• Coin with a diameter of 25mm (air) up to 30 cm
  
• Helmet 0.8m
• Maximum up to 2.0m
• Sound indication multi-voice selectable
  
• Visual indication LCD 16x2
• How IB works
• Selective search mode
• Sensor ring or DD diameter 10-30 cm
  
• Range of possible operating frequencies 6-12 kHz
• Supply voltage 7.0-9V
• Current consumption no more than 90 mA
• The weight of the structure, which is 850 grams in the photo on the left, can be disassembled into two parts.
  

Basic functionality

* Operating modes: dynamic, static (pinpointer)
* Selectable types of signal parameters indication
Indication of the numerical value of the VDI and graphical indication of the amplitude.
Indication in the form of histograms - VDI and amplitude
* Main sound indication 3-tone - color, black and overload
* Selectable sound indication for color
Changing the beep tone from VDI , sound discrimination
Changing the volume of a sound signal, a function of amplitude
Multi-tone PWM
* Supply voltage indication
* Visual and audio indication of overload.
* Availability of a discriminator: color -10 segments with a step of 10 gr.
Iron - 6 segments with increments of 20 gr.
* Sensitivity setting separately for “color” and “black” targets - from 0 to 9
* Availability of manual ground balance in static and automatic (dynamic).
* Cash switchable backlight
* Availability of debugging mode
Digital frequency setting in the range of 6-12 kHz in steps of ~10 Hz
Indication of X and Y values ​​and dX dY for setting MD
Software selectable phase shift in 45 degree increments. hardware - smoothly + -20 gr
* Convenient and intuitive interface.
* Automatic recording of all settings into non-volatile memory
* Easy setup for different IB sensors
* Ability to create a profile for a specific sensor. In total you can create 4 profiles.

Scheme

Capacitors (except for those for power supply) use film or ceramic type NPO, X7R!
Button 5 - “static”, is not necessary - it is intended for the amateur.

Coil connection

This is how the electronic unit may look in the housingZ5, batteries inside.

Manufacturing: Manufacturing managerboards (new board below) is not complicated and only requires the skill of installing SMD components, standard size resistors and capacitors 0805. Capacitors C and C1 are film. All microcircuits are in DIP package.
But the productionThe device sensor and subsequent adjustment of the device will require certain knowledge and skills. And it is only available to trained radio amateurs! Personally, this site helped me in this regard. http://www.md4u.ru/forum/ .

Data for DD coil frequency 8-10 kHz, diameter 20-25 cm
Number of turns of transmitting TX 40-50 turns of wire 0.4-0.5mm c1=0.3-0.47 uF
Number of turns of receiving RX 150-200 turns of wire 0.15-0.22 mm C0=0.022-0.033 uF Not resonance C0=0.

If the sensor ring , O You can read the description of the sensor manufacturing technology .
Number of turns of transmitting coil TX ~50 turns of wire 0.3-0.5mm
Number of turns of compensating coil CX (selectable) ~20 turns of wire 0.3-0.5mm
Number of turns of the receiving coil RX ~200 turns of wire 0.15-0.22mm

Changes to the sensor parameters are allowed (wire, number of turns, winding diameter). In general, there is a huge field for experimentation. It is also possible to use a “proprietary” sensor(s).

Settings

First, let's get acquainted with the "Description of operating modes - menu"
We start by setting up the “virtual zero” of the receiving path. We turn off the coils, place a jumper from C0 to ground and, with the button “released”, achieve zero (in fact, the voltage on the 23rd leg of the controller is about +2.5V) readings at the output of the receiving path; we do this procedure in the “NL,Up” settings mode (in firmware versions 7 and higher) using the “+” button, remembering the value using the “0” button. Voltage readings should not change by more than +-1! We can immediately adjust the “correctness” of the supply voltage readings, similar to what was described above with the “-” button. In earlier versions, adjustments are made using resistors R1/R2 and R8/R7.
Next, we infuse the coil; for ease of adjustment, it is recommended to set R5 and R17 to the maximum resistance values. Then select them to obtain satisfactory sensitivity-stability parameters.
If the sensor is resonant, it is necessary to configure the receiving RX (for non-resonant - without capacitor C0, we skip this setting) and the transmitting TX coils to resonance or a state close to resonance. To do this, connect the transmitting coil TX, go into the settings mode (long press the "mode" button), submode X,Y and use the "+" and "-" buttons to find the resonant frequency based on the maximum voltage on the TX coil. The amplitude voltage value should be about 10-30 V (depending on the parameters of the TX circuit). Let's remember this frequency - F . Next, we separate the (connected) receiving and transmitting coils (previously replacing capacitor C1 with a 10-50 ohm resistor) and using the same buttons we find the resonant frequency of the receiving circuit based on the maximum signal amplitude at pin 6 of D2, it may differ from F within +-20%. The further you move from the TX resonance frequency, the more stable the coil will be. (It is recommended to set the receiving RX ~10-20% lower. That is, if you have a TX -10 kHz, set the RX to 8.0-9 kHz.) Let me remind you the frequency F can range from 4 to 16 kHz. Recommended frequency 6-12 kHz.
You can select the resonant frequency by changing the number of turns of the windings or the value of the capacitances of capacitors C0 and C1. Next, the coils are placed in the housing and brought together by the compensating winding CX (if the coil is DD, then by moving the windings) according to the minimum voltage on the 6th pin of the D2 chip (You can also use a “Mole” voltmeter in X,Y mode - firmware 5 and higher). For normal operation it should not exceed 1B! In general, you should strive to get the minimum possible value. We set our previously found frequency F or close one.
Next we set up the phase: When a copper coin approaches, Y and X should increase; when a ferrous metal (iron) approaches, Y increases and X decreases. If this doesn’t happen, use the “0” button (discretely in increments of 45 degrees) and trimming resistor R16 (smoothly) to change the phase until we achieve the correct reaction to metals . If this procedure does not work, you need to select capacitance C0, C1 or change the transmitter frequency . Next, you need to set the exact phase for correct VDI measurement construction resistor R16. N It is better to adjust the phase according to the ferrite - X to minus, Y in place or slightly to +. You can also adjust the phase according to the “standard” - for example, 5 kopecks USSR VDI value about +55.VDI Foil ~ 0-10, pure copper +80-+90, all pieces of iron are in the minus. If possible, correct the convergence using a tuning turn(s). There is one more subtlety: after tuning, change the frequency + -30 Hz. find the place where the values ​​of X and Y change (jump) the least!
Sensitivity Setting: ResistorR0 andR17 selects the current of the TX coil. The higher the current, the greater the sensitivity, both to targets and, unfortunately, to the ground. Recommended current for a resonant system is 40-80 mA, non-resonant 80-100 mA. The current in TX can be approximately estimated by the difference in the consumption of the device with and without a coil, then for a resonant system 20-40 mA, non-resonant 40-60 - depends on the quality factor of the circuit. Resistor R5 selects the sensitivity of the device in the RX channel; the lower its value, the higher the sensitivity. Recommended resistor values ​​for a resonant system are ~1k, non-resonant is ~200ohm. The sensitivity of the device must be adjusted so that there are no false alarms at the threshold (sensitivity) of 1.2 (of course, provided there is no interference). There is no need to chase high sensitivity through the air, 20-25 cm is just right! for a sensor with a diameter of ~20 cm. For a sensor with a diameter of 25 cm, respectively, 25-30 cm.
Resistor R13 adjusts the contrast of the indicator and its value depends on the type of indicator.
Note: INIn setting mode, the “+” and “-” buttons change the frequency on TX in steps of ~10 Hz. The "0" button changes the phase with a period of 45º.

New pay under the dip microcircuit. The board is functional and tested! For more reliable operation, it is advisable to leave the top layer of foil on the board and connect it at several (marked in blue) points. When installing the indicator, see the description; power pins vary!
Chips: MCP601 can be replaced by NE5534, K140UD1408, LF357, LM308.Microcontrollerit's better to put ATMEGA8А it consumes less and works better.

How to easily flash a microcontroller, you can read about it

We program fuses like this

Who wants to listen to the sounds of "Mole" or other MD on FM radio, you can put together a simple circuit.

Adjust to the desired frequency - spreading and squeezing the turns of coil L1
We select R4 based on the absence of distortion.

New firmware 0.2 with two dynamic modes and vector cutting of the soil.. Don’t forget to periodically adjust the ground with the “0” button. First, a meter from the ground for a short time “0”, then at the search distance for a long time “0”, the value of the soil coefficient should be minus (-10 - 70)

15.09.2011
1. The circuit has been changed, one resistor has been removed (formerly R4) and some values ​​have been changed (red color)
2. The board has been changed taking into account the new scheme.
3.New firmware 0.3 for testing.
The measurement speed has been increased by 20%, the soil accounting algorithm in modes “D” and especially “G” has been changed, and the ability to measure supply voltages above 10V has been added.
New firmware and changes in the circuit diagram allow the use of non-resonant RX, but you will have to sacrifice ~20% range...

15.10.2011
1.New firmware 0.4 for testing and firmware 0.4a + firmware 0.4v (for transistor TX)
A fifth button has been added for the pinpointer mode, the ability to reduce the TX frequency to 4 kHz has been added, and the sensitivity of the channels has been increased (only in 0.4).

15.11.2011
1.New firmware 0.5 for testing.
2. In (X,Y) mode:
a) added, the function of measuring the voltage in mV of the coil unbalance.
b) added the ability to automatically adjust the minimum voltage value imbalance coil - long press "0" (use with understanding!).
c) the form for displaying the initial phase setting values ​​has been changed, now: 0-0g, 1-45g, 2-90g, 3-135g
3. The initial state of the 15th leg of the microcontroller at the moment of initialization +5V has been changed - for a circuit with parallel TX drive on a transistor, this does not affect operation with serial drive.

15.12.2011
1. New firmware 0.6 for testing. 0.6A - for old ones - ATmega8-XXX (or who has high voltage values ​​without a coil)
2 . The initialization screensaver has been changed; it now shows the profile number (4 in total), as well as the coil frequency and imbalance amplitude.
3. Added "profile" selection function. The profile number is selected in mode (dX, dУ) and takes effect after overload. The profile includes all selectable parameters: frequency, phase, filter, sound circuit and everything else.
4 .When you press the "0" button, the actual coil voltage in mV is now displayed, although the zeroing function has not changed.
5 .The filter in the “static” mode has been changed; now this mode will work successfully with DD coils.
6 .Mode "D" soil is taken into account "dynamically", mode "G" soil is taken into account "pseudostatically".

How do MDs work? - Very simple! -article

Video firmware 0.5, coil DD20 (resonance), frequency 12 kHz, Air tests phone quality...

Video "Mole brick and penny" firmware 0.6, DD20 coil (non-resonance), frequency 12 kHz, demonstrates automatic “cutting” of soil in dynamic mode, or how to check your device in winter..)

16.01.2012 I have corrected the article in light of recent observations...

15.02.2012 I corrected the circuit - inaccuracies were eliminated, resistors R17, R18 were added, some values ​​​​were changed, a connector for in-circuit programming was added (as an option), measuring and digital grounds were separated (they can be connected at one point through a resistor or inductor). All changes are not fundamental and do not bring significant improvements! The payment has been corrected.

23.02.2012
For "Men's Day" new firmware 0.7
1. Now thresholds for different profiles are also remembered.
2. Changed the type of discriminator - first black, then color.
3. Added a new mode in the settings - “NL,Up” Use the “+” button to select the coefficient for setting “0” (with the coil turned off, equivalent to selecting R1/R2) use the “-” button to select coefficient for setting "Supply voltage" ( equivalent to selecting R8/R7).
The settings are saved using the "0" button.
4. The voltmeter now also measures “negative” voltages.
5. Mode “G” is now again a “static” accounting of the soil; after correct adjustment to the ground, the depth is comparable to air...
6 Removed the auto-adjustment mode for the minimum coil balance voltage.
7. Other minor improvements.
I'm looking forward to interesting reports about the firmware!

CHRIST IS RISEN!
15.04.2012 - New firmware 0.8
1. Improved "histogram" mode. Allows you to identify objects with complex geometric shapes, etc.
2. In the “LED” backlight mode, the third parameter “SG” has been introduced, an economical mode for illuminated indicators and for those who like to search at night.
3. Improved "Static" mode. Now you can search in this mode.
4. The “Interference” mode has been introduced. It serves to localize the source of interference and evaluate his level . It is turned on by pressing the "Reg" button when turning on. Exit - turn off.
5. Other minor improvements.

05/15/2012 Firmware 0.8G -analogue 0.8, but you may have to tighten the phase -R16.

If you are interested in depth on real soil, learn to work in "G"
We work only after careful adjustment to the ground. You may have to do this often and several times.
The correctness of the setting can be assessed as follows: Soil coefficient from -10 to -80.


10/11/2012 Firmware 10. (more precisely, the two firmwares differ only in the initial phase. “10” as in “8G”, “10” as in “8” and below.)
Changed (simplified) menu. Now all four buttons in the main mode have two functions: “short” and “long” press. The "DEV" and "0" keys are the same as before. Key "-" short press - increase the threshold for "color" (like before) , long-term increase in filter number. "+" key: short increase in "black" threshold (like before) , long “on-off” of the indicator backlight.
In the main mode, the “fN” parameter appeared, where N is the filter number:
0 - filter as in firmware 8G and below for the “lightest soil” - air, sand..
1- Filter for light soil.
2- Filter for medium soil.
3- Filter for heavy soil.
Other improvements to improve the operation of the device.

12/15/2012 Firmware 12
Continuation of version 10.
The main menu has been slightly changed.
The settings are now hidden and are called up when you turn on the device and simultaneously press the "+" or "-" buttons
Exiting setting modes via “MODE” or “Off-On.” device.
Improved selection of "colored" targets at Fe0.
The VDI range has been increased to +-100.
The "color" range is now 2-tone 0-79 and 80-100.
Other software improvements that improve the operation of the device.

01/20/2013 Firmware 12A
Continuation of version 12
Improved "visualization algorithm" in main mode. Now the target can be scanned multiple times with accumulation (averaging) of the result of the displayed VDI.
Added "parameter display" function - button S5.

01/20/2013 Firmware 12B
Same as 12A, instead "parameter display", "pinpointer" - button S5
There is no interference level measurement mode.

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:

1. 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.
2. The size and dimensions of the search zone is an imaginary area in the ground in which the object will be detected.
3. Sensitivity is the ability to detect more or less small objects.
4. 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.
5. 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.
6. 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).
7. 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

1. 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.
2. 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.
3. 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.
4. 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 that 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 sound tone in the headphones can be changed by selecting the capacitance C1. Under the influence of the object, the tone decreases, unlike all other types, so initially you need to achieve a “mosquito squeak”, and not wheezing or grumbling. The device distinguishes live wiring from “empty” wiring; a 50 Hz hum is superimposed on the tone.

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

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

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

Transceiver

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

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

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

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

Cumulative phase

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

The device does not require adjustment; integrated timer 555 is an analogue of the domestic IC (integrated circuit) K1006VI1. All signal transformations occur in it; The search method is pulsed. The only condition is that the speaker needs 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 setup itself is not difficult: the newly assembled device beeps. We alternately bring an aluminum saucepan or a beer can to the coils. To one - the squeak becomes higher and louder; to the other - lower and quieter or completely silent. Here we add a little capacity to the trimmer, and in the opposite shoulder we remove it. In 3-4 cycles you can achieve complete silence in the speakers - the device is ready for searching.

More about "Pirate"

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

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

Video: “Pirate” metal detector

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

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

On the beats

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

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

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

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

It couldn't be simpler

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

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

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

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

1. We unfold the box with the lid into a book.
2. 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.
3. 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).
4. We turn on the calculator: the receiver should hum, wheeze, growl; in general, give the tone. We don't turn down the volume!
5. If there is no tone, carefully and smoothly adjust until it appears; We caught some of the harmonics of the calculator's strobe generator.
6. 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.
7. 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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