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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Metal detector on transistors with quartz. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / metal detectors Metal detectors based on beat frequency deviation (BFO) have a relatively low sensitivity when searching for metals with weak ferromagnetic properties (copper, tin, silver, etc.). Since the frequency difference (beat) is hardly noticeable using conventional indication methods, it is quite difficult to increase the sensitivity of BFO metal detectors. Naturally, this situation has become a good incentive to search for other circuit solutions. Many years ago, the author made a device based on the scheme of the original device, published in the journal "Radio-Electronics" (1967, No. 11). The main element used to analyze the presence of metal objects was quartz. The results of the analysis were evaluated visually. Schematic diagram The design offered to the attention of readers 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 analyzing the frequency deviation of the reference oscillator under the influence of metal objects that have fallen into the area of the search coil. The main distinguishing features of this device can be considered an interesting circuit design of the analyzer, made on a Q1 quartz element, as well as the use of a pointer device as an indicator. The basis of the circuit of the considered metal detector (Fig. 2.15) is a measuring generator, a buffer stage, an analyzer, a high-frequency oscillation detector and an indicator device.
The oscillatory circuit of the high-frequency generator, made on the transistor T1, consists of a coil L1 and capacitors C1-C4. The operating frequency of the RF generator depends on the deviation of the inductance of the L1 coil, which is also a search coil, as well as on the change in the capacitances of the tuning (C2) and adjusting (C1) capacitors. In the absence of metal objects in the coverage area of the coil L1, the frequency of oscillations excited in the RF generator must be equal to the frequency of the quartz element Q1, that is, in this case, 1 MHz. After a metal object is near the coil L1, its inductance will change. This will lead to a deviation in the oscillation frequency of the RF generator. Next, the RF signal is fed to the buffer stage, which ensures the matching of the generator with subsequent circuits. An emitter follower made on a transistor T2 is used as a buffer stage. From the output of the emitter follower, the RF signal through the adjusting resistor R8 and quartz Q1 goes to the detector, made on the diode D2. Due to the high quality factor of quartz, the slightest changes in the frequency of the measuring oscillator will lead to a decrease in the impedance of the quartz element. As a result, a low-frequency (LF) signal is supplied to the input of the DC amplifier (DCA), made on the transistor T3, the change in the amplitude of which provides a corresponding deviation of the arrow of the indicator device. The load of the UPT, made on the transistor T3, is a pointer device with a total deflection current of 1 mA. The metal detector is powered from a source B1 with a voltage of 9 V. Details and construction As in some of the designs discussed earlier, any breadboard can be used to make a metal detector with a quartz element. Therefore, the used parts are not subject to any restrictions related to overall dimensions. Installation can be both hinged and printed. The search coil L1 is an annular frame made of a piece of cable with an outer diameter of 8-10 mm (for example, a cable of the PK-50 brand). The central core of the cable should be removed, and instead of it, six cores of PEL-type wire with a diameter of 0,1-0,2 mm and a length of 115 mm should be stretched. The resulting multi-core cable must be bent on a suitable mandrel into a ring so that a gap of approximately 25-30 mm wide remains between the beginning and end of the formed loop. The end of the wire, which is the beginning of the first turn, should be soldered to the shielding braid of the cable, the beginning of the second turn - to the end of the first, and so on. The result is a coil containing six turns of wire. In the manufacture of coil L1, it is especially necessary to ensure that the ends of the shielding braid do not close, since in this case a short-circuited coil is formed. Additional rigidity of the design of the coil L1 can be given if it is placed between two discs made of plywood or getinaks with a diameter of 400 mm and a thickness of 5-7 mm. Instead of transistors of the 2N2924 type indicated in the diagram, almost any domestic low-power silicon transistors, for example, the KT315B type, can be used in this design. Instead of a 1N4001 (D2) diode, it is recommended to use any germanium diode of the D2 or D9 series with any letter index, and a 1N753 type zener diode can be replaced without problems, for example, with a 2S156A zener diode. Any quartz element with a frequency from 1 kHz to 900 MHz can be used as the Q1,1 element. As a power source V1, you can use, for example, a Krona battery or two 3336L batteries connected in series. The board with the elements located on it and the power supply are placed in any suitable plastic or wooden case. A variable resistor R8, connector X1 for connecting the search coil L1, switch S1 and indicator PA1 are installed on the housing cover. The search coil L1 should be installed at the end of a suitable handle 100-120 cm long. The coil is connected to the device board with a multicore shielded cable. Establishment The main condition for high-quality tuning of this device is the absence of large-sized metal objects at a distance of at least 1,5 m from the search coil L1. The direct adjustment of the metal detector should begin with setting the desired frequency of oscillations generated by the RF generator. The RF oscillation frequency must be equal to the frequency of the Q1 quartz element. It is recommended to use a digital frequency meter to make this adjustment. In this case, the frequency value is first roughly set by changing the capacitance of the capacitor C2, and then precisely by adjusting the capacitor C1. In the absence of a frequency meter, the setting of the RF generator can be carried out according to the readings of the PA1 indicator. Since quartz Q1 is a connection element between the search and indicator parts of the device, its resistance at the time of resonance is very high. Thus, the minimum reading of the pointer instrument PA1 will indicate the fine tuning of the oscillations of the RF generator to the frequency of the quartz. The sensitivity level of this device is regulated by resistor R8. Operating procedure In the practical use of this metal detector, the variable resistor R8 should set the arrow of the indicator PA1 to zero on the scale. In this case, to a certain extent, changes in operating modes due to battery discharge, changes in ambient temperature, or deviations in the magnetic properties of the soil are compensated. If, during operation, any metal object appears in the area of \u1b\u1bthe search coil LXNUMX, the arrow of the indicator PAXNUMX will deviate. Author: Adamenko M.V.
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