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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING MI-2 metal detector on transistors. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / metal detectors In the first half of the 70s of the last century, the MI-2 metal detector was developed and mass-produced in the Soviet Union, which was widely used in the national economy. The scheme and design of this device have been repeatedly refined and improved. One of the well-known variants of the MI-2 metal detector can be recommended to beginner radio amateurs for repetition. Schematic diagram The MI-2 metal detector is one of the many variants of the BFO (Beat Frequency Oscillator) type device, that is, it is a device based on the principle of analyzing the beats of two frequencies. At the same time, in this design, the assessment of the change in frequency is carried out by ear (Fig. 2.12).
The device circuit is based on measuring and reference oscillators, a capacitive stage, an emitter follower, a Schmitt trigger and headphones. The measuring generator is made on the transistor T1, connected according to the scheme with a common base. The operating frequency of this generator is determined by the parameters of the oscillatory circuit, which consists of the search coil L1 and capacitors C3, C4. The feedback voltage required for self-excitation is supplied from the collector of transistor T1 to the emitter circuit through a capacitive divider C3, C4. As a result, a sinusoidal signal with a frequency of 510 kHz is formed at the output of the measuring generator. The reference oscillator is made on the transistor T6 according to a scheme similar to that of the measuring oscillator. The operating frequency of this generator is determined by the parameters of the oscillatory circuit, which consists of an L3 coil with a brass trimmer core and capacitors C12, C13 and C14. Oscillations from the reference and measuring oscillators through the capacitors C5 and C11 are fed to the input of the mixer, which is made on the transistor T2. The collector circuit of the transistor T2 includes a circuit consisting of a coil L2 and a capacitor C6, in which the oscillations of the difference frequency are distinguished. The search coil L1, which is part of the oscillatory circuit of the measuring generator, is a sensor that reacts to the appearance of metal objects in the coverage area of the device. When the coil L1 approaches such an object, its inductance changes and, as a result, the frequency of the signal of the measuring generator changes. As a result, the frequency of the signal at the output of the mixing stage will also change. Since the mixer circuit, made on the elements L2 and C6, is tuned to the difference frequency of the measuring and reference oscillators in the absence of metal objects, a change in the signal frequency will also lead to a decrease in the signal amplitude at the mixer output. The operating frequency of the mixer circuit is 1 kHz. Next, the selected signal is fed to the emitter follower, made on the transistor T3 and serving to match the Schmitt trigger with the mixer. The Schmitt trigger is made on transistors T4, T5 and is an electronic relay that responds to changes in the amplitude of the input signal. The operating modes of transistors T4 and T5 are selected so that the trigger is triggered when the signal voltage at the input is more than 0,5 V. The generated acoustic signal is fed to the headphones BF1. The metal detector is powered from a source B1 with a voltage of 9 V, while the current consumption does not exceed 4-5 mA. Details and construction Structurally, the MI-2 metal detector consists of two blocks. The search block includes elements that form the measuring generator, the indication block includes a reference generator, a capacitive cascade, an emitter follower and a Schmitt trigger. Both blocks are interconnected by a shielded cable. There are no special requirements for the parts used in the assembly of the MI-2 metal detector. The only limitation is related only to the overall dimensions, since most of the device parts are mounted on two relatively small printed circuit boards. The details of the search block are placed on a printed circuit board with dimensions of 70x35 mm, made of one-sided foil-coated getinax or fiberglass (Fig. 2.13).
The details of the display unit are placed on a printed circuit board with dimensions of 150x75 mm, also made of one-sided foil-coated getinax or fiberglass (Fig. 2.14).
In the mass-produced MI-2 metal detector, resistors of the MLT-0,125 type were used, capacitors C1, C2, C8, C9, C15 and C16 - of the KLS-1 type; C5, C11, C13 - KSO-1; capacitors C3, C4, C12, C14 - type KSO-2; C6 - MBM or MBM-2; electrolytic capacitors C7 and C10 - type K50-3. Naturally, when repeating this device, you can use any similar parts from the modern element base. TON-1 type headphones are suitable as a source of an acoustic signal. The search coil L1 is made in the form of a ring with a diameter of about 300 mm. The turns of the coil are enclosed in an electrostatic screen made of a duralumin tube with a diameter of 8 mm and a wall thickness of 1 mm. To make a coil, it is necessary to make a bundle of ten pieces of PEV-2 wire with a diameter of 0,96 mm and a length of 1250 mm. First, the tourniquet must be dragged into a PVC tube 1000 mm long, and then into a duralumin tube 960 mm long. The duralumin tube with the wires in it must be bent according to the template into a ring. Ordinary aluminum foil can also be used as a screen. Pieces of wires are connected in series with the help of soldering on the block installed in the body of the search unit. In the manufacture of the L1 coil, care must be taken to ensure that the ends of the shielding tube do not close, since in this case a short-circuited coil is formed. Therefore, it is desirable to isolate the ends of the screen with a rubber tube. The coil L2 of the mixer is wound on an annular ferrite core M2000 NM-A-K38x24x7. It has 200 turns of PEV-2 wire with a diameter of 0,47 mm and is installed on the printed circuit board of the display unit. Coil L3 of the reference generator contains 135 turns of PELSHO wire with a diameter of 0,1 mm, which are wound on a frame with a diameter of 7-9 mm with a brass core. If necessary, a detailed description of the special design of the L3 coil can be found in Radio magazine No. 4, 1973. The body of the search block is made of duralumin. The search coil L1 and the search unit are fixed on the bottom of a special handle. The housing of the display unit is also made of duralumin. A connector for connecting a search unit (not indicated on the circuit diagram), an S1 switch, and an X1 connector for connecting BF1 headphones are installed on the housing cover. The cover should also have a hole for the L3 coil adjustment knob. As a power source B1, you can use, for example, two 3336L batteries connected in series. Establishment The main steps in setting up the MI-2 metal detector are setting the trigger threshold and selecting the reference oscillator frequency. The trigger threshold is set by selecting the resistance of the resistor R11. To do this, unsolder the output of capacitor C2 from the collector of transistor T8 and apply a signal from a sound generator with a voltage of 0,5 V at a frequency of 1 kHz to this capacitor. The value of the resistance of the resistor R11 must be chosen such that with a slight decrease in the amplitude of the signal of the sound generator, the sound in the headphones disappears, and the collector current of the transistor T5 becomes equal to zero. Rough adjustment of the frequency of the signal generated by the reference oscillator is performed by selecting the capacitance of the capacitor C12. More precisely, the frequency value is set by selecting the capacitance of the capacitor C18. These adjustments should be carried out under conditions when metal objects are removed from the search coil L1 at a distance of at least 1,5 m. The frequency of the reference oscillator is determined using a frequency meter or an oscilloscope. In this case, the capacitor C11 must be soldered from the emitter of the transistor T6. Then you need to set the center frequency of the reference oscillator. To do this, restore the connection of the capacitor C11 with the emitter of the transistor T6, disconnect the search unit from the display unit and measure the frequencies of the reference oscillator with a frequency meter when the L3 coil tuning knob is set to the extreme positions. The average frequency of the reference oscillator is determined as the arithmetic mean of the measured frequencies. If necessary, the capacitances of capacitors C12 and C13 are selected so that the average frequency of the reference oscillator differs from the frequency of the measuring oscillator by 1 kHz. After adjusting the frequencies of the measuring and reference oscillators by rotating the tuning core of the L3 coil, at the output of the mixing stage, you need to set the signal voltage level to a little more than 0,5 V. In this case, the trigger will switch with the frequency of the incoming signal, and a sound signal will be heard in the headphones. Operating procedure Carrying out search operations with the help of the MI-2 metal detector does not have any special features. If a metal object is in the coverage area of this device, then when the search coil L1 approaches it, a tone of varying frequency will be heard in the headphones, decreasing in volume. If the coil is still brought closer to a metal object, then the signal voltage at the mixer output will become less than the trigger threshold. The trigger will stop switching and the audio signal in the headphones will disappear. If necessary, during the search process, you can adjust the metal detector to the beat frequency by adjusting the position of the L3 coil core. In accordance with the data obtained during the practical use of the MI-2 metal detector, large metal objects (for example, a well cover) can be detected at a distance of 600-800 mm, small ones (for example, a screwdriver) - at a distance of 70-100 mm, and coins of an average value, the device starts to react from a distance of 30-50 mm. Author: Adamenko M.V.
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