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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Advanced metal detector on transistors with quartz. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / metal detectors In recent years, many respected European publishing houses have paid considerable attention to various technical devices used in search operations. Every year, new books with descriptions of various devices arrive on the shelves of bookstores. It should be recognized that, in general, these devices are difficult to assemble and adjust and can hardly be recommended for repetition by novice radio amateurs. Nevertheless, in one of the books published as part of the "Elektronicke hledace" series by the popular European publishing house "BEN", the author relatively recently, not without surprise, discovered a metal detector circuit that seemed very familiar. The main element with which the presence of metal objects is analyzed in this device is quartz. At the same time, the results of the analysis are evaluated both visually and aurally. 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. Upon careful study of the circuit diagram, you can see that this device is an improved version of the metal detector discussed in the previous section. One of the main distinguishing features of this design is still the analyzer, made on the Q1 quartz element. In addition, in an improved version of the metal detector, in addition to the pointer instrument, an acoustic signaling circuit is used as an indicator. Since in the proposed circuit (Fig. 2.16) the numbering of elements has been changed, a new element base is used, and an additional cascade has been added, the author considered it necessary to consider its features in more detail.
As in previous design, the basis of the scheme of this metal detector is a measuring generator, a buffer stage, an RF oscillation detector, an analyzer and an indicator device. The oscillatory circuit of the high-frequency generator, made on the transistor T1, consists of a coil L1 and capacitors C3-C6. The operating frequency of the RF generator depends on the deviation of the inductance of the search coil L1, as well as on changes in the capacitances of the tuning capacitor C4 and the adjusting capacitor C3. In the absence of metal objects near 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 in the coverage area of the search coil L1, its inductance will change. This will change 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 R7 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 signal is supplied to the input of the DC amplifier (the base of 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. When the switch S2 is closed, the audio signal generator, made on the transistor T4, is turned on in the load circuit. 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 (fig. 2.17) is similar to the coil used in the metal detector, which was discussed in the previous section. Instead of transistors of the BC108 type indicated in the diagram, almost any domestic low-power silicon transistors, for example, of 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.
As an element Q1, you can use any quartz element with a frequency of 900 kHz to 1,1 MHz. The power source for V1 can be 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 R7, switches S1 and S2, connectors X1 and X2, as well as an 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 C4, and then precisely by adjusting the capacitor C3. 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. Quartz Q1 is an element of communication between the measuring and indicating parts of the device, so its resistance at the time of resonance is 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 R7. Operating procedure In the practical use of this metal detector, the variable resistor R7 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 L2, the arrow of the indicator PAXNUMX will deviate. In this case, when the contacts of switch SXNUMX are closed, an audible signal will appear in the headphones. Author: Adamenko M.V.
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