this case, the receiver will catch the pulses reflected from the object.
And what's next? If there is an electrically conductive or ferromagnetic object near the opening of the "book", it will re-emit probing pulses, but their magnetic vector will turn. The magnetic antenna will “smell” them, the receiver will again give a tone. That is, we have already found something.
There are reports of another metal detector "for complete dummies" with a calculator, but instead of a radio, supposedly 2 computer disks, a CD and a DVD, are needed. Also - piezo headphones (precisely piezo, according to the authors) and a Krona battery. Frankly speaking, this creation looks like a techno-myth, like a memorable mercury antenna. But - what the hell is not joking. Here's a video for you:
try it, if you wish, maybe something will be found there, both in the subject and in the scientific and technical sense. Good luck!
There are hundreds, if not thousands, of schemes and designs of metal detectors. Therefore, in the appendix to the material, we also give a list of models, in addition to those mentioned in the test, which, as they say, are in circulation in the Russian Federation, are not overly expensive and are available for repetition or self-assembly:
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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.
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 ....
This metal detector was developed by Evgeny based on the terminator pro. The device showed itself from the best side, discrimination at a high level, low current consumption of the device, low cost and availability of parts, as well as the ability to work on heavy soils. The device board has been tested and works with a bang.
Specifications:
The principle of operation is inductively balanced
Operating frequency, kHz 8-15kHz
Operating mode dynamic
Precise Detection Mode (Pin-Point) No
- Discrimination mode, two-tone voice acting (low - iron, high - non-ferrous metal)
Food, V 9-12
There is a sensitivity level control
Ground balance available (manual)
Air detection depth with DD-250mm sensor
Coins 25mm - about 35cm
Gold ring - 30cm
Helmet 100-120cm
Maximum depth 150cm
Consumption current:
Silent approximately 35 mA
Metal detector scheme:
Soldering should be neat - there should be no clamps and sticky, after assembly, be sure to rinse the board with alcohol.
The printed circuit board was provided by the user under the nickname DeD, for which many thanks to him.
We are starting to make a metal detector unit.
Removed the fee.
We drill holes)) drill 0.8.
Another recommendation, 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)
Here is the finished board from the detail side.
In my assembly, I did not solder the battery discharge unit.
From the side of soldering.
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. Where there should be 4 volts, then there should be 4 volts (well, plus or minus a few millivolts), and so on for all points.
Further, in order to have the correct reaction to the threshold knob, you need to select the microcircuits ms7, ms8. No current should flow through resistors R27, R28; for this, the voltage at the output of MC8 must exactly match the voltage at the output of MC7. To do this, you pull out the MC8 from the socket and put the MC6 in place. You check the voltage on the 6th leg of the MC6-MS7 and on the 8th leg of the MC6-MS7, if they are equal in pairs, you pull out the microcircuit from the MC6 socket and put it in place of the MC8. In fact, the output voltage of MC7 is equal to the input voltage of MC8, then the current through R27, R28 does not flow and the circuit is thermally stable. The output voltage of the MC8, when it is selected as it should, is regulated using R60 within 0 ... + 1V. Usually, at a supply voltage of +4.040V, the voltage at the output of MC6, MC7 is about +1.950 ... + 2.002V. If the output voltage is strong from these levels
different, then either the chips are rubbish.
In my case, this turned out to be the most difficult stage in setting up. Since not all chips fit. I bought 14 pieces of HEF4069 chips. 8 pieces were HCF4069. And it worked only with CD4069 chips.
Coil making
The DD sensor is made according to the same principle as for all balancers, so I will focus only on the required parameters.
TX is the transmitting coil and RX is the receiving coil. number of turns: 30 turns with a wire folded in half wire diameter: 0.4 enameled winding and transmitting and receiving coils are wound with a double wire (that is, 4 ends of the wire should be obtained), we determine the winding arms with a tester and connect the beginning of one arm to the end of the other, it turns out middle output of the coil. Coils after winding are tightly wrapped 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. The middle output of TX is connected to the minus of the board (without this, the generator will not start), the middle output of RX is needed only for frequency tuning, after tuning for frequency (resonance) it is isolated and the receiving coil turns into a normal one (without output). The receiver for tuning is connected instead of the transmitter and is tuned 100Hz-150Hz below the transmitter. Each of the coils is tuned in frequency separately, there should not be any metal objects nearby!!! Coils can be shielded 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 case. Balancing is carried out by shifting the coils (as on wedding rings) relative to each other. The balance should be within 20-30mV but not higher than 100mV.
The device can operate from 7KHz to 20KHz. The lower the frequency - the deeper it will take the target, but at the same time there will be worse discrimination on some targets, and vice versa, the higher the frequency, the less depth but better the discrimination to some targets (such as gold for example). Therefore, I think that it is better to choose, as they say, the “golden mean” - this is about 10KHz - 14KHz.
The cable has 4 cores in a common screen, two wires for the transmitting coil and two for the receiving coil, a screen for the body.
Then we put C5 to start with 4n7, ran a ferrite over the coil (if there was a double beep, then everything is fine, if it is single, then the ends were transferred to TX in places), connected the oscilloscope probe to the C5 output and move the coils to achieve a minimum amplitude.
So the device works, on which TX or RX coil to solder additional capacitors when setting up the reaction to metals!
The chocolate foil is on one end of the scale, the copper is on the other end. This is where you get your bearings.
Here, for reference, the entire VDI scale, with the discrim knob set to a minimum, the device should see all non-ferrous metals, when winding the discrim, all metals should be cut in order to copper, copper should not be cut, if the device works like this, then it is configured correctly.
A few words about the work of BG.
The BG 100k resistor should have a resistance of -100k in the 7 o'clock position, and 0k in the 17 o'clock position. When rotated clockwise, the resistance decreases.
The soil is a weakly mineralized thing, so it is balanced at BG = 100 ... 80k, i.e. almost all the way to the left. Ferrite is a highly mineralized thing, so it must be balanced at BG = 15 ... 10k, i.e. hours by 15. In this case, the voice acting should be as follows: if the BG is adjusted from 7 to (approximately) 15 hours, then the voice acting on the ferrite should be high, this indicates that the ferrite is undercompensated. if the BG is regulated from 15 to 17 hours, then the voice acting should be low, this indicates that the ferrite is overcompensated (in fact, we demolished it to the iron area). During normal operation ON REAL low-mineralized ground, ferrite is ALWAYS undercompensated and is voiced in a high tone, although the tone is blurry and fuzzy. When setting the VDI scale, check the sound of the ferrite.
ALLMET mode setting
Resistor R8 type 3296W trimmer. Turn on the device in the ALLMET mode, take a nickel and a piece of ferrite and wind R8 so that the nickel has a high tone, and the ferrite has a low tone.
