DIY pulse metal detector with discrimination. DIY deep metal detector: diagram, instructions and reviews

Metal detectors are used to search for metal in the soil at a certain depth. This device can be assembled independently at home, having at least minimal experience in this matter, or following the clear instructions in the instructions. The main thing is the desire and availability of the necessary tools.

Detailed instructions for the Terminator 3 metal detector with your own hands

This type of design is designed to search for coins. The process of assembling it is completely simple. However, experience in assembling such a tool is still necessary. The Terminator is able to detect an object even if the target of capture is minimal.

To begin, you should prepare the necessary equipment, namely:

a multimeter that measures speed.
LC meter
Oscilloscope.

Next, you need to find a diagram broken down into nodes. Now you can make a printed circuit board into which jumpers, resistors, panels for microcircuits and other parts should be soldered in order. The next step is to clean the board with alcohol.. It is definitely worth checking for defects. You can check whether the board is in working condition as follows:

Turn on the power.
Turn down the sensitivity control until no sound is heard from the speaker.
Touch the sensor connector with your fingers.
When turned on, the LED should blink and then go out.

If all actions occurred, then everything was done correctly. Now you can make a coil. It is necessary to prepare a winding enamel wire with a diameter of 0.4 mm, which must be folded in half. A circle is drawn on a sheet of plywood with a diameter of 200 mm and 100 mm. Now you need to drive nails in a circle, the distance between them should be 1 cm.

Next, you can move on to winding the turns. At 200 mm you should make 30 of them, and at 100 - 48. Then the first coil should be soaked in varnish; when it dries, you can wrap it with thread. The thread can be removed, and by soldering the middle, you get a solid winding of 60 turns. Afterwards, the coil must be wrapped quite tightly with electrical tape.. And a 1 cm foil is placed on top, this will be a screen, and more electrical tape is wound on top of it. The ends should come out.

On the second coil it is also necessary to solder the middle. In order to start the generator, you need to connect the first coil to the board. The second coil should be wrapped with a wire of 20 turns, then we connect it to the board. Now you need to connect the oscilloscope minus to minus to the board, and the plus is connected to the coil. Be sure to look at what frequency it will be when you turn it on and remember it or write it down on paper.

Now the coils need to be placed in a special mold so that they can then be filled with resin. Next, the oscilloscope is connected to the board, with the negative pole, the amplitude should reach zero. The coils in the mold are filled with resin to approximately half the depth. When everything is ready, the metal discrimination scale is adjusted.

Parts list for the Terminator 3 metal detector

As parts for the trio metal detector you will need:

If you have these parts, you can assemble the Terminator Pro metal detector yourself.

Circuit diagram of a metal detector with metal discrimination

You can make a metal detector with metal discrimination yourself using the circuit for the Chance pulse device. The process of making a coil is quite simple.

The diagram itself can be found on the Internet. But still, experience in assembling such devices will be useful. Assembling the metal detector should begin with the board.

After the board is manufactured, the microcontroller needs to be flashed. And at the end of the work, we connect the metal detection device to the power supply.

Homemade equipment can be made without complex microcircuits, but using a simple transistor generator. The metal detector will be non-discriminatory. It will detect objects in the ground to a depth of 20 centimeters, and in dry sand - to a depth of 30 centimeters. In this device, the transmitting and receiving coils operate simultaneously.

Terminator 3 metal detector coil

To begin with, you should take winding enamel with a diameter of 0.4 mm. Fold it so that there are two ends and two beginnings. Next, you should wind from two reels at a time.

Now we need to make transmitting and receiving coils; for this, two circles of 200 mm and 100 mm are drawn on a plywood sheet. Nails are driven in along these circles, the distance between them should be 1 cm. 30 turns of enamel wire are wound onto a large mandrel. Then you should apply varnish to the coil and wrap it with thread, then remove it from the winding and solder the middle. This creates one middle wire and two outer wires.

The resulting coil should be wrapped with electrical tape and a piece of foil should be placed on top, and foil again on top. The ends of the windings should go outside.

Now it’s time to move on to the receiving coil. 48 turns are already wound here. To start the generator, you need to connect the transmitting coil to the board. The middle wire is connected to the negative. And the middle terminal of the take-up coil is not used. The transmitting coil requires a compensating coil, on which 20 turns are wound.

We connect the oscilloscope to the board like this: a probe with a minus to the minus of the board, and a plus probe to the coil. Be sure to measure the frequency of the coils and write it down.

After connecting the coils according to the diagram, they must be placed in a special container and filled with resin. The oscilloscope now sets the division time (10 ms and 1 volt per cell). Now you should reduce the amplitude to zero. We wind the turns until the volt value reaches zero. We make a compensating loop at the coil, which will be outside.

The mold should be filled halfway with resin. When everything hardens, you need to connect the oscilloscope and bend the loop inward. Then twist it until the amplitude value becomes minimal. Afterwards, you need to glue the loop, check the balance, and now you can fill the second half of the container with resin. The reel is ready for use.

Before you begin the repair, you should prepare the following tools:

Stationery knife;
Incandescent lamp;
A container for glue, preferably flat;
Special or epoxy resin;
Medium and fine sandpaper;
Small spatula.

First of all, you need to dry the coil using an incandescent lamp. And use a utility knife to widen the cracks on it. Squeeze the glue onto a flat surface and mix with a spatula. Apply this substance to the coil. In places of cracks, you can apply more resin. Now you should wait until it all hardens thoroughly. And then sand it, using first medium and then fine sandpaper. This procedure will help smooth out all the unevenness. In this fairly simple way, you can revive the oldest coil from a metal detecting device.

Printed circuit board for the Terminator 3 device

A printed circuit board for this type of equipment can be made and configured independently. The board diagram for Terminator 3 is available on the Internet. Once it is found, you can begin manufacturing the printed circuit board. After that, jumpers, SMD resistors and panels for microcircuits are soldered into it. The capacitors on the board must have high thermal stability.

DIY metal detector sensor

Before starting work, it is necessary to prepare a device that will accurately measure capacitance and inductance. Now you should take the housing for the reel and make PCB inserts into the ears. Pieces of fabric are used for compaction. The upper surface of the ears should be sanded. The fabric must be impregnated with epoxy resin. When everything is dry, you should sand everything and insert a sealed lead-in, thus making grounding. Next you need to apply a special Dragon varnish.

Now windings are made, which are tied with threads. All windings are placed in a coil and capacitors are glued. Everything can be connected and configured. A housing is required for pouring. Mandatory: there should be no metal nearby. After pouring, the epoxy should be sanded and dried thoroughly. The sensor is suitable for the Terminator 3 and Terminator 4 metal detectors, which are the most popular models of devices.

Metal detector Terminator 3: reviews

Many people consider this model of device popular. The positive qualities include:

Finding objects made of non-ferrous metal.
No false positives.

And the following are identified as negative features:

Rusty iron is detected rather poorly.
You may lose some of your finds.

The search depth of the device is higher than that of other similar models. Basically this is 30 centimeters using the example of a coin.

Metal detector Sokha 3: diagram and description

The metal detector has an operating frequency of 5 to 17 kHz. Its power supply is 12 Volts. Its ground balance is manual.

The circuit of this device is not entirely simple, since it contains two microcontrollers. The diagram can be found on the Internet. The device itself has good characteristics. However, due to the lack of detailed assembly information, difficulties may arise in the manufacture of the device.

Today I would like to present to your attention the circuit diagram of a metal detector, and everything related to it, what you see in the photograph.
The metal detector can operate in both the search mode for all metals and background discrimination.

Technical characteristics of the metal detector.

Operating principle: induction balanced
-Operating frequency, kHz 8-10kHz
-Dynamic operating mode
-Precise detection mode (Pin-Point) is available in static mode
-Power supply, V 12
-There is a sensitivity level regulator
-There is a threshold tone control
-Ground adjustment is available (manual)

Detection depth in the air with a DD-250mm sensor In the ground, the device sees targets almost the same as in the air.
-coins 25mm - about 30cm
-gold ring - 25cm
-helmet 100-120cm
-maximum depth 150cm
-Consumption current:
-No sound approximately 30 mA

And the most important and intriguing thing is the diagram of the device itself

To assemble the metal detector you need the following parts:

So that you do not have to heavily customize the device, do the assembly and soldering carefully; the board should not contain any clamps or sticky parts.

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 board

We start the assembly by soldering in jumpers, then resistors, then sockets for microcircuits and everything else. One more small recommendation, now regarding the manufacture of the device board. It is very 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 advisable to select those parts that are repeated at each amplification stage so that they have the most identical parameters as measured by the tester (that is, what readings in a specific cascade on one channel - the same readings on the same cascade and in another channel)

Making a coil for a metal detector

Today we would like to talk about the manufacture of a sensor in a finished housing, so photos are more than words.
We take the housing, fix the pressure gland in the right place and install the cable, call the cable and mark the connections.
Next we wind the coils. The DD sensor is manufactured 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.

After winding, the coils are tightly wrapped with thread and impregnated with varnish.
After drying, wrap tightly with electrical tape around the entire circumference. The top is shielded with foil; between the end and the beginning of the foil there should be a gap of 1 cm not covered by it, in order to avoid a short-circuited turn.

The coil can be shielded with graphite; to do this, mix graphite with nitro varnish 1:1 and cover the top with a uniform layer of tinned copper 0.4 wire wound on the coil (without gaps), connect the wire to the cable shield.

We put it in the case, connect it and roughly bring the coils into balance, there should be a double beep for ferit, a single beep for a coin, if it’s the other way around, then we swap the terminals of the receiving winding. Each of the coils is adjusted in frequency separately; there should be no metal objects nearby!!! The coils are tuned with an attachment for measuring resonance. We connect the attachment to the Eldorado board in parallel with the transmitting coil and measure the frequency, then with the RX coil and the selected condenser we achieve a frequency 600 Hz higher than that obtained 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 in steps of 0.5-1 nf in one direction or another, and in addition the moment when the device will see foil and copper at the minimum discriminance, and when the discrim is screwed on, the entire scale will be cut out one by one.

We finally reduce the coils to zero, fixing everything with hot glue. Next, to lighten the coil, we glue the voids with pieces of polystyrene foam, the foam sits on the hot glue, otherwise it will float up after filling the coil.

Pour the first layer of epoxy, without adding to the top 2-3mm

Fill in the second layer of resin with color. An aniline dye is a good choice for dyeing fabric; the powder comes in different colors and costs a penny. The dye must first be mixed with the hardener, then the hardener must be added to the resin; the dye will not dissolve in the resin immediately.

To assemble the board correctly, start by checking the correct power supply to all components.

Take the circuit and the tester, turn on the power on the board, and, checking the circuit, go through the tester at all points on the nodes where power should be supplied.
When the discriminator knob is positioned at a minimum, the device should turn all non-ferrous metals; when turning the discriminator, they should be cut out
All metals are in order up to copper, less should not be cut out, if the device works like this, it means it is configured correctly. The discrimination scale must be selected in such a way that it fits completely into a full turn of the discrimination 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 wires in a common screen, two wires to the transmitting coil and two to the receiving coil, the screen to the body.

COP IS PROHIBITED BY LAW

Updated 11/01/2018

Metal detector discrimination ( metal detector discrimination – English) is a function that allows you to distinguish (recognize) detected objects by type of metal and classify them in a very specific way.

How does a metal detector distinguish between metals? It is known that different metals have different electrical conductivities. The electronic unit (“brain”) of the metal detector analyzes the phase shift between the signal from the transmitting and receiving coils. The phase shift varies depending on the conductivity of the metal. Each type of metal has its own characteristic phase shift, unique to that metal.

This makes it easy to distinguish ferrous metals from non-ferrous ones, as well as non-ferrous metals from each other (aluminium, silver, copper, bronze, lead). The discrimination function is best implemented in metal detectors whose operation is based on the VLF (VLF/TR) principle...

Pulse metal detectors ( Pulse induction metal detectors – English) are not capable of the same degree of discrimination as VLF detectors. Many attempts have been made to create pulse metal detectors that can distinguish between iron, silver and copper, but all these attempts have had very limited success. This is due to the physics of the pulse signal.

Therefore, the use of pulsed metal detectors in search areas with a high concentration of metal debris is extremely difficult.

Simple and professional metal detector discrimination function

The simplest discrimination function (discriminator) of a metal detector is implemented in outdated models and simple entry-level devices. Selecting one of two modes, “all metals” or “non-ferrous metals,” allows the metal detector to respond to a phase shift of a certain magnitude (a certain electrical conductivity of the metal of the object), which is compared with the configured (set) discrimination level.

But at the same time, the metal detector does not distinguish between non-ferrous metals, and at a high level of discrimination it will practically not react to some coins and most jewelry.

Professional level metal detectors use a discriminator with range selection ( notch discriminator- English). Function notch is best implemented in microprocessor systems and allows the detector to be programmed so that it responds to certain groups of metals (electrical conductivity range) and excludes reaction to other metals.

However, it should be noted that absolutely accurate discrimination of a metal detector is impossible. This is due to the fact that different metals have similar conductivity values (for example, gold and aluminum).

That is, when searching, for example, on the beach, the device will give the operator a reaction signal to aluminum foil similar to the signal to gold jewelry.

The numerical value of the metal conductivity of the detected object is displayed on the indicator VDI(visual indicator of discrimination). This makes it possible to identify the object and make a decision about excavation.

Metal discrimination is useful in very littered search areas. If the scanned soil does not contain a lot of metal debris, then it is better not to use this function, since this mode reduces the detection depth by 10 - 20%. The "all metals" mode provides the highest sensitivity and the fullest use of the metal detector's capabilities.

Related tags: metal detector discrimination, metal discrimination, metal detector discrimination function, notch function, visual discrimination indicator, all metals mode, metal detector discriminator, what does a metal detector with discrimination mean, the principle of metal discrimination, how does a metal detector distinguish between metals

Metal detector circuit

Today I would like to present to your attention a diagram of a metal detector, and everything related to it, what you see in the photograph. After all, it is sometimes so difficult to find the answer to a question in a search engine - Diagram of a good metal detector

In other words, the metal detector has a name Tesoro Eldorado

The metal detector can operate in both the search mode for all metals and background discrimination.

Technical characteristics of the metal detector.

And the most important and intriguing thing is the diagram of the device itself

The picture is easily enlarged when you click on it

To assemble the metal detector you need the following parts:

So that you don’t have to spend a long time setting up the device, do the assembly and soldering carefully; the board should not contain any clamps.

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 board

We begin assembly soldering jumpers, then resistors, further sockets for microcircuits And all the rest. One more small recommendation, now regarding the manufacture of the device board. It is very desirable to have a tester that can measure the capacitance of capacitors. The fact is that the device These are two identical amplification channels, therefore the amplification through them should be as identical as possible, and for this it is advisable to select those parts that are repeated on each amplification stage so that they have the most identical parameters as measured by the tester (that is, what are the 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 a sensor in a finished housing, so the photo is more than words.
We take the housing, attach the sealed wire in the right place and install the cable, ring the cable and mark the ends.
Next we wind the coils. The DD sensor is manufactured 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.

After winding, the coils are tightly wrapped with thread and impregnated with varnish.

After drying, wrap tightly with electrical tape around the entire circumference. The top is shielded with foil; between the end and the beginning of the foil there should be a gap of 1 cm not covered by it, in order to avoid a short-circuited turn.

It is possible to shield the coil with graphite; to do this, mix graphite with nitro varnish 1:1 and cover the top with a uniform layer of tinned copper 0.4 wire wound on the coil (without gaps), connect the wire to the cable shield.

We put it into the case, connect it and roughly bring the coils into balance, there should be a double beep for the ferrite, a single beep for the coin, if it’s the other way around, then we swap the terminals of the receiving winding. Each of the coils is adjusted in frequency separately; there should be no metal objects nearby!!! The coils are tuned with an attachment for measuring resonance. We connect the attachment to the Eldorado board in parallel with the transmitting coil and measure the frequency, then with the RX coil and a selected capacitor we achieve a frequency 600 Hz higher than that obtained 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 in steps of 0.5-1 nf in one direction or another, and in addition the moment when the device will see foil and copper at a minimum of discrimination, and when the discrimination is turned up, the entire scale will be cut out in turn.

Pour the first layer of epoxy, without adding to the top 2-3mm

To assemble the board correctly, start by checking the correct power supply to all components.

, when screwing the discrim, they should be cut out

all metals in order up to copper should not be cut out if the deviceit works this way, which means it is configured correctly. The discrimination scale needs to be selected so that it fits completely into a full turn of the discrimination knob, this is done by selecting c10. When the capacity decreases, the scale stretches and vice versa.

A metal detector or metal detector is designed to detect objects that differ in their electrical and/or magnetic properties from the environment in which they are located. Simply put, it allows you to find metal in the ground. But not only metal, and not only in the ground. Metal detectors are used by inspection services, criminologists, military personnel, geologists, builders to search for profiles under cladding, fittings, to verify plans and diagrams of underground communications, and people of many other specialties.

Do-it-yourself metal detectors are most often made by amateurs: treasure hunters, local historians, members of military historical associations. This article is primarily intended for them, beginners; The devices described in it allow you to find a coin the size of a Soviet nickel at a depth of 20-30 cm or a piece of iron the size of a sewer manhole approximately 1-1.5 m below the surface. However, this homemade device can also be useful on the farm during repairs or at construction sites. Finally, having discovered a hundredweight or two of abandoned pipes or metal structures in the ground and selling the find for scrap metal, you can earn a decent amount. And there are definitely more such treasures in Russian land than pirate chests with doubloons or boyar-robber pods with efimkas.

Note: If you are not knowledgeable in electrical engineering and radio electronics, do not be intimidated by the diagrams, formulas and special terminology in the text. The essence is stated simply, and at the end there will be a description of the device, which can be made in 5 minutes on a table, without knowing how to solder or twist the wires. But it will allow you to “feel” the peculiarities of metal searching, and if interest arises, knowledge and skills will come.

A little more attention compared to the others will be paid to the “Pirate” metal detector, see fig. This device is simple enough for beginners to repeat, but its quality indicators are not inferior to many branded models costing up to $300-400. And most importantly, it showed excellent repeatability, i.e. full functionality when manufactured according to descriptions and specifications. The circuit design and operating principle of the “Pirate” are quite modern; There are enough manuals on how to set it up and how to use it.

Operating principle

The metal detector operates on the principle of electromagnetic induction. In general, the metal detector circuit consists of an electromagnetic vibration transmitter, a transmitting coil, a receiving coil, a receiver, a useful signal extraction circuit (discriminator) and an indication device. Separate functional units are often combined in circuitry and design, for example, the receiver and transmitter can operate on the same coil, the receiving part immediately releases the useful signal, etc.

The coil creates an electromagnetic field (EMF) of a certain structure in the medium. If there is an electrically conductive object in its area of action, pos. And in the figure, eddy currents or Foucault currents are induced in it, which create its own EMF. As a result, the structure of the coil field is distorted, pos. B. If the object is not electrically conductive, but has ferromagnetic properties, then it distorts the original field due to shielding. In both cases, the receiver detects the difference between the EMF and the original one and converts it into an acoustic and/or optical signal.

Note: in principle, for a metal detector it is not necessary that the object be electrically conductive; the soil is not. The main thing is that their electrical and/or magnetic properties are different.

Detector or scanner?

In commercial sources, expensive highly sensitive metal detectors, e.g. Terra-N are often called geoscanners. This is not true. Geoscanners operate on the principle of measuring the electrical conductivity of soil in different directions at different depths; this procedure is called lateral logging. Using logging data, the computer builds a picture on the display of everything in the ground, including geological layers of different properties.

Varieties

Common parameters

The operating principle of a metal detector can be implemented technically in different ways, depending on the purpose of the device. Metal detectors for beach gold prospecting and construction and repair prospecting may be similar in appearance, but differ significantly in design and technical data. To make a metal detector correctly, you need to clearly understand what requirements it must satisfy for this type of work. Based on this, The following parameters of search metal detectors can be distinguished:

Penetration, or penetrating ability, is the maximum depth to which an EMF coil extends in the ground. The device will not detect anything deeper, regardless of the size and properties of the object.
The size and dimensions of the search zone is an imaginary area in the ground in which the object will be detected.
Sensitivity is the ability to detect more or less small objects.
Selectivity is the ability to respond more strongly to desirable findings. The sweet dream of beach miners is a detector that beeps only for precious metals.
Noise immunity is the ability not to respond to EMF from extraneous sources: radio stations, lightning discharges, power lines, electric vehicles and other sources of interference.
Mobility and efficiency are determined by energy consumption (how many batteries will last), the weight and dimensions of the device and the size of the search zone (how much can be “probed” in 1 pass).
Discrimination, or resolution, gives the operator or control microcontroller the opportunity to judge the nature of the found object by the device’s response.

Discrimination, in turn, is a composite parameter, because At the output of the metal detector there is 1, maximum 2 signals, and there are more quantities that determine the properties and location of the find. However, taking into account the change in the reaction of the device when approaching an object, 3 components are distinguished:

Spatial – indicates the location of the object in the search area and the depth of its occurrence.
Geometric – makes it possible to judge the shape and size of an object.
Qualitative – allows you to make assumptions about the properties of the object’s material.

Operating frequency

All parameters of a metal detector are connected in a complex way and many relationships are mutually exclusive. So, for example, lowering the frequency of the generator makes it possible to achieve greater penetration and search area, but at the cost of increasing energy consumption, and worsens sensitivity and mobility due to an increase in the size of the coil. In general, each parameter and their complexes are somehow tied to the frequency of the generator. That's why The initial classification of metal detectors is based on the operating frequency range:

Ultra-low frequency (ELF) - up to the first hundred Hz. Absolutely not amateur devices: power consumption of tens of W, without computer processing it is impossible to judge anything from the signal, transportation requires vehicles.
Low frequency (LF) - from hundreds of Hz to several kHz. They are simple in circuit design and design, noise-resistant, but not very sensitive, discrimination is poor. Penetration - up to 4-5 m with power consumption from 10 W (so-called deep metal detectors) or up to 1-1.5 m when powered by batteries. They react most acutely to ferromagnetic materials (ferrous metal) or large masses of diamagnetic materials (concrete and stone building structures), which is why they are sometimes called magnetic detectors. They are little sensitive to soil properties.
High frequency (IF) – up to several tens of kHz. LF is more complex, but the requirements for the coil are low. Penetration - up to 1-1.5 m, noise immunity at C, good sensitivity, satisfactory discrimination. Can be universal when used in pulse mode, see below. On watered or mineralized soils (with fragments or particles of rock that shield EMF), they work poorly or do not sense anything at all.
High, or radio frequencies (HF or RF) - typical metal detectors “for gold”: excellent discrimination to a depth of 50-80 cm in dry non-conductive and non-magnetic soils (beach sand, etc.) Energy consumption - as before. n. The rest is on the verge of failure. The effectiveness of the device largely depends on the design and quality of the coil(s).

Note: mobility of metal detectors according to paragraphs. 2-4 good: from one set of AA salt cells (“batteries”) you can work for up to 12 hours without overworking the operator.

Pulse metal detectors stand apart. In them, the primary current enters the coil in pulses. By setting the pulse repetition rate within the LF range, and their duration, which determines the spectral composition of the signal corresponding to the IF-HF ranges, you can obtain a metal detector that combines the positive properties of LF, IF and HF or is tunable.

Search method

There are at least 10 methods of searching for objects using EMFs. But such as, say, the method of direct digitization of the response signal with computer processing is for professional use.

A homemade metal detector is built in the following ways:

Parametric.
Transceiver.
With phase accumulation.
On the beats.

Without receiver

Parametric metal detectors in some way fall outside the definition of the operating principle: they have neither a receiver nor a receiving coil. For detection, the direct influence of the object on the parameters of the generator coil - inductance and quality factor - is used, and the structure of the EMF does not matter. Changing the parameters of the coil leads to a change in the frequency and amplitude of the generated oscillations, which is recorded in different ways: by measuring the frequency and amplitude, by changing the current consumption of the generator, by measuring the voltage in the PLL loop (a phase-locked loop system that “pulls” it to a given value), etc.

Parametric metal detectors are simple, cheap and noise-resistant, but using them requires certain skills, because... the frequency “floats” under the influence of external conditions. Their sensitivity is weak; Most of all they are used as magnetic detectors.

With receiver and transmitter

The device of the transceiver metal detector is shown in Fig. at the beginning, to an explanation of the principle of operation; The principle of operation is also described there. Such devices allow achieving the best efficiency in their frequency range, but are complex in circuit design and require a particularly high-quality coil system. Transceiver metal detectors with one coil are called induction detectors. Their repeatability is better, because the problem of the correct arrangement of the coils relative to each other disappears, but the circuit design is more complicated - you need to highlight the weak secondary signal against the background of the strong primary one.

Note: In pulsed transceiver metal detectors, the problem of isolation can also be eliminated. This is explained by the fact that the so-called “catch” is “caught” as a secondary signal. the “tail” of the pulse re-emitted by the object. Due to dispersion during re-emission, the primary pulse spreads out, and part of the secondary pulse ends up in the gap between the primary ones, from where it is easy to isolate.

Until it clicks

Metal detectors with phase accumulation, or phase-sensitive, are either single-coil pulsed or with 2 generators, each operating on its own coil. In the first case, the fact is used that the pulses not only spread out during re-emission, but are also delayed. The phase shift increases over time; when it reaches a certain value, the discriminator is triggered and a click is heard in the headphones. As you approach the object, the clicks become more frequent and merge into a sound of increasingly higher pitch. It is on this principle that “Pirate” is built.

In the second case, the search technique is the same, but 2 strictly symmetrical electrically and geometrically oscillators operate, each with its own coil. In this case, due to the interaction of their EMFs, mutual synchronization occurs: the generators work in time. When the general EMF is distorted, synchronization disruptions begin, heard as the same clicks, and then a tone. Double-coil metal detectors with synchronization failure are simpler than pulse detectors, but less sensitive: their penetration is 1.5-2 times less. Discrimination in both cases is close to excellent.

Phase-sensitive metal detectors are the favorite tools of resort prospectors. Search aces adjust their instruments so that exactly above the object the sound disappears again: the frequency of clicks goes into the ultrasonic region. In this way, on a shell beach, it is possible to find gold earrings the size of a fingernail at a depth of up to 40 cm. However, on soil with small inhomogeneities, watered and mineralized, metal detectors with phase accumulation are inferior to others, except parametric ones.

By the squeak

Beats of 2 electrical signals - a signal with a frequency equal to the sum or difference of the fundamental frequencies of the original signals or their multiples - harmonics. So, for example, if signals with frequencies of 1 MHz and 1,000,500 Hz or 1.0005 MHz are applied to the inputs of a special device - a mixer, and headphones or a speaker are connected to the output of the mixer, then we will hear a pure tone of 500 Hz. And if the 2nd signal is 200-100 Hz or 200.1 kHz, the same thing will happen, because 200 100 x 5 = 1,000,500; we “caught” the 5th harmonic.

In a metal detector, there are 2 generators operating on beats: a reference and a working one. The coil of the reference oscillating circuit is small, protected from extraneous influences, or its frequency is stabilized by a quartz resonator (simply quartz). The circuit coil of the working (search) generator is a search generator, and its frequency depends on the presence of objects in the search area. Before searching, the working generator is set to zero beats, i.e. until the frequencies match. As a rule, a complete zero sound is not achieved, but is adjusted to a very low tone or wheezing, this is more convenient to search for. By changing the tone of the beats one judges the presence, size, properties and location of an object.

Note: Most often, the frequency of the search generator is taken several times lower than the reference one and operates on harmonics. This allows, firstly, to avoid the harmful mutual influence of generators in this case; secondly, adjust the device more accurately, and thirdly, search at the optimal frequency in this case.

Harmonic metal detectors are generally more complex than pulse detectors, but they work on any type of soil. Properly manufactured and tuned, they are not inferior to impulse ones. This can be judged at least by the fact that gold miners and beachgoers will not agree on what is better: an impulse or a beating one?

Reel and stuff

The most common misconception of novice radio amateurs is the absolutization of circuit design. Like, if the scheme is “cool”, then everything will be top-notch. Regarding metal detectors, this is doubly true, because... their operational advantages greatly depend on the design and quality of manufacture of the search coil. As one resort prospector put it: “Findability of the detector should be in the pocket, not the legs.”

When developing a device, its circuit and coil parameters are adjusted to each other until the optimum is obtained. Even if a certain circuit with a “foreign” coil works, it will not reach the declared parameters. Therefore, when choosing a prototype to replicate, look first of all at the description of the coil. If it is incomplete or inaccurate, it is better to build another device.

About coil sizes

A large (wide) coil emits EMF more effectively and will “illuminate” the soil more deeply. Its search area is wider, which allows it to reduce “being found with its feet.” However, if there is a large unnecessary object in the search area, its signal will “clog” the weak one from the small thing you are looking for. Therefore, it is advisable to take or make a metal detector designed to work with coils of different sizes.

Note: typical coil diameters are 20-90 mm for searching for fittings and profiles, 130-150 mm for “beach gold” and 200-600 mm “for large iron”.

monoloop

The traditional type of metal detector coil is called. thin coil or Mono Loop (single loop): a ring of many turns of enameled copper wire with a width and thickness 15-20 times less than the average diameter of the ring. The advantages of a monoloop coil are a weak dependence of the parameters on the type of soil, a narrowing search zone, which allows, by moving the detector, to more accurately determine the depth and location of the find, and design simplicity. Disadvantages - low quality factor, which is why the setting “floats” during the search process, susceptibility to interference and vague response to the object: working with a monoloop requires considerable experience in using this particular instance of the device. It is recommended that beginners make homemade metal detectors with a monoloop in order to get a workable design without any problems and gain search experience with it.

Inductance

When choosing a circuit, in order to ensure the reliability of the author’s promises, and even more so when independently designing or modifying it, you need to know the inductance of the coil and be able to calculate it. Even if you are making a metal detector from a purchased kit, you still need to check the inductance by measurements or calculations, so as not to rack your brains later: why, everything seems to be working properly, and not beeping.

Calculators for calculating the inductance of coils are available on the Internet, but a computer program cannot provide for all practical cases. Therefore, in Fig. an old, decades-tested nomogram for calculating multilayer coils is given; a thin coil is a special case of a multilayer coil.

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

We take the inductance value L from the description of the device and the dimensions of the loop D, l and t from the same place or according to our choice; typical values: L = 10 mH, D = 20 cm, l = t = 1 cm.
Using the nomogram we determine the number of turns w.
We set the laying coefficient k = 0.5, using the dimensions l (height of the coil) and t (its width) we determine the cross-sectional area of the loop and find the area of pure copper in it as S = klt.
Dividing S by w, we obtain the cross-section of the winding wire, and from it the diameter of the wire d.
If it turns out d = (0.5...0.8) mm, everything is OK. Otherwise, we increase l and t when d>0.8 mm or decrease when d<0,5 мм.

Noise immunity

The monoloop “catches” interference well, because is designed exactly the same as a loop antenna. You can increase its noise immunity, firstly, by placing the winding in the so-called. Faraday shield: a metal tube, braid or foil winding with a break so that a short-circuited turn does not form, which will “eat up” all the EMF coils, see fig. on right. If on the original diagram there is a dotted line near the designation of the search coil (see diagrams below), this means that the coil of this device must be placed in the Faraday shield.

Also, the screen must be connected to the common wire of the circuit. There is a catch here for beginners: the grounding conductor must be connected to the screen strictly symmetrically to the cut (see the same figure) and brought to the circuit also symmetrically relative to the signal wires, otherwise noise will still “crawl” into the coil.

The screen also absorbs some of the search EMF, which reduces the sensitivity of the device. This effect is especially noticeable in pulse metal detectors; their coils cannot be shielded at all. In this case, increasing noise immunity can be achieved by balancing the winding. The point is that for a remote EMF source, the coil is a point object, and the emf. interference in its halves will suppress each other. A symmetrical coil may also be needed in the circuit if the generator is push-pull or inductive three-point.

However, in this case it is impossible to symmetry the coil using the bifilar method familiar to radio amateurs (see figure): when conductive and/or ferromagnetic objects are in the field of the bifilar coil, its symmetry is broken. That is, the noise immunity of the metal detector will disappear just when it is most needed. Therefore, you need to balance the monoloop coil by cross-winding, see the same fig. Its symmetry is not broken under any circumstances, but winding a thin coil with a large number of turns in a crosswise manner is hellish work, and then it is better to make a basket coil.

Basket

Basket reels have all the advantages of monoloops to an even greater extent. In addition, basket coils are more stable, their quality factor is higher, and the fact that the coil is flat is a double plus: sensitivity and discrimination will increase. Basket coils are less susceptible to interference: harmful emf. in crossing wires they cancel each other out. The only negative is that basket coils require a precisely made, rigid and durable mandrel: the total tension force of many turns reaches large values.

Basket coils are structurally flat and three-dimensional, but electrically a three-dimensional “basket” is equivalent to a flat one, i.e. creates the same EMF. The volumetric basket coil is even less sensitive to interference and, which is important for pulse metal detectors, the pulse dispersion in it is minimal, i.e. It's easier to catch the variance caused by the object. The advantages of the original “Pirate” metal detector are largely due to the fact that its “native” coil is a voluminous basket (see figure), but its winding is complex and time-consuming.

It is better for a beginner to wind a flat basket on his own, see fig. below. For metal detectors “for gold” or, say, for the “butterfly” metal detector described below and a simple 2-coil transceiver, a good mount would be unusable computer disks. Their metallization will not harm: it is very thin and nickel. An indispensable condition: an odd, and no other, number of slots. A nomogram for calculating a flat basket is not required; the calculation is carried out as follows:

They are set with a diameter D2 equal to the outer diameter of the mandrel minus 2-3 mm, and take D1 = 0.5D2, this is the optimal ratio for search coils.
According to formula (2) in Fig. calculate the number of turns.
From the difference D2 – D1, taking into account the flat laying coefficient of 0.85, the diameter of the wire in insulation is calculated.

How not to and how to wind baskets

Some amateurs take it upon themselves to wind large baskets using the method shown in Fig. below: make a mandrel from insulated nails (pos. 1) or self-tapping screws, wind them according to the diagram, pos. 2 (in this case, pos. 3, for a number of turns that is a multiple of 8; every 8 turns the “pattern” is repeated), then foam, pos. 4, the mandrel is pulled out and the excess foam is cut off. But it soon turns out that the stretched coils cut the foam and all the work went to waste. That is, in order to wind it reliably, you need to glue pieces of durable plastic into the holes of the base, and only then wind it. And remember: independent calculation of a volumetric basket coil without appropriate computer programs is impossible; The technique for a flat basket is not applicable in this case.

DD coils

DD in this case does not mean long-range, but a double or differential detector; in the original – DD (Double Detector). This is a coil of 2 identical halves (arms), folded with some intersection. With an accurate electrical and geometric balance of the DD arms, the search EMF is contracted into the intersection zone, on the right in Fig. on the left is a monoloop coil and its field. The slightest heterogeneity of space in the search area causes an imbalance, and a sharp strong signal appears. An DD coil allows an inexperienced seeker to detect a small, deep, highly conductive object when a rusty can lies next to it and above it.

DD coils are clearly oriented “to gold”; All metal detectors marked GOLD are equipped with them. However, on shallow, heterogeneous and/or conductive soils, they either fail altogether or often give false signals. The sensitivity of the DD coil is very high, but the discrimination is close to zero: the signal is either marginal or there is none at all. Therefore, metal detectors with DD coils are preferred by searchers who are only interested in “pocket-fitting”.

Note: More details about DD coils can be found further in the description of the corresponding metal detector. The DD shoulders are wound either in bulk, like a monoloop, on a special mandrel, see below, or with baskets.

How to attach the reel

Ready-made frames and mandrels for search coils are sold in a wide range, but sellers are not shy about mark-ups. Therefore, many hobbyists make the base of the coil from plywood, on the left in the figure:

Multiple designs

Parametric

The simplest metal detector for searching for fittings, wiring, profiles and communications in walls and ceilings can be assembled according to Fig. The ancient transistor MP40 can be replaced without any problems with the KT361 or its analogues; To use pnp transistors, you need to change the polarity of the battery.

This metal detector is a parametric type magnetic detector operating on LF. The tone of the sound in the headphones can be changed by selecting the capacitance C1. Under the influence of the object, the tone decreases, unlike all other types, so initially you need to achieve a “mosquito squeak”, and not wheezing or grumbling. The device distinguishes live wiring from “empty” wiring; a 50 Hz hum is superimposed on the tone.

The circuit is a pulse generator with inductive feedback and frequency stabilization by an LC circuit. A loop coil is an output transformer from an old transistor receiver or a low-power “bazaar-Chinese” low-voltage power one. A transformer from an unusable Polish antenna power source is very suitable; in its case, by cutting off the mains plug, you can assemble the entire device, then it is better to power it from a 3 V lithium coin cell battery. Winding II in Fig. – primary or network; I – secondary or step-down by 12 V. That’s right, the generator operates with transistor saturation, which ensures negligible power consumption and a wide range of pulses, making searching easier.

To turn a transformer into a sensor, its magnetic circuit must be opened: remove the frame with windings, remove the straight jumpers of the core - the yoke - and fold the W-shaped plates to one side, as on the right in the figure, then put the windings back on. If the parts are in working order, the device starts working immediately; if not, you need to swap the ends of any of the windings.

A more complex parametric scheme is shown in Fig. on right. L with capacitors C4, C5 and C6 is tuned to 5, 12.5 and 50 kHz, and the quartz passes the 10th, 4th harmonics and fundamental tone to the amplitude meter, respectively. The circuit is more for the amateur to solder on the table: there is a lot of fuss with the settings, but there is no “flair”, as they say. Provided as an example only.

Transceiver

Much more sensitive is a transceiver metal detector with an DD coil, which can be made at home without much difficulty, see Fig. On the left is the transmitter; on the right is the receiver. The properties of different types of DD are also described there.

This metal detector is LF; search frequency is about 2 kHz. Detection depth: Soviet nickel - 9 cm, tin can - 25 cm, sewer hatch - 0.6 m. The parameters are “three”, but you can master the technique of working with DD before moving on to more complex structures.

The coils contain 80 turns of PE wire 0.6-0.8 mm, wound in bulk on a mandrel 12 mm thick, the drawing of which is shown in Fig. left. In general, the device is not critical to the parameters of the coils; they would be exactly the same and located strictly symmetrically. Overall, a good and cheap simulator for those who want to master any search technique, incl. "for gold." Although the sensitivity of this metal detector is low, the discrimination is very good despite the use of DD.

To set up the device, first turn on headphones instead of the L1 transmitter and check by the tone that the generator is working. Then L1 of the receiver is short-circuited and by selecting R1 and R3, a voltage equal to approximately half the supply voltage is set on the collectors VT1 and VT2, respectively. Next, R5 sets the collector current VT3 within 5..8 mA, opens L1 of the receiver and that’s it, you can search.

Cumulative phase

The designs in this section show all the advantages of the phase accumulation method. The first metal detector, primarily for construction purposes, will cost very little, because... its most labor-intensive parts are made... from cardboard, see fig.:

The device does not require adjustment; integrated timer 555 is an analogue of the domestic IC (integrated circuit) K1006VI1. All signal transformations occur in it; The search method is pulsed. The only condition is that the speaker needs a piezoelectric (crystalline) one; a regular speaker or headphones will overload the IC and it will soon fail.

Coil inductance is about 10 mH; operating frequency – within 100-200 kHz. With a mandrel thickness of 4 mm (1 layer of cardboard), a coil with a diameter of 90 mm contains 250 turns of PE 0.25 wire, and a 70 mm coil contains 290 turns.

Metal detector “Butterfly”, see fig. on the right, in its parameters it is already close to professional instruments: the Soviet nickel is found at a depth of 15-22 cm, depending on the soil; sewer hatch - at a depth of up to 1 m. Effective in case of synchronization failures; diagram, board and type of installation - in Fig. below. Please note that there are 2 separate coils with a diameter of 120-150 mm, not DD! They must not intersect! Both speakers are piezoelectric, as before. case. Capacitors - heat-stable, mica or high-frequency ceramic.

The properties of the “Butterfly” will improve, and it will be easier to configure it if, firstly, you wind the coils with flat baskets; inductance is determined by the given operating frequency (up to 200 kHz) and the capacitances of the loop capacitors (10,000 pF each in the diagram). Wire diameter is from 0.1 to 1 mm, the larger the better. The tap in each coil is made from a third of the turns, counting from the cold (lower in the diagram) end. Secondly, if individual transistors are replaced with a 2-transistor assembly for K159NT1 amplifier circuits or its analogues; A pair of transistors grown on the same crystal has exactly the same parameters, which is important for circuits with synchronization failure.

To set up the Butterfly, you need to accurately adjust the inductance of the coils. The author of the design recommends moving the turns apart or moving them or adjusting the coils with ferrite, but from the point of view of electromagnetic and geometric symmetry, it would be better to connect 100-150 pF trimming capacitors in parallel with 10,000 pF capacitors and twist them in different directions when tuning.

The actual setup is not difficult: the newly assembled device beeps. We alternately bring an aluminum saucepan or a beer can to the coils. To one - the squeak becomes higher and louder; to the other - lower and quieter or completely silent. Here we add a little capacity to the trimmer, and in the opposite shoulder we remove it. In 3-4 cycles you can achieve complete silence in the speakers - the device is ready for searching.

More about "Pirate"

Let's return to the famous "Pirate"; It is a pulse transceiver with phase accumulation. The diagram (see figure) is very transparent and can be considered a classic for this case.

The transmitter consists of a master oscillator (MG) on the same 555 timer and a powerful switch on T1 and T2. On the left is the ZG version without an IC; in it you will have to set the pulse repetition rate on the oscilloscope to 120-150 Hz R1 and the pulse duration to 130-150 μs R2. Coil L is common. A limiter on diodes D1 and D2 for a current of 0.5 A saves the QP1 receiver amplifier from overload. The discriminator is assembled on QP2; together they make up the dual operational amplifier K157UD2. Actually, the “tails” of re-emitted pulses accumulate in container C5; when the “reservoir is full,” a pulse jumps at the output of QP2, which is amplified by T3 and gives a click in the dynamics. Resistor R13 regulates the filling speed of the “reservoir” and, consequently, the sensitivity of the device. You can learn more about “Pirate” from the video:

Video: “Pirate” metal detector

and about the features of its configuration - from the following video:

Video: setting the threshold of the “Pirate” metal detector

On the beats

Those who want to experience all the delights of the beating search process with replaceable coils can assemble a metal detector according to the diagram in Fig. Its peculiarity, firstly, is its efficiency: the entire circuit is assembled on CMOS logic and, in the absence of an object, consumes very little current. Secondly, the device operates on harmonics. The reference oscillator on DD2.1-DD2.3 is stabilized by ZQ1 quartz at 1 MHz, and the search oscillator on DD1.1-DD1.3 operates at a frequency of about 200 kHz. When setting up the device before searching, the desired harmonic is “caught” with a varicap VD1. Mixing of the working and reference signals occurs in DD1.4. Third, this metal detector is suitable for working with replaceable coils.

It is better to replace the IC 176 series with the same 561 series, the current consumption will decrease and the sensitivity of the device will increase. You can’t just replace old Soviet high-impedance headphones TON-1 (preferably TON-2) with low-impedance ones from the player: they will overload the DD1.4. You need to either install an amplifier like the “pirate” one (C7, R16, R17, T3 and a speaker on the “Pirate” circuit), or use a piezo speaker.

This metal detector does not require any adjustments after assembly. The coils are monoloops. Their data on a 10 mm thick mandrel:

Diameter 25 mm – 150 turns PEV-1 0.1 mm.
Diameter 75 mm – 80 turns PEV-1 0.2 mm.
Diameter 200 mm – 50 turns PEV-1 0.3 mm.

It couldn't be simpler

Now let's fulfill the promise we made at the beginning: we'll tell you how to make a metal detector that searches without knowing anything about radio engineering. A metal detector “as simple as shelling pears” is assembled from a radio, a calculator, a cardboard or plastic box with a hinged lid and pieces of double-sided tape.

The metal detector “from the radio” is pulsed, but to detect objects it is not dispersion or delay with phase accumulation that is used, but the rotation of the magnetic vector of the EMF during re-emission. On the forums they write different things about this device, from “super” to “sucks”, “wiring” and words that are not customary to use in writing. So, in order for it to be, if not “super,” but at least a fully functional device, its components—the receiver and the calculator—must meet certain requirements.

Calculator you need the most tattered and cheapest, “alternative”. They make these in offshore basements. They have no idea about the standards for electromagnetic compatibility of household appliances, and if they heard about something like that, they wanted to choke it from the bottom of their hearts and from above. Therefore, the products there are quite powerful sources of pulsed radio interference; they are provided by the calculator's clock generator. In this case, its strobe pulses on the air are used to probe space.

Receiver We also need a cheap one, from similar manufacturers, without any means of increasing noise immunity. It must have an AM band and, which is absolutely necessary, a magnetic antenna. Since receivers that receive short waves (HF, SW) with a magnetic antenna are rarely sold and are expensive, you will have to limit yourself to medium waves (SV, MW), but this will make setup easier.

We unfold the box with the lid into a book.
We paste strips of adhesive tape onto the back sides of the calculator and the radio and secure both devices in the box, see fig. on right. Receiver - preferably in a cover so that there is access to the controls.
We turn on the receiver and look for an area at maximum volume at the top of the AM band(s) that is free from radio stations and as clean as possible from ethereal noise. For CB this will be around 200 m or 1500 kHz (1.5 MHz).
We turn on the calculator: the receiver should hum, wheeze, growl; in general, give the tone. We don't turn down the volume!
If there is no tone, carefully and smoothly adjust until it appears; We caught some of the harmonics of the calculator's strobe generator.
We slowly fold the “book” until the tone weakens, becomes more musical, or disappears altogether. Most likely this will happen when the lid is turned about 90 degrees. Thus, we have found a position in which the magnetic vector of the primary pulses is oriented perpendicular to the axis of the ferrite rod of the magnetic antenna and it does not receive them.
We fix the lid in the found position with a foam insert and an elastic band or supports.

Note: depending on the design of the receiver, the opposite option is possible - to tune to the harmonic, the receiver is placed on the turned on calculator, and then, by unfolding the “book,” the tone softens or disappears. In this case, the receiver will catch pulses reflected from the object.

What's next? If there is an electrically conductive or ferromagnetic object near the opening of the “book,” it will begin to re-emit probing pulses, but their magnetic vector will rotate. The magnetic antenna will “sense” them, and the receiver will again give a tone. That is, we have already found something.

Something weird at last

There are reports of another metal detector “for complete dummies” with a calculator, but instead of a radio, it supposedly requires 2 computer disks, a CD and a DVD. Also - piezo headphones (precisely piezo, according to the authors) and a Krona battery. Frankly speaking, this creation looks like a technomyth, like the ever-memorable mercury antenna. But - what the hell is not joking. Here's a video for you:

try it if you wish, maybe you’ll find something there, both in the subject matter and in the scientific and technical sense. Good luck!

As an application

There are hundreds, if not thousands, of metal detector designs and designs. Therefore, in the appendix to the material we also provide a list of models, in addition to those mentioned in the test, which, as they say, are in circulation in the Russian Federation, are not overly expensive and are available for repetition or self-assembly:

Clone.

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