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A simple metal detector with a minimum number of elements. Do-it-yourself deep metal detector: diagram, instructions and reviews. Simple magnetic detectors

Deep type metal detectors are able to detect objects in the ground at a great distance. Modern modifications in stores are quite expensive. However, in this case, you can try to make a metal detector with your own hands. For this purpose, it is first of all recommended to familiarize yourself with the design of the standard modification.

Modification scheme

When assembling a metal detector with your own hands (the diagram is shown below), you need to remember that the main elements of the device are a damper on a microcontroller, a capacitor and a handle with a holder. The control unit in the devices consists of a set of resistors. Some modifications are made on drive modulators that operate at a frequency of 35 Hz. Directly racks are made with narrow and wide plate-shaped plates.

Assembly instructions for a simple model

Assembling a metal detector with your own hands is quite simple. First of all, it is recommended to prepare a tube and attach a handle to it. Installation requires high conductivity resistors. The operating frequency of the device depends on many factors. If we consider modifications on diode capacitors, then they have high sensitivity.

The operating frequency of such metal detectors is about 30 Hz. The maximum detection distance of an object is 25 mm. Modifications are capable of working on lithium-type batteries. Microcontrollers for assembly will be required with a polar filter. Many models fold on open type sensors. It is also worth noting that experts do not recommend using high sensitivity filters. They greatly reduce the accuracy of detecting metal objects.

Model series "Pirate"

You can make a Pirate metal detector with your own hands only on the basis of a wired controller. However, first of all, a microprocessor is prepared for assembly. To connect it, you will need Many experts recommend the use of grid capacitors with a capacitance of 5 pF. Their conductivity should be maintained at a level of 45 microns. After you can start soldering the control unit. The stand must be strong and support the weight of the plate. Plates larger than 5.5 cm in diameter are not recommended for 4V models. System indicators are optional. After fixing the block, it remains only to install the batteries.

Using reflex transistors

Making a do-it-yourself metal detector with reflex transistors is quite simple. First of all, experts recommend installing a microcontroller. Capacitors in this case are suitable for a three-channel type, and their conductivity should not exceed 55 microns. At a voltage of 5 V, they have a resistance of approximately 35 ohms. Resistors in modifications are used mainly of the contact type. They have a negative polarity and cope well with electromagnetic vibrations. It is also worth noting that during assembly it is allowed to use the maximum plate width for such a modification is 5.5 cm.

Model with convection transistors: expert reviews

You can assemble a metal detector with your own hands only on the basis of a collector controller. In this case, capacitors are used at 30 microns. If you believe the reviews of experts, then it is better not to use powerful resistors. In this case, the maximum capacitance of the elements should be 40 pF. After installing the controller, it is worth taking up the control unit.

These metal detectors receive good reviews for reliable protection against wave interference. For this purpose, two diode-type filters are used. Modifications with display systems are very rare among homemade modifications. It is also worth noting that power supplies must operate at low voltage. Thus, the battery will last a long time.

Using Chromatic Resistors

With your own hands? A model with chromatic resistors is quite easy to assemble, but it should be borne in mind that capacitors for modifications are only allowed to be used on fuses. Experts also point out the incompatibility of resistors with feed-through filters. Before starting the assembly, it is important to immediately prepare a tube for the model, which will be a handle. Then the block is installed. It is more expedient to select modifications for 4 microns, which operate at a frequency of 50 Hz. They have a small scattering coefficient and high measurement accuracy. It is also worth noting that seekers of this class will be able to successfully work in conditions of high humidity.

Model with a pulsed zener diode: assembly, reviews

Devices with pulsed zener diodes are distinguished by high conductivity. If you believe the reviews of experts, then home-made modifications are able to work with objects of different sizes. If we talk about the parameters, then their detection accuracy is approximately 89%. It is worth starting the assembly of the device with a rack blank. Then the handle for the model is mounted.

The next step is to install the control unit. Then the controller is mounted, which is powered by lithium batteries. After installing the unit, you can start soldering the capacitors. Their negative resistance should not exceed 45 ohms. Expert reviews indicate that modifications of this type can be made without filters. However, it should be borne in mind that the model will have serious problems with wave interference. This will damage the capacitor. As a result, the battery in models of this type is quickly discharged.

Application of the low frequency transceiver

Low-frequency transceivers in models significantly reduce the accuracy of the devices. However, it is worth noting that modifications of this type are able to successfully work with small objects. At the same time, they have a small self-discharge parameter. In order to assemble the modification with your own hands, it is recommended to use a wired controller. The transmitter is most often used on diodes. Thus, conductivity is provided at around 45 microns with a sensitivity of 3 mV.

Some experts recommend installing mesh filters, which increase the security of models. To raise the conductivity, only transitional type modules are used. The main disadvantage of such devices is the burnout of the controller. With such a breakdown, it is problematic to repair the metal detector with your own hands.

Using a high frequency transceiver

On high-frequency transceivers, you can assemble a simple metal detector with your own hands only on the basis of a transitional controller. Before starting the installation, a rack for the plate is prepared as a standard. The conductivity of the controller is on average 40 microns. Many specialists do not use contact filters when assembling. They have high heat losses and are capable of operating at 50 Hz. It is also worth noting that lithium batteries are used to assemble the metal detector, which recharge the control unit. The sensor itself for modifications is installed through a capacitor, in which the capacitance should not exceed 4 pF.

Longitudinal resonator model

There are often devices with longitudinal resonators on the market. They stand out from their competitors with high accuracy in determining objects, and at the same time they can work in high humidity. In order to independently assemble the model, a stand is prepared, and a plate should be used with a diameter of at least 300 mm.

It is also worth noting that a contact controller and one expander are required to assemble the device. Filters are used only on a mesh lining. Many experts recommend installing diode capacitors that operate at a voltage of 14 V. First of all, they discharge the battery a little. It is also worth noting that they have good conductivity compared to field counterparts.

Using Selective Filters

Making such a deep metal detector with your own hands is not easy. The main problem is that a conventional capacitor cannot be installed in the device. It is also worth noting that the modification plate is selected from 25 cm in size. In some cases, the racks are installed with an expander. Many experts advise starting the assembly with the installation of the control unit. It must operate at a frequency of no more than 50 Hz. In this case, the conductivity depends on the controller that is used in the equipment.

Quite often, it is selected with a lining to increase the security of the modification. However, such models often overheat, and are not able to work with high accuracy. To solve this problem, it is recommended to use conventional adapters that are installed under the capacitor units. A do-it-yourself coil for a metal detector is made from a transceiver unit.

Application of contactors

Contactors are installed in the devices together with control units. Racks for modifications are used in short lengths, and plates are selected at 20 and 30 cm. Some experts say that devices should be assembled on impulse adapters. In this case, capacitors can be used with low capacitance.

It is also worth noting that after installing the control unit, it is worth soldering a filter that can operate at a voltage of 15 V. In this case, the model will maintain a conductivity of 13 microns. Transceivers are most often used on adapters. Before turning on the metal detector, the level of negative resistance is checked on the contactor. The specified parameter averages 45 ohms.

Transmitter

The transmitting part consists of a rectangular pulse generator on the IC1 chip - NE555 (domestic analogue of KR1006VI1) and a powerful key on the T1 transistor - IRF740 (IRF840). For its buildup, there is a transistor T2 - 2N3904. The load T1 is the search coil L1. To adjust the duration and frequency of the pulse, we select the resistance R10 and R11, respectively.

Receiver

The receiving node is assembled on the IC2 chip - TL074. It consists of four low-noise operational amplifiers. At the input of the first stage of the amplifier there is a signal limiter on the diodes VD1, VD2, connected in anti-parallel. At the output of the last amplifier, an LED is turned on, which lights up when there is metal in the coil field.

After the first amplification stage, there is a passive filter that cuts out the useful part of the incoming pulse.

A sound generator is assembled on the IC3 - NE555 chip, which works together with the LED when metal appears. The transistor T3 - 2N3906 controls the generator.

Diode VD3 IN4001 together with a fuse (0.5A) are needed to protect the circuit from power reversal.

search coil

Coil L1 (250μH) is wound on a mandrel 180 - 200 mm and contains 27 turns of PELSHO wire in lacquer and silk insulation, if this is not the case, then PEV (PEL, PETV, etc.), with a diameter of 0.3 - 0.8 mm. The wire can be taken from transformers, chokes, a deflecting system, or a degaussing loop of an unusable color TV. The coil can be wound on a round mandrel, such as a bucket or saucepan. Then remove from the mandrel and wrap several layers of electrical tape. To make a coil, you can use a plastic lid from a bucket or an embroidery hoop, in which the wire fits very well.

The coil frame must NOT contain metal! The coil itself in this type of metal detector is also NOT wrapped in foil!

The wire connecting the coil and the board must be thick and preferably shielded, and also not have connections and connectors. In a pulse, the current reaches large values and all of the above affects the sensitivity of the device.

Setting up the metal detector

Setting up this metal detector is much more complicated than the one considered earlier on a single K561LA7 chip.

Solder the board with clean rosin or an alcohol-rosin solution. After soldering with a toothbrush, wash off the remnants of rosin with alcohol. After installation, ALWAYS double-check the correctness of installation according to the circuit diagram.

A properly assembled metal detector works right away, but to achieve maximum sensitivity, you will need a lot of effort and patience, and an oscilloscope and a frequency meter would not hurt to set it up. You will also need a multimeter. When switching on, check the current consumed by the device. At 9V - 30 mA, at 12V - 42mA.

To power the device, it is better to take batteries. I took from an old laptop battery. 4 pcs of 3V = 12V.

First, it is recommended to wind the coil about 30 turns, then adjust the maximum sensitivity with resistors. Headphones need to achieve R6 and R16 RARE CRACKLES. Then wind 2 turns - then adjust until crackling. For example, I wound 2 turns and try the first stage to adjust the gain (R6), then run the filter adjustment (R14, C8), then the gain control of the second stage (R20), the third (R22).

As long as you can control the sound, ignore the LED. When winding the turns, the current will increase, but the sensitivity needs to be “caught” to the maximum. If there are many turns, it will be weak, and with small turns, it will also be weak. You need to find the "golden mean".

Resistors R6 - gain threshold of the first stage(voltage table below) together with regulators "Filter" And "Gain" achieve maximum sensitivity rare crackling in the headphones! ) And R24 - sound generator threshold to make the LED and tone oscillator in the headphones appear at the same time. Regulators "Filter" And "Gain" set the threshold for the beginning of the glow of the LED.

With a multimeter, you can measure the voltage (V) at the terminals of the op-amp (without the presence of metal in the coil field / with the presence of metal) (metal detector power supply + 12V):

IC1 (NE555)

IC2 (TL074)

0 / 4,1
0,8 / 4,3
0,8 / 4,3
0,1 / 4,3
4 / 3,6
4,0 / 3,6

IC3 (NE555)

7,1 / 6,3
11,5 / 10,1
7,1 / 6,3
7,1 / 6,3

If you have an oscilloscope, you can see:

Transmitter operation

generator frequency on IC1 pin 3 (tuning R11 - 120 - 150Hz);
the duration of the control pulse at the gate T1 (tuning R10 - 130-150 μs).

receiver operation

The passage of transmitter pulses at the receiver test points (outputs of operational amplifiers Pins 1, 14, 8 and 7.

A tone appears at the output of the sound generator microcircuit (pin 3), with a frequency of about 800 - 1000 Hz. The tone frequency is determined by the capacitor C13 and the resistance R27.

To increase the volume at the output of the microcircuit, there is a transistor T4 - 2N3906. The volume in the headphones is set by the resistance R31, connected in series with the headphone.

Printed circuit board of the metal detector "Vintik"

The metal detector circuit is assembled on a printed circuit board made of foil fiberglass according to the above figure.

Location of parts on the board

Working with a metal detector

When turned on, the regulators R14 "Filter" and R16 "Gain" set the threshold for the start of the LED glow. Setting to maximum sensitivity: we find a position in which clicks are barely heard in the speaker!

Schematic diagram of the modified impulse metal detector "VINTIK-PI"

The scheme is different from the previous one:

By adding a delay node on a NE555 chip and a key on a BF245 field effect transistor instead of a filter. The pulse duration is regulated by a tuning resistor from 50 to 100 µs. In the previous version, the required part of the pulse was cut out by a passive filter on R9, R12, R14, C8, C9, C10; now this is done by a keyed delay node (NE555 and BF245). With this solution, the task of setting the metal detector filter is simplified, and the sensitivity is also increased by 5-7 cm, the current consumption has increased to 65 mA (depending on the coil).
Added power control circuit on a free element (IC 2.2) TL074. When the power drops below 12V, the LED lights up. From 12 V to 10 V, the circuit is still operational with a slight adjustment of the “gain” regulator. Sensitivity also decreases when the power is reduced.
The volume control scheme has been changed. Now you can connect both headphones and a low-power speaker to the output. When an earphone is connected, the speaker is turned off.
This circuit uses a "basket type" search coil, consisting of three turns of a "twisted pair" computer cable (without a shield). With its help, it is possible to obtain a greater sensitivity of the device.

You can discuss the proposed metal detectors at.

If you want to assemble a circuit, but do not have the necessary details, you can

Metal detectors or metal detectors are a diverse family of measuring instruments whose operation is based on differences in the electromagnetic radiation of objects.

Using a metal detector

Professional highly sensitive metal detectors are used in the daily work of various checkpoints, they are used to conduct search and inquiry activities of police and rescue services.

A huge army of amateur treasure hunters around the world practice long and leisurely trips with metal detectors. Sometimes such entertainment brings income and even fame.

In our time, the industry of detector (recognizing) devices for all occasions has already been established, differing not only in the principles of operation, but also in a wide range of prices and technical characteristics.

Simple magnetic detectors

The principle of operation of the simplest metal detector is based on electromagnetic induction - the device contains an electromagnetic coil, which, due to oscillations and distortions of its field, captures nearby electrically conductive and iron-magnetic materials, while creating an audible or visual signal.

The first experience of assembling a metal detector at home can be the beginning of a serious hobby: new design solutions and even inventions in this field of applied radio electronics are not excluded even at an amateur level.

The diagram shows the structure of the simplest low-frequency magnetic detector.

Hundreds of different developments are used in the production of metal detectors. In order to implement one of them on your own, you will need to make a printed circuit board with your own hands, purchase the necessary coils, transistors, resistors, capacitors, etc., and assemble the device.

Metal detector from improvised means

Another option is to assemble a metal detector from improvised means, more suitable for humanitarians and novice techies with a passion for finding treasures and lost artifacts.

During the operation of such a home-made device, electromagnetic waves emitted by the calculator are caught on the AM band of the receiver.

The indicator of the presence of an object in this device is the rotation of the electromagnetic field during re-emission, which changes the parameters of the sound signal. A photo of such a do-it-yourself metal detector can be found on the Internet and at the end of our material.

To use such a prefabricated version, you need not a detailed diagram or assembly instructions, but compliance with certain requirements for the two main components of a home-made detector, namely, a properly working calculator and a radio receiver.

Both devices must be from the category of the cheapest, the receiver must have an AM band and a magnetic antenna, and the calculator must emit impulse radio interference during operation.

To work on the model, you will also need a suitable size plastic box with an opening lid, like a book, which will become the body of the finder.

For these purposes, an old CD box is ideal. You will need double-sided tape to attach the parts.

Assembling a metal detector

Fixing devices inside the case: a strip of adhesive tape is attached to the back side of the devices, then the calculator is placed at the base of the box, the receiver is on the inside of the cover.
Receiver setup: you need to turn on the receiver at maximum sound and select the upper position of the AM band, free from broadcasting radio stations and interference.
Adjustment of the calculator: when the calculator is turned on, the receiver should react with a sharp noise with a hum or wheeze, if this is not the case, you need to adjust the range.
Fixing the position: we begin to smoothly close the box until the sound disappears or becomes more uniform and fix the box doors in this position using a foam cube, rubber bands, etc.
The metal detector is ready. If there is a product with electromagnetic radiation nearby, the receiver will beep.

By combining the elements of other radio devices in the simplest detector, it will be possible to observe the principle of operation of metal detectors in action and enjoy your first search expedition.

Note!

Such a detector, assembled at home, can be tested on the search for coins or metal construction debris lying in the surface layer of the earth in almost any area, on any open ground.

DIY metal detector photo

Note!

If you have lost a ring, a key, a screwdriver... and you know the approximate place of loss, then do not despair! You can assemble a metal detector with your own hands or ask a friend of a radio amateur to assemble simple do-it-yourself metal detector. Below is a diagram of a metal detector that is easy to manufacture and proven over the years, which (with certain skills) can be made in one day. The simplicity of the described metal detector is that it is assembled on just one very common microcircuit K561LA7 (CD4011BE). The setup is also simple and does not require expensive measuring instruments. An oscilloscope or frequency meter is enough to tune the generators. If everything is done without errors and from serviceable elements, then these devices will not be needed.

The sensitivity of this metal detector:

jar metal lid “sees” up to 20 cm, cell phone up to 15 cm, “krone” battery up to 10 cm, 5 ruble coin up to 8 cm.

At this distance, the tone of the generator in the headphones barely changes; at a closer distance, the tone increases. The larger the metal area, the greater the detection distance. Distinguish between diamagnets and ferromagnets.

For metal detector manufacturing we will need:

Chip K561LA7 (or K561LE5, analogue of CD4011);
Transistor - low-power low-frequency, for example - KT315, KT312, KT3102, analogues: BC546, BC945, 2SC639, 2SC1815, etc.);
Diode - any low-power, for example - kd522B, kd105, kd106, analogues: in4148, in4001, etc .;
Variable resistor - 3 pcs (1 kOhm, 5 kOhm, 20 kOhm with a switch or a switch separately);
Fixed resistor - 5 pcs (22 Ohm, 4.7 kOhm, 1.0 kOhm, 10 kOhm, 470 kOhm);
Ceramic, and even better mica capacitors - 5 pcs: 1000 pF -3 pcs, 22 nF -2 pcs, 300 pF);
Electrolytic capacitor (100.0 microfarad x 16V) - 1 pc;
Wire PEL, PEV, PETV, etc., with a diameter of 0.4-0.7 mm;
Low-impedance headphones (from the player);
Battery 9V.

Metal detector scheme

The appearance of the metal detector board

In a case from an old pocket radio (you can use a case from a soap dish, from a sponge for cleaning shoes or in the housing from the electrical junction box.

Attention! To eliminate interference and the influence of human hands when touching the regulators, the cases of variable resistors must be connected to the minus of the board.

With a properly soldered metal detector circuit, serviceability and the correct value of the elements, a correctly made search coil, the device works without problems. If, when you turn on the headphones for the first time, you don’t hear a squeak and a change in frequency when adjusting the “FREQUENCY” knob, then you need to choose a resistor (10 kOhm) , in series with the regulatorand / or a capacitor in this generator (300 pF). Thus, we make the frequencies of the exemplary and search generators the same.

When the generator is excited, the appearance of whistling, hissing, distortion, solder a 1000 pF capacitor (1H0 aka 102) with a pin. 6 chips per case.

Using an oscilloscope or frequency meter, look at the signal frequencies at pins 5 and 6 of the K561LA7. Achieve their equality by the above described setup method. The very operating frequency of the generators can range from 80 to 200 kHz.

A protective diode (any low-power one) is needed to prevent damage to the microcircuit when the battery is turned on by mistake (which often happens :).

Making a metal detector coil

Coils are wound on a mandrel with a diameter of 15-25 cm (for example, a bucket or on a shuttle made of thick wire or plywood - the smaller the diameter, the lower the sensitivity, but the greater the selectivity of small metals). Choose for what purpose you need it.

A wire is used in lacquer insulation PEL, PEV, PETV ..., with a diameter of 0.4 - 0.7 mm (well suited for old color TVs with a kinescope degaussing loop or a deflecting system) and contains about 100 turns (you can wind from 80 to 120 turns ). We wrap tightly with electrical tape over the wire.

Then we wrap the coil over the electrical tape with a strip of foil, leaving a 2-3 cm area not wrapped. Foil can be taken from some types of cables or, in extreme cases, cut into strips 2 cm wide from chocolate foil 🙂

Once again tightly wrap with electrical tape.

Photo of the finished coil. It remains to wrap the top with electrical tape.

We fix the resulting finished coil on a dielectric (for example, non-foil textolite or getinaks). Next, we attach it to the holder.

We connect the coil with the circuit with a double shielded wire (screen on the case). The wire can be taken from old cords for dubbing from a tape recorder to a tape recorder or a low-frequency (audio-video) cord for connecting a TV to a DVD, etc.

Proper operation of the metal detector: when turned on with the “frequency” regulator in the headphones, we set a low-frequency rumble, when approaching the metal, the frequency changes.

It is possible the second option, so that the hum in the ears “does not stand”, set zero beats, i.e. combine two frequencies. Then there will be silence in the headphones, but as soon as we bring the coil to the metal, the frequency of the search generator changes and a squeak appears in the headphones. The closer to the metal, the higher the frequency in the headphones. But the sensitivity of this method is not great. The device will react only when the generators are strongly detuned, for example, when brought to the lid of a can.

Location of parts on the board for a microcircuit in a DIP package

The location of parts on the board for the microcircuit in the SMD package

Zotov A., Sergey V., Volgograd region

This metal detector scheme can be discussed at our

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Multiple metal detector options from the set

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Metal detector kit

(in set all necessary parts and circuit board)

When developing this metal detector, the task was to create a small-sized, highly economical device with good repeatability and high performance, using widely used and inexpensive parts.

An analysis of most common circuits showed that they are all powered by a source with a voltage of at least 9 V (that is, "Krona"), and this is both expensive and uneconomical. So, the metal detector assembled on the K561LE5 chip works from one battery for no more than 6-8 hours.

Search coils for most devices are either tapped or have multiple windings. The sensitivity of simple metal detectors is low, while more complex ones require the use of quartz resonators or other scarce parts. As a result, A. Melnikov developed a circuit (Fig. 2.44, a) of a beating metal detector, as the easiest to set up and use.

Rice. 2.44. Metal detector with low-voltage power supply a - circuit diagram; b - a circuit diagram assembled entirely on silicon transistors of the KT3156 type; c - an example of hanging installation

circuit diagram

The functionally developed scheme includes:

two generators (the coil of the oscillatory circuit of one of them is the search one);
balanced mixer;
audio frequency amplifier loaded on headphones.

For such low-voltage devices, barrier oscillators are most suitable, which operate stably from a supply voltage of 0.8 V or more (for silicon transistors).

Another advantage is that at their output the constant component of the voltage (relative to the collector connected to the device case) is 0.65 V and is stabilized (the emitter-base junction of the transistor plays the role of a stabistor). This effect is used to stabilize the operating point of a balanced mixer.

The sound amplifier is single-transistor. For such a sound amplifier, it is desirable to use a transistor with a current transfer coefficient of at least 200.

Generators operate at different frequencies:

search - approximately at a frequency of 100 kHz;
reference - at a frequency of 200 or 300 kHz.

Thus, the balanced mixer extracts the beats between the frequency of the reference oscillator and the 2nd or 3rd harmonic of the search. This solution makes it possible to drastically reduce the phenomenon of "capture" of the frequency of the search generator, which in simple circuits does not allow setting the beat frequency below 200 Hz, and also increases the sensitivity - changing the frequency of the search generator by 10 Hz changes the beat frequency by 20 (or 30) Hz.

Of course, you can increase the frequency of the reference oscillator even more, but in this case, the beat level becomes very small, which means that:

the sound volume is reduced;
instability increases, making it difficult to work.

It should be noted that the temperature stability of the circuit is not high, but in practice this does not greatly affect the results:

firstly, the same loop capacitors are used in oscillatory circuits, therefore their frequency changes in the same way, in the same direction, and as a result, the beat frequency does not change;
secondly, some simple design measures can increase the thermal stability of the metal detector.

Namely, the following requirements are relevant:

the search coil must be rigid;
correct shielding must be applied;
the board and case must be rigidly fixed to the rod.

The wire from the coil to the circuit must be shielded, it is advisable to use a thin television cable. The bar itself should be made of dry wood or fiberglass. It is advisable to fill the board in the case with paraffin. This will not only protect against moisture, but also from rapid temperature changes.

Search coil manufacturing

The search coil is made of a twisted pair cable, which is used for local networks. The cable must be shielded, category 5, preferably for outdoor installation (it has thicker insulation and the coil is tougher).

Four turns of cable should be laid in a ring with an outer diameter of approximately 25 cm, and:

lay first two outer turns on top of each other;
then wrap in four places with electrical tape;
then wind two turns inside.

All this then needs to be cut in the middle and wrapped with electrical tape. For such a winding, it is better to use cloth tape. From both ends of the cut, strip the insulation by about one and a half centimeters, irradiate the ends of the wires. The shielding foil must be cut off.

On the one hand, bite the wire that goes along with the foil, and on the other, connect it to one of the wires of the cable. This wire will be the output of the beginning of the winding. It should be noted that the screen must in no case form a short-circuited coil!

Further, the cable conclusions must be connected in accordance with-series, it is almost impossible to make a mistake, because all eight wires are of different colors. The result should be a coil of 32 turns with decent moisture resistance and rigidity.

Making another version of the coil

Another version of the coil is wound with a winding wire with a thickness of at least 0.3 mm. You can drive a few nails into the board at a distance of 40 cm and wind the wire (34 turns) around them, then carefully remove the coil and wrap it with electrical tape. Then the coil must be shielded. It is best to wrap it with foil taken from an old electrolytic capacitor.

It must be borne in mind that there is an alkaline electrolyte inside electrolytic capacitors, so it is advisable to unwind the foil from the capacitor under running water so that the electrolyte does not corrode your fingers. The foil must be wound so that it does not form a short-circuited coil; there should be a gap of about 1 cm between the beginning and end of the winding.

It is useless to try to solder the wire to the foil - it is aluminum and does not tin, so you need to wind several turns of bare tinned wire over the foil - this will be the screen output. It can already be connected to one of the ends of the coil. Next, connect this end to the braid of the shielded wire coming from the coil to the board, and on the board, with a common wire.

The second end of the coil must be connected to the central core wire and on the board with the base of the first transistor of the search generator. Rewind the tape over the foil.

Features of the application of various coils

The performance characteristics of the metal detector depend on the size of the coil. A coil with a diameter of 35 cm confidently catches a track from a tractor caterpillar at a depth of 80 cm, but does not detect coins, rings, nails and other trifles. This option is perfect for searching for ferrous metal, when massive pieces of iron (scrap metal) are of interest.

To search for rings, coins on the beach, you need a coil with a diameter of 15 cm. A small coil with a diameter of 15 cm consists of 6 turns of cable or 50 turns of wire. The depth of detection of coins is about 15 cm. The coil with a diameter of about 25 cm is a compromise solution, it has 40-45 turns.

Element base

Metal detector parts are the most affordable. Resistors and capacitors of almost any type, transistors in generators can be used KT315 (better with the letters B, G, E, some copies with the letters A and C refused to work - the current transfer coefficient is low). KT3102, KT368 work great.

Balanced mixer transistors must be germanium. Any transistor receiver from the 70s and 80s will provide you with plenty of them. Suitable P416 with any letter, P422, P423, P401, GT309, GT322, GT313. The selectors of SKM-24 TVs have GT346A transistors.

Since the operating frequencies of the circuit are not very high, even P27, P28, MP39B, MP42B, which were used in playback amplifiers for reel-to-reel tape recorders, will do. In amplifier 34, it is desirable to use a transistor with the highest current transfer ratio available.

The reference oscillator coil is wound on standard IF circuit fittings from Chinese radio tape recorders and receivers. Some coils have a built-in capacitor that needs to be removed. The coil is carefully unwound, and if it has more than 85 turns, then it is carefully wound with the same wire. If there are fewer turns, then 85 turns are wound with any winding wire. The wire must be thin enough, otherwise the required number of turns will not fit. In extreme cases, you can wind 75 turns.

The capacitance of the loop capacitors does not have to be exactly observed, it is only desirable to use capacitors of the same rating and type in both generators - for better thermal stability. The capacitance of 4700 pF can be from 3300 to 5100 pF, instead of 2200 pF, 1500 or 1800 pF can be used.

Mounting the device

The printed circuit board was not developed, it turned out to be more reasonable to abandon the printed wiring and assemble the device on a thin (0.5 mm) piece of textolite, connecting the parts to each other with their own conclusions. An example of such an assembly, which occupies less than half the size of a matchbox, is shown in Fig. 2.44, c.

We used transistors KT3102 and GT322 in metal cases.

Scheme of an alternative metal detector

When manufacturing several metal detectors, the problem suddenly arose with the search for old germanium transistors. And in case radio amateurs did not have them at hand, a circuit was developed, assembled entirely on silicon transistors, such as KT315B. Despite some decrease in sensitivity, the circuit showed good performance. The scheme is shown in fig. 2.44b.