Without a doubt, many novice radio amateurs will be interested in the design of a simple metal detector, the basis for which was a diagram that was repeatedly published in domestic and foreign specialized publications in the mid-70s of the last century. Using this metal detector, made with only two transistors, you can detect metal objects located several tens of centimeters away from the search coil.
Schematic diagram
This design is one of the variants of metal detectors of the FM (Frequency Meter) type, that is, it is a device based on the principle of measuring the frequency deviation of a reference oscillator under the influence of metal objects that fall within the range of the search coil. In this case, the frequency change is assessed by ear (Fig. 2.4).
Rice. 2.4. Schematic diagram of a simple metal detector using two transistors
The basis of the device circuit is a high-frequency generator and a receiver, which records changes in the frequency of the generator when approaching metal objects.
The high-frequency generator is assembled on transistor T1 according to a capacitive three-point circuit. The oscillatory circuit of the reference oscillator consists of a chain of series-connected capacitors C1, C2 and C3, to which coil L1 is connected. The operating frequency of the RF generator is determined by the inductance of this coil, which is also a search coil.
One of the features of this device is that it uses a heterodyne-type receiver as an analyzer, which is made with just one transistor. In this case, the cascade on transistor T2 combines the functions of a local oscillator and a detector. The heterodyne is assembled according to a three-point capacitive circuit. The advantage of this scheme is the possibility of using an inductor without taps, which, although slightly, simplifies the design. The local oscillating circuit contains an inductor L2 and a capacitance made up of series-connected capacitors C4, C5 and C6. The local oscillator frequency can be changed by rotating the tuning core of coil L2.
From the collector of transistor T2, the detected signal is supplied to headphones BF1.
If there is a metal object near coil L1, its inductance will change. This will lead to a change in the frequency of the reference oscillator, which will be immediately registered by the metal detector receiver. As a result, the signal tone in BF1 phones will change.
Details and design
All parts of a simple metal detector with two transistors, with the exception of the search coil L1, local oscillator coil L2, connector X1 and switch S1, are located on a printed circuit board measuring 70x40 mm (Fig. 2.5), made of single-sided foil getinax or textolite.
There are no special requirements for the parts used in this device. It is advisable to use any small-sized capacitors and resistors that can be placed on a printed circuit board without any problems. As can be seen from the circuit diagram, this metal detector uses outdated RF transistors such as P422, P401 or P402. Instead, you can use any modern pnp RF transistors designed for operation in the input stages of radio receivers.
The L1 search coil used in the reference generator is a rectangular frame with dimensions of 175x230 mm, on which 32 turns of PEV-2 wire with a diameter of 0.35 mm or, for example, PELSHO with a diameter of 0.37 mm are wound.
Two cylindrical paper frames contain pieces of ferrite rod type 400NN or 600NN with a diameter of 7 mm. The length of the first of them, permanently fixed, is about 20-22 mm. The second rod is movable and is used to adjust the inductance of the coil. Its length is 35-40 mm. The frames of the rods are wrapped in paper tape, onto which 55 turns of PELSHO wire with a diameter of 0.2 mm are wound. You can also use wire type PEV-1 or PEV-2.
Coil L2 (Fig. 2.6) should be installed at a distance of 5-7 mm from the plane of location of the turns of coil L1.
Headphones with a resistance of 800-1200 Ohms can be used as a source of sound signals. The well-known TON-1 or TON-2 phones are also suitable, but when using them, both capsules must be connected not in series, but in parallel, that is, connect the plus of one capsule to the plus of the other, and the minus to the minus. In this case, the total resistance of the phones should be approximately 1000 Ohms.
Rice. 2.5. Printed circuit board (a) and arrangement of elements (b) of a simple metal detector on two transistors
A simple metal detector with two transistors is powered from source B1 with a voltage of 4.5 V. As such a source, you can use, for example, a so-called square battery of type 3336L or three elements of type 316, 343 connected in series.
The printed circuit board with the elements located on it and the power supply are placed in any suitable plastic or wooden case. Switch S1 and connector X1 for connecting BF1 headphones are installed on the housing cover.
Coils L1 and L2 are connected to the board with a flexible stranded insulated wire.
Setting up
The metal detector should be set up in conditions where metal objects are removed from the L1 search coil at a distance of at least 1.5 m.
Rice. 2.6. L2 coil design
After turning on the power, you should check the voltages at the emitters of the transistors. There should be a voltage of -2.1 V at the emitter of transistor T1, and about -1 V at the emitter of transistor T2.
Next, by slowly moving the tuning core of the L2 coil, it is necessary to achieve the appearance of a loud, clean low-frequency signal in the phones. If the generator is initially configured, for example, at a frequency of 465 kHz, then phones will hear a signal with a frequency of about 500 Hz.
When coil L1 approaches a metal object, which can be used during the tuning process, for example, a tin can, the tone of the low-frequency signal in the headphones will change. The beginning of the signal tone change must be at least approximately recorded. After this, moving the core of the L2 coil to more accurately adjust the local oscillator frequency, the greatest sensitivity of the device should be achieved.
This completes the process of setting up a simple metal detector with two transistors.
Operating procedure
Carrying out search work using this device does not have any special features. If there is a metal object in the range of search coil L1, the pitch in the headphones will change. When approaching some metals, the signal frequency will increase, and when approaching others, it will decrease. By changing the tone of the beat signal, having some experience, you can easily determine what metal, non-ferrous or so-called black, the detected object is made of.
Metal detectors are quite useful devices; just remember the military specialty of a miner. And, of course, associations of seekers of ancient treasures with gold buried in the ground pop up. But in ordinary everyday life, such devices are also necessary, be it searching for pipes in the ground in various areas, cables, hatches and other industrial metal things. But what is closer to you readers is something like searching for hidden wiring in the wall or some ill-fated stud. We will look at such a simple and proven metal detector circuit for such purposes here, so that you can assemble it with your own hands and be happy, proud, and benefit.
Let's start with the types of metal detectors. Based on the principles of their work, they are divided into several types.
The most complex and sensitive, but also the most expensive, are built on the principle radio signal transmission/reception. The complexity and high cost lies not only in the abundance of electronic components of the circuit, but also in the need for qualified configuration of the circuits.
There are several more types based on different principles: induction, frequency meters, pulse, generation attenuation, beat method, pulse induction, resonance disruption... I tried to delve into various descriptions about them and, frankly, I got confused, since the descriptions, or rather, types into which metal detectors are divided differ. Yes, it hardly makes sense for us to know these subtleties of classifications, what the hell with them, with these classifications! The essence of all devices is generally the same: a change in the frequency of the generator when a metal object enters the field of a coil (or two coils, or one of two coils). This change in frequency is usually very insignificant, and the second essence of a particular circuit is to catch this slightest change and convert it into something. As a rule, it is converted into a sound signal from a generator, with a change in its frequency, to control the direction of the metal.
Here is a diagram of this simple metal detector, which can be repeated by an amateur without much experience. 
Sensitivity of this metal detector:
* Coin detection - 10-15 cm (with good adjustment, some grab it up to 50 cm!)
* Steel scissors - 20-25 cm
* Large objects - 1-1.5 meters
The circuit consists of two high-frequency generators, each with one transistor (VT1 and VT2). The frequency of the left generator (VT1) changes when metal enters the L1 field, and the frequency of the right one (VT2) remains unchanged. The values of the elements of both generators are selected so that the frequencies of the generators differ only slightly. The generators operate at a radio frequency (more than 100 kHz), and such a sound is neither audible to our ears nor reproduced by a speaker. But their small difference, for example, 160 kHz and 161 kHz is equal to 1 kHz - these are vibrations already audible to the ear. And both generator coils (L1, L2) are inductively coupled (located close), so both signals from the generators with a difference of 1 kHz are combined and we hear the so-called amplitude beats frequency 1 kHz.
Turning on the power, selecting resistor R2 on the emitter VT1 achieves a voltage of -2.1V relative to the total positive. Then the same is done with resistor R4 on the emitter VT2 to -1V. Next, by slowly moving the moving core of the L2 coil, the generator is adjusted so that a loud, clear low-frequency sound appears in the headphones.
Coil L1— rectangular frame 175x230 mm, 32 turns, PEV-2 0.35 (drawing below)
Coil L2- its design is shown in the figure below. In two paper cylinders (6) there are rods with a diameter of 7 mm made of ferrite 400NN or 600NN: one (1) 20-22mm long (fixed invariably), the other (2) - 35-40mm (for adjusting the frequency of the generator). Winding: 55 turns with a diameter of 0.2 mm.
This is roughly how frame L1 is made, with L2 placed inside it (as close to the edge of L1 as possible).
It should be added that the frame should be made as rigid as possible; after winding, the wire should be impregnated with varnish or epoxy resin. L2 is also rigidly attached inside it.
As you understand, this is the most labor-intensive and important part of the work; its implementation will require all your efforts and abilities. Your skillful hands should be fully demonstrated! The convenience of the work, its clarity and, as a consequence, the result and, in general, the pleasure you will receive in the process or engage in sexual intercourse with your device will depend on how you perform it.
For L2, you can try to find and use something like these from old radios. There are plastic frames of threaded coils and ferrites screwed into them, which have a recess for a screwdriver at their end.
Transistors: almost any p-n-p operating at the required frequencies (more than 100 kHz), selection with more high gain— the sensitivity of the metal detector will be higher. Even the ancient P401, P422 will do if you have old museum receivers a la “Spidola” lying around.
Foreign analogues: SFT316, SFT357, 2N1524, 2N1526, 2SA108, 2SA109, 2SA110, 2SA111, 2SA112, 2SA351, 2SA352, 2SA353, 2SA354, 2SA355, SFT316, SFT354, SFT357, 2 N1524, 2N1526, 2SA108, 2SA109, 2SA110, 2SA111, 2SA350 , 2SA351.
Transistors can be used with the same success n-p-n transition, you just need to change the polarity of the battery when using them.
Capacitors: C1, C2 and C5, C6, preferably all of the same type, so that the tuned frequency “runs away” less when the temperature changes. The rest doesn't matter.
Headphones- but it’s more difficult with them (indicated BF1 in the diagram): for this circuit you need high-resistance (type TON-1, TON-2, TA-4, TA-56, TG-1, etc.) winding resistance of about 1600 Ohms. They were written about in the article about. Any modern ones have about ten ohms, so the sound in them will be very quiet.
Therefore, if you do not find high-impedance headphones (which is more likely), then it makes sense to assemble a circuit with a cascade using a composite transistor KT503E-KT502E. In this scheme, you can already use modern headphones. It also adds a 150 Ohm variable resistor R10. By changing its resistance, you can change the current of the entire circuit, thus smoothly adjusting the frequency in the field, if necessary, rather than resorting to the inconvenient adjustment of the L2 coil.
The left part of the diagram, as you can see, is the same, an additional part has been added to the right. Replacement: KT503 with KT315 or KT342, and transistor KT502 with KT603, KT608, KT626.
Good luck with your treasure hunt! 🙂 But seriously, by making such a metal detector compact and taking it with you on a trip to the sea, it can really help you out if suddenly one of your close ladies drops a gold earring or pendant on the beach, which sometimes happens. Yes, and in the home arsenal of a household man there is a place for such a device.
Have you seen my electromagnetic pendulum yet?
This metal detector is capable of detecting: large metal objects (iron bucket, manhole cover, water pipe) at a depth of up to one meter, as well as small objects (coins or screws) at a depth of 15-20 cm.
The device is built on the basis of the most common parts that are available in the supplies of any radio amateur. The metal detector is made according to the well-known and widely used principle in such devices of beats between the frequencies of two high-frequency generators. 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 its action zone.
The schematic diagram of the metal detector is shown in Fig. 1. The reference oscillator is made on transistor VT1. Its oscillation frequency is determined by the parameters of the L1C3 circuit and is about 1 MHz.
The search generator is made on transistor VT2; it also produces 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 have practically no significant effect on its inductance.
The coil of the search generator circuit is wound on a larger frame in the form of a frame. It has no core. As a result, its inductance changes greatly as it approaches a metal object, which in this case begins to act as a moving core.
Signals from both generators are sent to a diode mixer using diode VD1. As a result, the product of subtraction of generator frequencies is obtained on capacitor C12.
The closer the values of these frequencies, the lower the audio tone on this capacitor, and the more different the frequencies of the generators are, the higher the sound tone in speaker B1, to which the signal is received (the product of the diode mixer).
The signal enters through a low-frequency amplifier using transistors VTZ-VT6.
Using a variable capacitor C7, the search generator can be configured in such a way that, in the absence of metal objects nearby, the sound tone in the speaker is the lowest.
Then, as coil L2 approaches the metal, the frequency of the generator at VT2 begins to change. The frequency difference between the generators increases, and therefore the tone in the dynamics will rise. When the metal is precisely located, the sound will turn into a piercing squeak.
Coil L1 should be wound on a ferrite rod with a diameter of 8 mm, for example, from a magnetic antenna of a radio receiver. The length of the rod is reduced to 30 mm.
First, you need to put a frame on the rod - a sleeve glued together 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 is a search coil. It must be wound on a frame, which is a frame measuring 120 x 220 mm, made of plexiglass, plastic or wood.
Winding should be done with 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 (attached to an aluminum tube or wooden strip).
The device itself (the main unit containing a generator on VT1 and an ultrasonic sounder 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:
The design may be different, it all depends on capabilities and desires.
Capacitor C7 can have a minimum capacitance of no more than 10 pF, and a maximum of no less than 150 pF.
Transistors KT315 can be replaced with KT3102 or KT312, KT316. Transistors MP35 can be replaced with MP35-MP38, and transistor MP39 with 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. The battery is 9 V, you can use a “Krona” or a suitable battery.
Attention:Power supply from a 220 V mains source is not advisable, because this creates an alternating current background and reduces the sensitivity of the device as a whole.
The setting consists of adjusting coil L1 in such a way that when the rotor of capacitor C7 is 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 starting the search will be made by capacitor C7.
If there are no oscillations from the generator at VT1, you need to select the value of C4 and/or adjust the operating mode of the cascade by selecting the value of R2. If the generator at VT2 is not excited, you need to adjust C8 and adjust the operating 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.
The article presents a circuit diagram of a simple but powerful 1.5 volt metal detector, very easy to replicate. The generators are assembled according to a circuit that has a number of useful properties, one of which is the stability of the output voltage (both direct and alternating) when the supply voltage changes. The oscillatory circuit of the search generator on transistor VT1 includes coil L1. It operates at a frequency of about 100 kHz, which is optimal for this type of metal detector. Its frequency can be changed within small limits using a variable capacitor C2. The second generator (on transistor VT2) is exemplary and operates at a frequency of about 300 kHz.
The generator signals through resistors R2, R4 are fed to a balanced mixer, where the difference in frequencies (beats) of the third harmonic of the search generator signal and the first harmonic of the reference generator is separated. This is done to increase sensitivity - when the frequency of the search generator changes at a frequency of 10 Hz, the beat frequency changes by 30 Hz, which is more noticeable to the ear.
The signal from the output of the mixer through capacitor C8 is supplied to the input of the ultrasonic sounder and, after amplification, to the headphones BF1, BF2. Capacitor C7 suppresses signals with generator frequencies.
When the search generator coil approaches a metal object, the generation frequency changes, therefore the tone of the signal in the headphones will also change. By the nature of the change in tone, one can judge the material from which this item is made.
Most of the parts are mounted on a printed circuit board made of single-sided foil fiberglass.
You can use transistors of the KT312, KT315, KT3102 series with any letter indices. In a balanced mixer, you can use only germanium transistors of the GT309, GT313, GT322, GT346 series or earlier - P416, P422, P423 with any letter indices. In an UZMCH, the transistor should have the highest possible current transfer coefficient, for example, KT3102BM - KT3102EM, KT342BM, KT342VM - the volume of the sound signal depends on this. Power switch - any small one. Headphones are suitable with a resistance from 8 to 32 Ohms, they are connected in series. To connect them, you can install a socket on the body of the metal detector. The device is powered by a galvanic cell or AA or AAA battery, the maximum current consumption is about 12 mA.
To wind the L2 coil, a frame from the IF circuit (455 kHz) of a foreign-made receiver is used. It consists of a ferrite “dumbbell” (on which 66 turns of PEV-2 wire with a diameter of 0.06...0.1 mm are wound) and a ferrite cup covering it, the movement of which regulates the inductance of the coil. The frame is enclosed in a metal screen.
The sensitivity of the device to metal objects of different sizes depends on the size of the search coil itself. For searching for large objects (a metal sheet measuring 80x80 cm, a manhole cover for a sewer well), a coil with a diameter of about 30 cm is more suitable. With it, a maximum detection depth of such objects is achieved up to 60 cm.
For searching for small objects, a coil with a diameter of about 120 mm is better suited. Such a coil contains 56 turns of PEL wire with a diameter of 0.2...0.5 mm.
It is more technologically feasible to make a coil of an even larger diameter (for example, 300 mm) from a multicore shielded twisted pair cable, which is used for laying computer local networks. The cable must contain four such “pairs”, and the coil must contain four turns of such cable. First, wind two outer turns and fasten them in four places with insulating tape. Then two inner ones are wound and they are also wrapped with insulating tape, preferably on a fabric basis. The ends of the cable are cut so that there is an “overlap” of 5 mm...10 mm, and the outer insulation is removed from them by 15 mm, and the ends of the wires are stripped by 5 mm and tinned.
All radio components of the device are domestic and have foreign analogues:
L1 - coil
R1 - 1 kOhm
R2 - 10 kOhm
R3 - 1 kOhm
R4 - 10 kOhm
R5 - 1 kOhm
R6 - 1 kOhm
R7 - 100 kOhm
C1 - 2200
C2 - 10...240
C3 - 4700
C4 - 0.047 µF
C5 - 2200
C6 - 4700
C7 - 0.047 µF
C8 - 2.2 uF x 16 volts
VT1 - KT315B
VT2 - KT315B
VT3 - GT322B
VT4 - GT322B
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 yourself can save quite a bit of money.
The circuits of modern metal detectors can be built according to different operating principles; we list the most popular of them:
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 and gain first skills in searching for treasures and products made of multi-colored metals.
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