We make a metal detector for gold with our own hands: diagrams and step-by-step instructions. A simple transistor metal detector Scheme of a sensitive metal detector on field-effect transistors

This metal detector is capable of detecting: large metal objects (an iron bucket, a manhole cover, a water pipe) at a depth of up to one meter, as well as small objects (coins or screws) at a depth of up to 15-20 cm.

The device is built on the basis of the most common parts that are available in the stocks of any radio amateur. The metal detector is made according to the principle of beats between the frequencies of two high-frequency generators, which is well-known and widely used in such devices. The frequency of one of them (reference) is constant, and the frequency of the second (search) changes under the influence of external metal objects that change the inductance of its coil when it enters the zone of its action.

circuit diagram

The schematic diagram of the metal detector is shown in fig. 1. The reference oscillator is made on the transistor VT1. The frequency of its oscillations is determined by the parameters of the L1C3 circuit and is about 1 MHz.

The search generator is made on the transistor VT2, it also generates a signal of approximately the same frequency. The difference is that the reference oscillator circuit uses a small coil with a ferrite core.

Fig 1. Schematic diagram of a simple homemade metal detector.

Therefore, external metal objects practically do not have a significant effect on its inductance.

The search generator circuit coil is wound on a larger frame in the form of a frame. She has no core. As a result, its inductance changes greatly when it approaches a metal object, which in this case begins to act as a moving core.

The signals from both generators are fed to the diode mixer on the diode VD1. As a result, the product of subtracting the frequencies of the generators is obtained on the capacitor C12.

The closer the values of these frequencies, the lower the sound tone on this capacitor, and the more the frequencies of the generators differ, the higher the tone of the sound in the speaker B1, which receives the signal (the product of the diode mixer).

The signal comes through a low-frequency amplifier based on VTZ-VT6 transistors.

With the help of a variable capacitor C7, the search generator can be adjusted in such a way that, in the absence of metal objects nearby, the sound tone in the speaker is the lowest.

Then, when the coil L2 approaches the metal, the frequency of the generator on VT2 begins to change. The frequency difference of the generators increases, and therefore, the tone in the dynamics will rise. With the exact location of the metal, the sound will turn into a shrill squeak.

Details and design

Coil L1 should be wound on a ferrite rod with a diameter of 8 mm, for example, from a radio magnetic antenna. The rod length is reduced to 30 mm.

First, a frame must be put on the rod - a sleeve glued from whatman paper, which moves along it with some friction.

Coil L1 should contain 110 turns of PEV wire with a diameter of 0.2-0.3 mm. The tap must be made from the 16th turn, counting from the VT1 collector.

Coil L2 - search. It must be wound on a frame, which is a frame measuring 120 x 220 mm, made of plexiglass, plastic or wood.

Winding must be carried out with a PEV wire with a diameter of 0.4 x 0.6 mm. The coil should contain 45 turns with a tap from the 10th, counting from the VT2 collector.

The coil must be connected to the main unit with a three-core shielded wire. The coil should be located at a distance of about 1 meter from the main unit (fixed on an aluminum tube or wooden rail).

The device itself (the main unit containing a generator on VT1 and an ultrasonic frequency converter with a speaker and a battery) can be mounted in a housing from a radio receiver. From the same receiver it is advisable to use:

speaker;
variable capacitor;
coil rod L1.

The design may be different, it all depends on the possibilities and desires.

Capacitor C7 can be with a minimum capacitance of not more than 10 pF, and a maximum capacitance of at least 150 pF.

Transistors KT315 can be replaced with KT3102 or KT312, KT316. Transistors MP35 can be replaced by MP35-MP38, and transistor MP39 by MP39-MP42.

Diodes D9 - with any letter, or D2, D18, GD507. Speaker - any resistance from 4 ohms to 100 ohms, for example, a speaker from a radio receiver or headphones. 9 V battery, you can use the "Krona" or a suitable battery.

Attention:Power supply from a mains source of 220 V is not desirable, because in this case an alternating current background appears and the sensitivity of the device as a whole decreases.

Setting

The setting consists in adjusting the L1 coil in such a way that, with the rotor of the capacitor C7 in the middle position and in the absence of external metal objects, the sound of the lowest tone is heard in the speaker.

In the future, during operation, adjustment before the start of the search will be performed by capacitor C7.

In the absence of oscillations from the generator on VT1, you need to select the value of C4 or (and) adjust the operation mode of the cascade by selecting the value of R2. If the generator on VT2 is not excited, you need to adjust C8 and adjust the operation mode of the transistor by selecting the value of R6.

The device is highly sensitive, and working with it requires certain skills. So you need to practice.

When working, it is important to take into account that when approaching ferrous metals (iron, steel, cast iron), the frequency of the generator on VT2 decreases, and when approaching non-ferrous metals, it increases.

Despite its extreme simplicity, the circuit works well and provides performance that is no worse than more complex circuits (detects a large coin at a distance of 10 cm). The circuit uses elementary non-deficient parts, it can be assembled using parts from an old radio.

However, assembly and adjustment have their own characteristics, which must be taken into account, and when using this metal detector, there is an inconvenience in adjusting the operating frequency of the generator (it is not very convenient to select the position of the tuning rod by fractions of a millimeter in a forest or field), besides, the published scheme has such a drawback, like using high impedance headphones, which are hard to find these days. On ordinary headphones for audio equipment, the sound is almost inaudible. I propose a description of the circuit, modified taking into account these shortcomings, and a simple method for manufacturing the metal detector itself.

The circuit is supplemented with an amplifier (it is highlighted with a dotted line) assembled on a composite transistor (T3 + T4). This allows you to use conventional headphones with a resistance of 30 ... 60 Ohms and at the same time provide an economical mode of operation. P416B transistors can be replaced with P416, P401, P402, P422 (these were used in old transistor radios), KT503 with KT315 or KT342, and KT502 with KT603, KT608, KT626. Printed circuit boards and transistor pinouts are shown in Figure 2. Generators are assembled on the first board, the board is small and placed in a small plastic box, which is then glued to the frame coil L1 (its design will be discussed later). The second board contains an additional amplifier with a headphone jack and contact pads for three AA batteries. Pads (petals) can be cut out of tin from a can, give them the desired shape and solder to the pads of the printed circuit board. In addition, on this board there is a variable resistor with a resistance of 100 ... 150 Ohm, with which you can change the supply current of the generators within a small range and thus fine-tune the frequency, which is very convenient in the "field" conditions. This board is also placed in a small box (metal can also be used here) and mounted on the metal detector rod next to the handle. The boards will be connected using a conventional unshielded three-core cable and appropriate suitable connectors (you can use SG-3, SG-5 or any others). In the diagram shown here, the headphone jack also serves as a power switch (shown in Figure 2.). For this, a mono-plug is used, the “long” contact of which closes the power supply circuit of the circuit. But you can, of course, put a simple switch, if the dimensions of the case allow. Then the "minus" of the battery power must be supplied through this switch to the "minus" connector of the cable and the headphone jack.

This prototyping board was made for general circuit setup, but it can also be used as a working board by placing it in an appropriate case. But I want to offer a simpler and more aesthetic option for making a frame, and besides, it does not require a case. To do this, you need a piece of plastic cable channel (box), which is used for laying electrical wiring. It is available at any electrical supply store. The minimum cross-sectional size is needed - 7 x 12, 10 x 15 mm. The cover is removed from the cable channel and the walls are cut with a sharp thin knife at distances equal to the sides of the frame (175 x 230 mm). Then the channel is bent at the sites of these incisions and its ends are glued together. All this is shown in the figure below:

32 turns of PEV (PEL) wire 0.3 ... 0.35 are wound onto the resulting rectangular frame. Then the frame is closed with a lid. You can fill the turns in the channel with epoxy, this will give the frame more rigidity. This will be coil L1. Coil L2 contains two pieces of a ferrite rod with a diameter of 8 mm - one 20 ... 25 mm long, the other 35 ... 40 mm. The rod can also be taken from an old radio receiver (it is used there as an antenna for the MW and LW bands). The necessary pieces of the rod can be broken off by clamping it in a vise to the desired length (clamp carefully, through cardboard or rubber gaskets, as it is very fragile!). Ferrite segments are inserted into cardboard “sleeves”, you can use a case from a felt-tip pen of a suitable diameter. Both "sleeves" are glued together and wrapped with several layers of paper tape, also impregnated with glue, to give rigidity to the structure. Then a turn to turn of 55 turns of PELSHO 0.2 wire is wound from above (I tried PEL 0.2 ... 0.3, no worse) and fixed with glue or electrical tape. The design of the coil is shown in Fig.3. A short piece of ferrite is fixed permanently, and a long piece remains movable to adjust the frequency of the generator. Next, the printed circuit board with the generators and the L2 coil is placed in a suitable plastic case (not metal!) And glued to one of the short sides of the frame coil from the inside so that the distance between the coils L1 and L2 is no more than 5 ... 8 mm. Between the coils, electromagnetic interconnection must be ensured. Power to the generators and the audio signal output are provided through a connector and a piece of connecting cable about 1 m long.

Batteries and the amplifier are located in a separate case near the handle of the device. The design is shown in the photo. The dimensions of the case depend mainly on the type of batteries used (any round - large or small). You can use one earphone, since you are unlikely to achieve a special stereo effect J. But it is better to use two, and turn them on in parallel. This way you can still increase the volume of the sound, although the current consumption from the batteries will increase slightly. In general, with normal headphones of average quality, the volume is quite sufficient.

Establishment

First check the operating modes of transistors T1 and T2. On the base of T1 there should be -2.1 V, on the base of T2 -1V (in relation to the positive wire of the circuit). If necessary, these voltages can be adjusted by selecting resistors R2 and R4, respectively. A deviation of 10 15% from the specified voltages is allowed. Then we put the R10 engine in the middle position and, by moving the movable core L2, we achieve low-frequency sound in the headphones. The lower the frequency of the sound, the more sensitive the device will be. In this position, we fix the core with glue or paraffin. In the future, the frequency of the generator can be adjusted within a small range by resistor R10. The bar (handle) of the device must be wooden or plastic ...

You can download printed circuit boards in LAY format below

List of radio elements

Designation	Type	Denomination	Quantity	Note	My notepad
T1, T2	bipolar transistor	P416B	2	P416, P401, P402, P422	To notepad
T3	bipolar transistor	KT503E	1	KT315, KT342	To notepad
T4	bipolar transistor	KT502E	1	KT603, KT608, KT626	To notepad
C1, C5	Capacitor	1000 pF	2		To notepad
C2, C6	Capacitor	3300 pF	2		To notepad
C3	Capacitor	300 pF	1		To notepad
C4	Capacitor	100 pF	1		To notepad
C7, C8	Capacitor	0.01uF	2		To notepad
C9	Capacitor	0.33uF	1		To notepad
R1, R6, R7	Resistor	1 kOhm	3		To notepad
R2, R3, R5	Resistor	4.7 kOhm	3		To notepad
R4	Resistor

An incredibly simple metal detector circuit, which even a novice radio amateur can master. The sensitivity of such a device is comparable to mass-produced models. All this is achieved through the use of a conventional medium-wave radio receiver.

The design and operation of a home-made metal detector is as follows: a high-frequency generator is assembled on a single transistor according to the “three-point” scheme. The portable radio is tuned to one of the oscillator's harmonics, and responds to any changes in the local oscillator generation. The radio receiver has good selectivity due to which the high sensitivity of the entire metal detector is achieved.

It will take

Transistor n-p-n structure, any will do, such as 2N2222, BC640, etc.
Capacitors: 1 nF - 2 pieces, 100 nF, 47 uF.
Resistors: 470 kΩ, 4.7 kΩ.
An old but working AM or AM receiver.
Wire 0.2-0.5 mm.

Scheme

The generator is built according to the classical scheme, powered by a voltage of 9 V. The oscillation frequency depends on the resonance in the oscillatory circuit, in the role of which the search coil is used.

Coil making

We take an enameled copper wire from the primary winding of any 220 V transformer.

The coil consists of 16 turns wound on a diameter of 12 centimeters. Before winding, we put nylon ties in order to fix the coil after winding.

We wound, we tighten the ties. We tin the ends of the wires.

Assembling a metal detector

So that the coil does not deform at the time of the search, we attach it to a piece of hard plastic with several additional ties.

We assemble a diagram on any piece of textolite.

We cut out a handle for a metal detector from soft plastic (dense foam).

Glue everything with super glue.

We solder the coil to the board, the batteries with the switch. We check the operation of the generator using a radio receiver.

We attach the receiver itself to the handles, for this we cut out the size of the box for it and glue it with super glue.

Glue the box onto the handle.

This completes the assembly.

Tuning and testing

The sensitivity of the device, as mentioned earlier, is quite comparable with inexpensive industrial models of metal detectors. But for this you need to make a setting.
If you are using a manually tuned receiver, turn it on to the medium wave range and search for harmonics, periodically checking for a change in tone as metal objects are placed in the coil.

If you have a receiver with automatic search, then everything is simpler - just search by clicking the auto search button.
Once the setup is completed, the metal detector will respond to any metal.

He is even able to find a coin at a distance of 1-2 centimeters. Naturally, the larger the object, the greater the distance at which you can find it.

A metal detector is an electronic device for searching and distinguishing metals, metal objects that can be hidden at different depths under a layer of sand, earth, in the walls of rooms and various structures.

Schematic diagrams of metal detectors made on transistors, microcircuits and microcontrollers are given. A factory-made metal detector is a fairly expensive device, so making a homemade metal detector on your own can save a lot of money.

Schemes of modern metal detectors can be built according to different principles of operation, we list the most popular of them:

Beat method (measuring the change in the reference frequency);
Induction balance at low frequencies;
Induction balance on spaced coils;
impulse method.

Many novice radio amateurs and treasure hunters are wondering: how to make a metal detector yourself? It is advisable to start your acquaintance with assembling a simple metal detector circuit, this will allow you to understand the operation of such a device, get the first skills in searching for treasures and products made of multi-colored metals.

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The metal detector is designed to detect a metal object (well cover, pipe section, hidden wiring). The metal detector consists of a parallel voltage regulator (transistors V1 V2) at a high (about 100 kHz) frequency generator on a transistor V4, an RF oscillation detector (V5) and ...

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The metal detector allows you to detect any metal object at a distance of up to 20 cm. The detection range depends only on the area of the metal object. For those for whom this distance is not enough, for example, treasure hunters, we can recommend increasing the size of the frame. This should also increase the depth of detection. The schematic diagram of the metal detector is shown in the figure. The circuit is assembled on transistors operating in the mode ...

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The scheme of a homemade metal detector on beats, which is built on five microcircuits. Finds a coin 0.25mm at a depth of 5cm, a pistol at a depth of 10cm, a metal helmet at 20cm. A schematic diagram of a beat detector is shown below. The circuit consists of the following units: a crystal oscillator, a measuring oscillator, a synchronous detector, a Schmidt trigger, an indication device...

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The circuit shown in the figure is a classic metal detector. The operation of the circuit is based on the principle of superheterodyne frequency conversion, which is usually used in a superheterodyne receiver. A schematic diagram of a metal detector with an integrated ULF, it uses two radio frequency generators, the frequencies of which are 5.5 MHz. The first radio frequency generator is assembled on a BF494 type T1 transistor, frequency ...

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This metal detector, despite the small number of parts and ease of manufacture, has a fairly high sensitivity. It can detect large metal objects, such as a heating battery, at a distance of up to 60 cm, while small ones, for example, a coin with a diameter of 25 mm, at a distance of 15 cm. The principle of operation of the device is based on a change in frequency in the measuring generator under the influence of nearby metals and. ..

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A simple compact metal detector is needed to detect various metal objects in the walls under a layer of plaster (for example, pipes, wires, nails, fittings). This device is completely autonomous, powered by a 9 volt battery type "Krona", consuming 4-5 mA from it. The metal detector has sufficient sensitivity to detect: pipes at a distance of 10-15 cm; wiring and nails at a distance of 5-10...

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The scheme of a small-sized, highly economical metal detector with good repeatability and high performance, using widely available 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, assembled on a K561LE5 chip ...

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If you have a long-wave transistor receiver in good condition, you can easily assemble a simple attachment to it - a metal detector. The metal detector circuit is a conventional LC oscillator, at a frequency of about 140 kHz. The coil of the oscillating circuit L1 is 12 cm in diameter, contains 16 turns of wire (any insulated mounting or varnished winding will do, with a diameter of 0.25 - 0.5 mm). The turns are laid on a platform of plywood of a suitable size and fixed, for example, with glue - "cold welding" or "liquid nails".

Resistors and capacitor - any type, low-power high-frequency transistor, reverse conduction.
Suitable - KT315, KT3102 with any letter. The circuit is assembled on a board made of getinax or textolite, printed wiring is not necessary, the parts can be connected with any insulated mounting wire.

After assembly, the circuit, together with the power source, is located next to the coil on a plywood platform, with a wooden handle of a convenient length. The receiver is attached to the handle and tuned to a reception frequency close to 140 kHz, until a sound resembling a squeak appears. When the coil approaches a metal object, its tone will change.

Despite the simplicity of the scheme, in terms of its sensitivity, such a metal detector is practically not inferior to industrial designs.
With it, metal objects such as a gold ring or a coin can be detected at a depth of up to 20 cm.