best reviews metal detectors for gold
Interesting information about treasure hunters
Improvised metal detectors simple and more complex
A metal detector is designed to detect objects that are electrically and/or magnetically different 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, forensics, military, geologists, builders to search for profiles under the cladding, fittings, reconciliation of plans-schemes of underground communications, and people of many other specialties.

Metal detectors with their own hands are most often made by amateurs: treasure hunters, local historians, members of military-historical associations. They, beginners, and intended primarily this article; the devices described in it allow you to find a coin with a Soviet heel at a depth of 20-30 cm or a piece of iron with a sewer hatch about 1-1.5 m below the surface. However, this homemade device can be useful on the farm during repairs or construction. Finally, if you find a centner or metal structure in the ground and scrap the find, you can get a decent amount. And such treasures in the land of the Russian are exactly more than pirate chests with duplicates or boyar-robber cubes with efimki.

Note: if you are not knowledgeable in electronics, do not be put off by the diagrams, formulas and special terminology in the text. The very essence is presented simply, and at the end will be a description of the device, which can be done in 5 minutes on the table, not knowing not what to solder, and wiring twist. But it will allow to “feel” the features of the search for metals, and there will be interest - will come and knowledge with skills.
A little more attention than the rest will be paid to the metal detector “Pirate”, see Fig. This device is simple enough for repeat beginners, but in their quality indicators is not inferior to many branded models price up to $300-400. And most importantly - it showed excellent repeatability, i.e. full performance in the manufacture of descriptions and specifications. The scheme and the principle of the Pirate are quite modern; there are enough manuals to set it up and use it.

How to act



The metal detector operates on the principle of electromagnetic induction. In general, the metal detector scheme consists of an electromagnetic oscillation transmitter transmitting the coil, a receiving coil, a receiver, a useful signal (discriminatory) circuit and an indication device. Individual functional nodes often combine schemetechnically and constructively, for example. the receiver and transmitter can work on one reel, the receiving part immediately emits a useful signal, etc.

The principle of the metal detector



The coil creates an electromagnetic field (EMF) of a certain structure in the environment. If there is an electrically conductive object in the area of its action, poses. And in the rice. it is caused by vortex currents or Fuco currents, which create its own EMF. As a result, the structure of the field of the coil is distorted, poses. B. If the object is not electrically conductive, but has ferromagnetic properties, it distorts the original field by shielding. In both cases, the receiver captures the difference between eMF and the original and converts it into an acoustic and/or optical signal.

Note: in principle, for a metal detector, it is not necessary for the object to be an electrical conductor, the ground is not. The main thing is that their electrical and/or magnetic properties differ.

Detector or scanner?



In commercial sources expensive highly sensitive metal detectors, for example. Terra-N is often referred to as geoscanners. That’s not true. Geoscanners operate on the principle of measuring the electrical conductivity of the ground in different directions at different depths, this procedure is called lateral logging. According to the logging computer builds on the display a picture of everything in the ground, including different properties geological layers.

Variety



Common options



The principle of the metal detector can be embodied technically in different ways, according to the purpose of the device. Metal detectors for beach gold prospectors and construction and repair search outwardly can be similar, but significantly differ in scheme and technical data. To make the metal detector right, you need to be clear about what requirements it must meet for this kind of work. On this basis, the following parameters of metal detectors can be highlighted:

The permination, or penetrating ability, is the maximum depth to which the EMF coil in the ground extends. Deeper the device will not detect anything at any size and properties of the object.
The size and size of the search area is an imaginary area in the ground in which the object will be detected.

Sensitivity is the ability to detect more or less small objects.
Selectivity is the ability to respond more strongly to desirable findings. The sweet dream of beach diggers is a detector that only squeaks to precious metals.

Interference resistance - the ability not to respond to EMF foreign sources: radio stations, thunderstorm discharges, power lines, electric vehicles, etc. sources of interference.

Mobility and efficiency are determined by energy consumption (how many batteries will be enough), mass sizes of the device and the size of the search area (how much can be “probed” for 1 pass).

Discrimination, or permissability, gives the operator or microcontroller the ability to judge the nature of the object found by the device’s reaction.
Discrimination, in turn, the parameter is composite, because at the output of the metal detector there is 1, maximum 2 signals, and the values that determine the properties and location of the find, more. However, given the change in the device’s response as it approaches the object, it highlights three components:

Spatial - testifies to the location of the object in the search area and the depth of its location.
Geometric - gives an opportunity to judge the shape and size of the object.
High-quality - allows you to make assumptions about the properties of the material of the object.
Operating frequency

All parameters of the metal detector are complex and many relationships are mutually exclusive. So, for example. lowering the frequency of the generator allows for more pernity and search area, but at the cost of increasing energy consumption, and impairs sensitivity and mobility due to the increasing size of the coil. In general, each parameter and their complexes are somehow tied to the frequency of the generator. Therefore, the initial classification of metal detectors is based on the range of working frequencies:

Super-low frequency (SD) - up to the first hundred Hz. Absolutely not amateur devices: energy consumption from dozens of w, without computer processing on the signal can not judge anything, to move you need vehicles.

Low-frequency (HF) - from hundreds of Hz to several kHz. Simple scheme-technicaland and constructive, jammable, but little sensitive, discrimination is bad. The permination - up to 4-5 m with energy consumption from 10 w (i.e. deep metal detectors) or up to 1-1.5 m when powered by batteries. React most acutely to ferromagnetic materials (black metal) or large masses of diamagnetic (concrete and stone construction structures), so sometimes called magnetodetectors. The properties of the soil are not sensitive.

Increased frequency (HR) - up to a few dozen kHz. It is more difficult to do so, but the requirements for the reel are low. The permination - up to 1-1.5 m, interference resistance on the triple, good sensitivity, satisfactory discrimination. Can be universal when used in pulse mode, see Below. On flooded or mineralized soils (with fragments or particles of rock shielding EMF) work poorly or do not feel anything at all.
High, or radio frequencies (HPh or RF) are typical metal detectors “for gold”: excellent discrimination to a depth of 50-80 cm in dry non-conducting and non-magnetic soils (beach sand, etc.) Energy consumption - as in the pre. The rest is on the verge of “failure.” The efficiency of the device largely depends on the design and quality of the coil (coil).

Note: mobility of metal detectors by pp. 2-4 is good: from one set of salt elements (“battery”) AA and without overwork the operator can work up to 12 hours.

Pulse metal detectors stand alone. Their primary current enters the coil with pulses. By setting the frequency of the pulses within the NH, and their duration, which determines the spectral composition of the signal corresponding to the ranges of HR-HF, it is possible to obtain a metal detector, combining the positive properties of HF, HR and HF or rebuilt .

Search method



There are at least 10 methods of searching for items using EMF. But such as, say, the method of directly digitizing the response signal with computer processing - the lot of professional application.
How to choose a metal detector Answering the main questions
The homemade metal detector is more technically built in the following ways:

Parametric.
Receiver-transmitting.
With the accumulation of phases.
On the beat.
No receiver
Parametric metal detectors in some way fall out of the definition of the principle of action: they have no receiver, no receiving coil. Detection uses the direct effect of the object on the parameters of the generator coil - inductiveness and goodness, 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 vibrations produced, which is fixed in different ways: by measuring the frequency and amplitude, by changing the current of the generator consumption, by measuring the voltage in the PHA (phase system) auto-adjustment of the frequency, “pulling” it to a given value) and others.

Parametric metal detectors are simple, cheap and jammable, but using them requires certain skills, as the frequency “floats” under the influence of external conditions. Their sensitivity is weak; most often used as magneto detectors.

With a receiver and transmitter


The device of the metal detector is shown on rice. In the beginning, to explain the principle of action; it also describes the principle of work. Such devices allow to achieve the best efficiency in its range of frequencies, but are complex scheme-technically, require a particularly high-quality coil system. Receiving metal detectors with one coil are called induction. Their repetition is better, because the problem of the correct location of coils relatively 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: Pulse-transmitting metal detectors also manage to get rid of the problem of discharge. This is explained by the fact that as a secondary signal “catch” t. naz. “tail” of the pulse reeworked by the object. The primary pulse due to variance in overexizing is blurred, and part of the secondary pulse is in between the primary, from where it is easy to distinguish.

Until clicking


Metal detectors with phase accumulation, or phase-sensitive, are either single-cut pulses, or with 2 generators, each working on its own coil. In the first case, the fact is used that the pulses during re-radiation are not only blurred, but also delayed. Over time, the phase shift increases; when it reaches a certain size, the discriminator is triggered and a click is heard in the headphones. As you get closer to the object, clicks become more frequent and merge into an increasingly high-tone sound. It is on this principle that the Pirate is built.

In the second case, the search technique is the same, but work 2 strictly symmetrical electrically and geometrically generators, each on its own coil. At the same time, due to the interaction of their EMF there is a mutual synchronization: generators work in time. When the common EMF is distorted, synchronization breaks begin, audible as the same clicks, and then the tone. Two-kathush metal detectors with a breakdown of synchronization are easier pulsed, but less sensitive: the pernation of them in 1.5-2 times less. Discrimination in both cases is close to excellent.

Phase-sensitive metal detectors are the favorite tools of resort miners. Search aces set up their devices so that the sound disappears again over the object: the frequency of clicks goes into the ultrasonic area. In this way, on the shell beach it is possible to find gold earrings the size of a helpto at a depth of up to 40 cm. However, on the ground with small heterogeneities, watered and mineralized, metal detectors with the accumulation of phase shall be inferior to others, except parametric.

By squeak
The beatings of 2 electrical signals are a signal with a frequency equal to the sum or difference of the main frequencies of the initial signals or multiples of them - harmonics. So, for example. if the entrances of a special device - a mixer - send signals with frequencies of 1 MHz and 1,000 500 Hz or 1,0005 MHz, and to the output of the mixer to connect headphones or speaker, we will hear a clean 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 x 1,000 500; we “caught” the 5th harmonica.

There are two generators in the metal detectors: the support and the worker. The loop of the oscillating contour of the support is small, protected from foreign influences, or its frequency is stabilized by a quartz resonator (simply - quartz). The contour coil of a working (search) generator is searchable, and its frequency depends on the presence of items in the search area. Before searching, the working generator is set up for zero beatings, i.e. before the frequencies are matched. A complete zero sound as a rule do not achieve, and adjust to a very low tone or wheeze, so it is more convenient to look. By changing the tone of the beatings are judged on the presence, size, properties and location of the object.

Note: most often the frequency of the search generator is taken several times lower than the support generator and work on harmonics. This allows, first, to avoid harmful in this case the mutual influence of generators; secondly, to adjust the device more precisely, thirdly, to search at the optimal frequency in this case.

Metal detectors on harmonics in general are more difficult pulsed, but work on any ground. Properly crafted and configured, they are not inferior to impulse. This can be judged at least by the fact that gold-diggers-beachers do not agree on what is better: pulse or beatings?

Reel and so on



The most common misconception of novice radio amateurs is the absolutization of circuitry. Like, if the scheme is “cool,” then everything will be type-top. Regarding metal detectors, this is doubly wrong, because their operational merits strongly depend on the design and quality of the production of the search coil. As a resort prospector put it: “Finding a detector should pull a pocket, not a leg.”

When developing the device, its layout and parameters of the coil are adjusted to each other before receiving the optimum. A certain scheme with a “foreign” reel, if it will work, it will not reach the stated 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.

On the size of the coil



The large (wide) coil more effectively emits EMF and will “enlighten” the soil deeper. Its search area is wider, which reduces “kicking. However, if there is a large unnecessary object in the search area, its signal will “score” weak from the desired little thing. Therefore, it is desirable to take or make a metal detector, designed to work with coils of different sizes.

Note: typical coil diameters of 20-90 mm for the search for rebar and profiles, 130-150 mm “on beach gold” and 200-600 mm “on the big iron”.

Monoloop



The traditional type of metal detector coil is a naz. Thin Coil or Mono Loop: A ring of many turns of enamelled copper wire is 15-20 times smaller than the average diameter of the ring. The advantages of the monoloop coil - a weak dependence of parameters on the type of soil, narrowing down the search area, which allows, moving the detector, more precisely to determine the depth and location of the find, and constructive simplicity. Disadvantages - a small goodness, why in the search “floats” setting, susceptibility to interference and vague reaction to the object: working with monolooprequires considerable experience of using this particular instance of the device. Improvised metal detectors beginners are recommended to do with monoloops, so that without any problems to get a workable design and acquire with it search experience.

Inductance


When choosing a scheme to make sure that the author’s promises are valid, and even more so when designing or refining yourself, you need to know the inductiveness of the coil and be able to calculate it. Even if you make a metal detector from a shopping set, inductives still need to be checked by measurements or calculation, so as not to puzzle then: why, everything is kind of healthy, and not squeaking.

Calculators for calculating the inductive coils are available on the Internet, but the computer program can not provide all cases of practice. That’s why it’s rice. given an old, decades-tested nomogram for calculating multi-layered coils; thin coil is a special case of multi-layered.

Nomogram for calculating multi-layered coils



To calculate the search monoloop, the nomogram is used as follows:

Take the inductive value of L from the description of the device and the dimensions of the loop D, l and t from there or by choice; typical values: L, 10 mGn, D, 20 cm, l t 1 cm.
By nomogram we determine the number of w turns.
We set the styling factor k q 0.5, the size of l (the height of the coil) and t (the width of it) determine the area of the section of the loop and find the area of pure copper in it as S q klt.
Dividing the S into w, we get a section of the winding wire, and on it - the diameter of the wire d.
If it turned out to be d q (0.5… 0.8) mm, everything is OK. Otherwise, increase l and t at d’0.8 mm or reduce at d’0.5 mm.
Immunity
Faraday screen

The monoloop catches the interference well, because it is arranged in the same way as a frame antenna. You can increase its interference resistance, first, by placing the wind-up in the so-called. Faraday shield: metal tube, braid or foil winding with a rupture, so as not to form a short-closed revolution, which will “eat” all the EMF coils, see Fig. Right. If there is a dotted line on the original diagram near the search coil designation (see diagrams below), it means that the coil of this device must be placed in faraday’s screen.

Also necessarily the screen connects with the common circuit wire. There is a flood of newcomers: the ground conductor must be connected to the screen strictly symmetrically cut (see the same figure) and bring it to the scheme also symmetrically relative to signal wires, otherwise the interference will still “go” into the coil.

The screen absorbs and some of the search eMF, which reduces the sensitivity of the device. This effect is especially noticeable in pulsed metal detectors; their coils can’t be shielded at all. In this case, increased interference can be achieved by symmetrying the remp. The bottom line is that for a remote source of EMF, the coil is a point object, and erin s.c. the interference in its halves will crush each other. A symmetrical coil may also be needed if the generator is a two-act or inductive three-point.

Ways to symmetry inductive coils



However, it is impossible to symmetry the coil in the usual bifillary way (see pic.) in this case: when in the field of bifillary coil of conductive and/or ferromagnetic objects its symmetry is disturbed. That is, the interference resistance of the metal detector will disappear just when it is most needed. Therefore, you need to symmetry the monoloop with cross winding, see. Same rice. Its symmetry is not disturbed under any circumstances, but to roll a thin coil with a lot of turns in a cross way is a hellofish work, and then it is better to make a basket coil.