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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Mine detector. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur There are many designs of mine detectors or, as they are often called in the literature, metal detectors, but we will get acquainted with only three, differing in circuit solutions and capabilities. The first design is a mine detector with two transistors (Fig. 69). It was developed by the Moscow radio amateur V. Vasiliev. The principle of operation of a mine detector, like most similar designs, is that when a metal object approaches the inductor of the generator, the frequency of the generator changes. The closer the object and the larger it is, the stronger its influence on the frequency of the generator.
The mine detector generator is made on the V1 transistor according to the capacitive three-point circuit. Generation is formed due to positive feedback between the emitter and base circuits of the transistor. The frequency of the generator depends on the capacitance of the capacitors C1-C3 and the inductance of the coil L1. When the coil approaches a metal object, its inductance changes - it increases if the metal is ferromagnetic, for example iron, and decreases if the metal is non-ferrous - copper, brass. But how to track the change in frequency? For this, a receiver assembled on the second transistor is used. This is also a generator, assembled, like the first one, according to the capacitive three-point scheme. Its frequency depends on the capacitance of the capacitors C4-C6 and the inductance of the coil L2 and does not differ much from the frequency of the first generator. The required frequency difference is selected with a coil trimmer. In addition, the cascade on the V2 transistor also combines the function of a detector that selects low-frequency oscillations from high-frequency oscillations entering the base of the transistor. The load of the detector are headphones B1; capacitor C8 shunts the load for high frequency oscillations. The oscillatory circuit of the receiver is inductively connected to the circuit of the generator, therefore, in the collector circuit of the transistor V2, currents with the frequency of both generators flow, as well as a current of the difference frequency, in other words, the beat frequency. If, for example, the frequency of the main generator is 460 kHz, and the frequency of the receiver generator is 459 kHz, then the difference will be 1 kHz, i.e. 1000 Hz. This signal is heard in phones. But as soon as the L1 search coil is brought closer to the metal, the sound frequency in the phones will change - depending on the type of metal, it will either decrease or become higher. This will serve as a signal for the detection of "mines". Instead of those indicated in the diagram, P401, P402 and other high-frequency transistors are suitable. Headphones are high-resistance, TON-1 or TON-2, but their capsules must be connected in parallel so that the total resistance is 800 ... 1200 Ohms. The sound volume in this case will be slightly higher. Resistors - MLT-0,25, capacitors - KLS-1 or BM-2. Coil L1 is a rectangular frame measuring 175x230 mm, consisting of 32 turns of PEV-2 0,35 wire (PELSHO 0,37 wire is suitable). The design of the L2 coil is shown in fig. 70. In two paper cylindrical frames 6, sections of a rod with a diameter of 7 mm made of ferrite 400NN or 600NN are placed: one (1) 20 ... 22 mm long, permanently fixed, the other (2) - 35 ... 40 mm (movable - for coil adjustment). The frames are wrapped with paper tape 3, on top of which a coil L2 (5) is wound - 55 turns of PELSHO wire (you can PEV-1 or PEV-2) with a diameter of 0,2 mm. The coil leads are fixed with rubber rings 4.
Power source - 3336L battery, switch S1 - toggle switch, connector X1 - two-slot block. Transistors, capacitors and generator resistors are mounted on a board (Fig. 71) made of insulating material. The board is connected to the coils, the battery, the switch and the connector with a stranded wire in isolation. The board and other details of the mine detector are placed in a plywood glued case with dimensions of 40x200x350 mm. Coil L1 is attached to the bottom of the case, and coil L5 is placed inside the coil at a distance of 7 ... 2 mm from its turns. A board is attached next to this coil. The connector and switch are attached to the outside of the side stack of the case. From above, a wooden handle about a meter long is attached to the case (preferably with glue).
Establishing a mine detector begins with measuring the operating modes of transistors. Turning on the power, measure the voltage at the emitter of the first transistor (relative to the common wire - plus power) - it should be 2,1 V. More precisely, this voltage can be selected by resistor R2. Then the voltage at the emitter of the second transistor is measured - it should be 1 V (it is more precisely set by selecting the resistor R4). After that, by slowly moving the tuning core of the L2 coil, a loud, clear low-frequency sound is achieved in the headphones. Bringing a tin can closer to the search coil, the beginning of the change in the tone of the sound is fixed. As a rule, this happens at a distance of 30 ... 40 cm. By more precise adjustment of the frequency of the second generator, the highest sensitivity of the device is achieved. The next design is a three-transistor mine detector (Fig. 72). It is capable of detecting tin cans or iron sheets with an area of at least 150 cm2 at a depth of up to 30 cm.
Let's analyze the work of a mine detector according to its concept. A generator is assembled on the transistor V1, which generates oscillations with a frequency of 80 ... 100 kHz. Generation is formed due to feedback between the collector coil L1 and the coil L2 connected to the base of the transistor. The oscillation frequency depends on the inductance of the coil L1 and the capacitance of the capacitor C2. According to the same scheme, the second generator was assembled on the transistor V2 with a frequency approximately equal to the first. The coupling coils (L3 and L4) of both generators are connected in series and connected to the output stage, assembled on the transistor V3. Headphones B1 are included in its collector circuit (via connector X1). The frequency of the first generator is constant (if there is no metal object near the search coil L1), the frequency of the second one is changed by adjusting the inductance of the coil L6. Alternating currents will flow through the headphones at the frequency of both generators and the beat frequency. If the frequency of the second generator is smoothly adjusted to the frequency of the first, a low-frequency sound will first be heard in the headphones, which will gradually decrease, and then "zero beats" will appear - the sound in the phones will disappear. Now it is worth bringing the coil of the first generator to a metal object - and the sound will be heard in the phones again. Its height will be the greater, the closer the coil is to the object, and also the larger the object itself. In this design, you can use transistors of the MP39-MP42 series with any letter index and a static current transfer coefficient of 30 ... 40. It is desirable to take mica capacitors (KSO-1 or KSO-2), resistors - MLT-0,25. Headphones - TON-1 or TON-2. The power source is a 3336L battery or three small-sized D-0,25 batteries connected in series. In the first version, the device will work 100 ... 150 hours, in the second - 40 ... 50 hours (and then the batteries will have to be charged). Connector XI - two-socket block, power switch - any design. Coils L4-L6 of the second generator are wound with wire PEV-1 0,2 on a frame made of insulating material, which is then placed in a carbonyl core SB-28a (SB-4a). First, a coil of L6-260 turns is wound on the frame with a tap from the 60th turn, counting from the top according to the output scheme. Next, a coil of L5-40 turns is wound, and lastly - L4 (2 turns). To make it more convenient to rotate the tuning core, a tuning knob is screwed onto it (Fig. 73).
For the coils of the first generator, a frame is first made (Fig. 74). It consists of a plywood disk 3 with a diameter of 445 and a thickness of 5...6 mm and cheeks 1 and 4 cut out of thin plywood. The cheeks are glued or nailed to the disk, and a wooden handle 5 is attached to the upper cheek XNUMX of such length that it is convenient to use the device when searching for "mines" near the ground.
Coils 2 are placed between the cheeks. First, a coil L1-55 of turns of PEV-1 0,6 wire is wound with a tap from the 15th turn, counting from the top according to the output scheme. A coil L2-10 turns of PEV-1 0,25 is wound on top of it. Coil L3 is wound last - it contains 2 turns of wire PEV-1 0,25. The top conclusions of the coils according to the scheme (these can be, for example, their beginnings - when winding, of course, all the coils in one direction) connect together and make a common conclusion with a flexible mounting wire in insulation 100 ... 120 mm long. Solder the conductors of the same length to the other terminals of the coils. Then solder all the conductors to the contacts of the bar installed on the upper cheek near the handle. Place capacitor C2 here as well. After that, cover the coils with several layers of varnish and wrap electrical tape over them between the cheeks. Place the rest of the parts in the case (Fig. 75), on the upper wall of which fix the power switch and the coil of the second generator, and on the side wall - a socket for the headphone plug. Attach the case to the handle in a place convenient for work and connect the leads of the coils of the first generator to the corresponding parts. Here it is better to use a homemade cable. To make it, take three multi-colored mounting wires and pass them inside a metal shield, such as a metal braid of a shielded wire. Put a PVC or rubber tube on top of the cable and attach the cable to the handle. Connect the metal braid to the common wire of the coils, and the multi-colored conductors to the remaining leads.
Establishing a mine detector comes down to determining the frequency of the first generator and adjusting the second one. The easiest way to do this is with any broadcast receiver with an antenna socket. First, turn off the second generator by unsoldering, for example, the emitter output of transistor V2 from the plus of the power source. With the headphones turned on, connect their bottom output (in other words, the transistor collector) through a 15 ... 20 pF capacitor to the receiver's antenna jack. After turning on the power of the mine detector, turn the radio tuning knob. At several points on the longwave range scale, you will hear a characteristic noise in the loudspeaker or you will see a narrowing of the sector of the tuning indicator (it is usually found in tube radios). The frequency difference between two adjacent points will correspond to the frequency of the generator. Similarly, the frequency of the second generator is checked by turning off the first one. With the middle position of the tuning core, it is necessary to set the frequency of the second generator equal to the frequency of the first by selecting the capacitor C5. Then both generators are turned on, "zero beats" are achieved by rotating the tuning core, and then the core is turned back a little so that a low tone sound is heard in the headphones. This setting corresponds to the maximum sensitivity of the device. Bring the searchcoil close to a metal object and the pitch will change. During the search, the mine detector must be carried at a close distance from the surface of the earth and rocked from side to side. Then, by the largest change in tone in the headphones, it is easy to determine the exact location of the "mine". And one more design - a mine detector with seven transistors (Fig. 76). It was developed by Moscow radio amateurs L. Bulgak and A. Stepanov. Such an abundance, in comparison with previous designs, of transistors made it possible to achieve a relatively high sensitivity, stability in operation and a clear distinction between ferrous and non-ferrous metals.
The operation of the mine detector is based on the principle of beating the frequencies of two generators, already known to you, one of which is a reference, and the other is tunable. The approach of the remote coil of the oscillatory circuit to the metal is accompanied by a change in its inductance, and hence the frequency of the generator. A ferrous metal object (ferromagnet) increases the inductance of the coil and, accordingly, reduces the frequency of the generator. Non-ferrous metal, on the contrary, increases the frequency of the generator. The reference oscillator signal is mixed with the tunable oscillator signal, after which the resulting beat signal is fed to the amplifier and then to the headphones. Even small changes in the frequency of the tunable oscillator are felt in phones as a change in the frequency of the sound. Since measures have been taken in the mine detector to increase the stability of the generator frequencies, it became possible to work at a beat frequency of 1 ... 10 Hz. And this increases the sensitivity of the device and reduces the current it consumes from the power source. For example, the device detects nails at a depth of up to 15 cm, and larger objects - up to half a meter. The tunable generator is made on the transistor V1 according to the capacitive three-point circuit, and the transistor is connected according to the common base circuit (in other words, the base is connected at high frequency to a common wire). Generation occurs due to positive feedback between the collector and emitter circuits. The frequency of the generator depends on the inductance of the coil L1 (it is remote) and the capacitance of the capacitors C1-C3. The frequency of the generator can be adjusted with a variable resistor R7, from the engine of which a constant voltage is supplied to the zener diode VXNUMX, which in this case plays the role of a varicap. A varicap is a capacitor whose capacitance depends on the voltage applied across its terminals. Zener diodes, as well as some diodes, have the same property to change their capacitance under the influence of voltage, if reverse voltage is applied to them (plus on the cathode, minus on the anode). Naturally, this voltage should not exceed the allowable voltage specified in the reference data. In our case, the capacitance of the zener diode changes when the constant voltage across it changes with a variable resistor. The reference oscillator is made on the transistor V2, also according to the capacitive three-point scheme. Its frequency depends on the inductance of the coil L2 and the capacitance of the capacitors C6, C7, C9. The operation mode of the generator transistors is set by resistors R1-R4. The high-frequency signals of the generators are mixed on the resistor R5. The amplitude of the resulting signal changes with the beat frequency: it is equal to the difference in the frequencies of the signals. To isolate the low-frequency envelope of the signal, a detector was used, made according to the voltage doubling scheme on diodes V4, V5. The load of the detector is resistor R6; capacitor C11 is installed to filter the high-frequency component. The low-frequency signal from the load of the detector is fed through the capacitor C12 to the pre-amplifier, assembled on the transistor V6. From the load of the cascade (resistor R10), the signal is fed further to the amplifier - the shaper of rectangular pulses on the transistor V7. Resistors R11 and R12 set such a mode of operation of the transistor, in which it is at the opening threshold. As a result, on the load of the cascade (resistor R13), instead of a sinusoidal signal, rectangular pulses are emitted, which are then differentiated by the capacitor C14 and turn into pointed peaks. Their duration does not depend on the repetition rate and duration of rectangular pulses. The positive peaks of the generated signal drive the transistor V9. Fixed-duration rectangular pulses appear on the collector load of the cascade (resistors R16 and R17), which are fed from the variable resistor R16 engine (this is the volume control) to the output stage assembled on transistors V10, V11. This cascade is loaded on headphones B1 connected via sockets X2 and X3. In a mine detector, you can use the K159NT1 chip with any letter index. In extreme cases, two KT315G transistors with the same or possibly close static current transfer coefficient and reverse collector current will do. Instead of KT342B transistors, KT315G, KT503E, KT3102A - KT3102E are suitable. We replace the KT502E transistor with KT361, and KT503E - with KT315 with any letter index. But in this case, the headphones must be high-resistance (TON-1, TON-2). If the phones are low-resistance, the V11 transistor should be more powerful, for example, KT6OZB, KT608B. The zener diode, in addition to that indicated on the diagram, can be D803-D813, KS156A. Diodes V4, V5 - any of the D2, D9, D10 series, and V8 - any silicon. Fixed resistors - MLT-0,125, variable R7 - SP-1, R16 - of any type, but combined with the power switch S1. Electrolytic capacitors - K50-6, the rest - KSO, PM, MBM or similar. Particular attention should be paid to the selection of capacitors operating in generators; they must have high temperature stability. Coil L2 is wound on a ferrite or carbonyl iron core, such as SB-12a or SB-23-lla. Its inductance should be 4 mH. To ensure such inductance, the number of turns for the SB-12a core should be 420, and for the SB-23-11a core - 250, the PEV-1 wire is 0,1. Some parts of the mine detector are mounted on a board (Fig. 77), on which mounting studs are installed for soldering the leads of the parts.
The base of the coil core L2 is glued to the board. After installation, the board is placed in a case (Fig. 78) made of plywood. Case dimensions 115x170x40 mm. On the front panel of the case, variable resistors, an input connector X1 (SG-3) and jacks for connecting a head phone (a two-socket socket can be installed) are installed.
The remote coil L1 is made in the form of a ring (Fig. 79) with a diameter of 160 nm. It contains 100 turns of PEV-1 0,3 wire. To wind the coil, it is convenient to use any suitable frame, the turns are stacked in bulk, and then the coil is removed and screened - wrapped with foil so that there is a gap of about 10 mm between the ends of the screen. After that, the coil is impregnated with epoxy glue or coated with epoxy putty. Conductors in polyvinyl chloride insulation are pre-soldered to the coil terminals, another such conductor is attached to the foil. After the glue or putty has hardened, the surface of the resulting coil is cleaned with sandpaper and a plywood or plastic jumper is attached to the coil. A rack is installed on the jumper, to which a rod is attached - they hold the coil for it when searching for "min". The fastening of the rod to the rack should be such that it is possible to change the angle between the rod and the coil.
A cable about a meter long is soldered to the conductors-outputs of the coil, at the other end of which a SSH-3 connector is installed - they are connected to the coil to the input connector. In this case, the device itself is either worn on the shoulder (a belt is attached to the corners on the body), or attached to the bar. The final stage of work is the establishment of a mine detector. Turning on the device, the engine of the variable resistor R7 is set to the middle position and by rotating the tuning core of the coil L2, clicks with a frequency of 1 ... 5 Hz are achieved in the phones. If necessary, select the capacitor C6. The selection of the resistor R8 achieves the highest signal volume. It should be remembered that the tuning core of the L2 coil can set the frequency of the reference oscillator both above and below the frequency of the tunable oscillator. In turn, the direction of change in the frequency of sound signals depends on this, depending on the type of metal detected. Therefore, it is advisable to check the setting practically by bringing the device closer to a particular metal object in order to know about it in the future. During the search for "mines", the sound frequency in the phones may change due to the discharge of the battery, a significant change in the ambient temperature (for example, in sunny and cloudy weather), changes in the magnetic properties of the soil. Therefore, the final adjustment of the device is carried out at the moment the remote coil approaches the ground - for this, a variable resistor R7 is installed. Author: B.S. Ivanov
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