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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Metal detector with increased sensitivity on transistors. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / metal detectors The operation of the metal detector, the scheme and design of which are discussed in this section, is based on the principle of analyzing changes in the beats of oscillations of two generators, the frequency of one of which is stable, and the frequency of the second changes when a metal object appears in the detector's coverage area. When working on this device, an attempt was made to create a metal detector free from a number of disadvantages inherent in other similar designs. Despite the fact that the scheme of this device was developed more than 20 years ago, its advantages include a relatively high sensitivity, stability in operation, as well as the ability to distinguish between non-ferrous and ferrous metals. The applied circuit solutions ensured increased stability of the operating frequencies of the generators, which made it possible to estimate the beat frequencies in the range from 1 to 10 Hz. As a result, the sensitivity of the device has increased, and the current consumed by it has also decreased. Schematic diagram As already mentioned, the proposed design is one of the many options for metal detectors of the BFO (Beat Frequency Oscillator) type, that is, it is a device based on the principle of analyzing the beats of two frequencies (Fig. 2.10).
The device is based on measuring and reference oscillators, RF oscillation detector, pre-amplifier, first limiting amplifier, differentiating circuit, second limiting amplifier and low-frequency amplifier. Two simple LC oscillators based on transistors T1 and T2 were used as measuring and reference oscillators. These transistors are part of the K159NT1G microcircuit, which is a pair of transistors with identical parameters placed in one package. The use of a transistor assembly can significantly increase the temperature stability of the generator frequencies. Each generator is assembled according to a capacitive three-point circuit, while transistors T1 and T2 are connected according to a common base circuit. The excitation of oscillations is provided by the introduction of positive feedback between the collector and emitter of each transistor. The operating frequency of the generators is determined by the parameters of the frequency-setting circuits connected between the collectors and emitters of transistors T1 and T2. In this case, the frequency-setting elements of the first generator, which performs the functions of a measuring generator, are the search coil L1 and capacitors C1, C2 and C3. The operating frequency of the second, reference, generator is determined by the parameters of the inductor L2, as well as capacitors C6, C7 and C9. In this case, both generators are tuned to an operating frequency of 40 kHz. With the help of resistors R1-R4, the operating modes of transistors T1 and T2 are set in direct current. In the process of setting up the device by changing the capacitance of the capacitor C6, a rough adjustment of the reference oscillator to the selected harmonic of the beat frequency is carried out. In this case, the capacitance of the capacitor C6 can vary from 100 to 330 pF. Fine tuning of the beat frequency is performed by a variable resistor R7, with which the bias on the zener diode D1, which in this circuit acts as a varicap, changes. When approaching the search coil L1 of the oscillatory circuit of the tunable generator to a metal object, its inductance changes, which causes a change in the operating frequency of the generator. In this case, if there is an object made of ferrous metal (ferromagnet) near the coil L1, its inductance increases, which leads to a decrease in the frequency of the generator. Non-ferrous metal reduces the inductance of the coil L1, and the operating frequency of the generator increases. The RF signal generated as a result of mixing the signals of the measuring and reference oscillators is isolated on the load resistor R5. In this case, the signal amplitude changes with the beat frequency, which is equal to the frequency difference of the RF signals. The low-frequency envelope of the RF signal is detected by a special detector made on diodes D2 and D3 according to the voltage doubling scheme. In this case, the capacitor C11 provides filtering of the high-frequency component of the signal. From the load of the detector, which is the resistor R6, the low-frequency beat signal is fed through the capacitor C12 to a preamplifier made on the transistor T3. From the collector of transistor T3, the amplified signal is fed through capacitor C13 to the first limiting amplifier, made on transistor T4 and providing the formation of rectangular pulses. With the help of a divider made up of resistors R11 and R12, a bias voltage is applied to the base of transistor T4 at which the transistor is at the opening threshold. The sinusoidal signal arriving at the base of transistor T4 is limited on both sides. As a result, on the load of the cascade, the role of which is played by the resistor R13, rectangular pulses are formed, which are further differentiated by the circuit C14, R14, R15 and converted into pointed peaks. In this case, at the place of the front of each pulse, a peak of positive polarity is formed, and at the place of the decline, a peak of negative polarity is formed. It should be noted that the duration of these peaks does not depend on the repetition rate of rectangular pulses and their duration. Positive peaks are fed to the base of transistor T5, and negative peaks are cut off by diode D4. Transistor T5, like transistor T4, operates in a key mode and limits the input signal so that short rectangular pulses of a fixed duration are formed on the collector load formed by resistors R16 and R17. Capacitor C15 filters the output signal and improves the tone of the signal in the BF1 headphones. From the resistor R16, which is the volume control, the signal goes to the amplifying stage, made on transistors T6 and T7, connected according to the so-called composite transistor circuit. With this inclusion, an equivalent to a high-power pnp conductivity transistor with a high current transfer coefficient is formed. Then the amplified signal goes to the BF1 headphones. The method used in this design for generating a pulse signal from a sinusoidal one makes it possible to reduce the power consumed by the amplifier, especially in the output stage, since transistors T5, T6 and T7 are closed in the pauses between pulses. The metal detector is powered from a source B1 with a voltage of 4,5 V, while the current consumption does not exceed 2 mA. Details and construction There are no special requirements for the parts used when assembling a metal detector with increased sensitivity. The only limitation is related to the overall dimensions, since most of the parts of this device are mounted on a printed circuit board measuring 70x110 mm, made of one-sided foil-coated getinax or fiberglass. The printed circuit board is designed to use fixed resistors MLT-0,125, capacitors KSO, PM, MBM, K50-6 or similar (Fig. 2.11). When repeating this design, as a transistor assembly (transistors T1 and T2), you can use the K159NT1 chip with any letter index. However, at present it is not always possible to find it. Therefore, if necessary, instead of a transistor assembly, it is recommended to use two transistors of the KT315G type with the same or possibly close parameters (static current transfer coefficient and initial collector current).
In amplifying stages (transistors T3, T4 and T5), instead of transistors of the KT342B type, transistors of the KT315G, KT503E or KT3102A - KT3102E types can be installed. A transistor of the KT502E (T6) type can be completely replaced by a KT361, and a transistor of the K503E (T7) type by a KT315 with any letter indices. But in this case, the headphones must be high-resistance (such as TON-2 or TEG-1). When using low-resistance phones, the transistor T7 must be more powerful, for example, type KT603B or KT608B. As a zener diode D1, you can also use zener diodes of the D808-D813 or KS156A type. Diodes D2 and D3 can be any of the D1, D9 or D10 series. Coil L2 contains 250 turns of wire PEV-2 with a diameter of 0,1 mm, wound on the magnetic circuit SB-23-11a. In its manufacture, you can use other cores. The main thing is that the inductance of the finished coil is 4 mH. The measuring coil L1 contains 100 turns of PEV-1 wire with a diameter of 0,3 mm and is made in the form of a torus with a diameter of 160 mm. This coil is easier to make on a rigid frame, but you can do without it. In this case, any suitable round object, such as a jar, can be used as a temporary frame. The turns of the coil are wound in bulk, after which they are removed from the frame and shielded with an electrostatic screen, which is an open aluminum foil tape wound over a bundle of turns. The gap between the beginning and the end of the tape winding (the gap between the ends of the screen) must be at least 10 mm. In the manufacture of coil L1, care must be taken to ensure that the ends of the shielding tape do not close, since in this case a short-circuited coil is formed. To increase the mechanical strength, the coil can be impregnated with epoxy glue. Solder the conductors of a two-core shielded cable about a meter long to the coil terminals, at the other end of which a SSH-3 connector or any other suitable small-sized connector is installed. The cable sheath must be connected to the coil screen. In the working position, the coil connector is connected to the mating connector located on the device body. The high sensitivity metal detector is powered from a source B1 with a voltage of 4,5 V. As such a source, you can use, for example, the so-called 3336L square battery or three elements of the 316, 343 type 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. Variable resistors R7 and R16, connector X1 for connecting the search coil L1, switch S1, and connector X2 for connecting headphones BF1 are installed on the housing cover. Establishment As with the adjustment of other metal detectors, the adjustment of this device must be carried out in conditions where metal objects are removed from the L1 search coil at a distance of at least 1,5 m. The direct adjustment of the metal detector should begin with the selection of the desired beat frequency. To do this, it is recommended to use an oscilloscope or a digital frequency meter. When working with an oscilloscope, its probe must be connected to the connection point of resistors R1, R4, R5 and capacitor C8, that is, to the detector input. The waveform at this point resembles the waveform of a modulated RF signal. Further, by adjusting the L2 coil and selecting the capacitances of the capacitors C2 and C6, it is necessary to ensure that the modulation frequency (beat frequency) is approximately 10 Hz. When using a digital frequency meter to set up a metal detector, the frequency meter should be connected first to the collector circuit of transistor T1, and then to the collector of transistor T2. Selecting the parameters of the previously mentioned elements (the inductance of the coil L2, the capacitance of the capacitors C2 and C6), it is necessary to ensure that the difference in the frequencies of the signals on the collectors of transistors T1 and T2 is approximately 10 Hz. Further, by selecting the resistor R8, the maximum gain of the cascade, made on the transistor T3, is set. In the absence of an oscilloscope and a frequency counter, the selection of the desired beat frequency can be performed without them. In this case, you must first set the slider of the resistor R7 to the middle position, and then, by rotating the tuning core of the L2 coil, make clicks appear in the phones with a frequency of approximately 1-5 Hz. If you cannot set the desired frequency, you should select the capacitance of the capacitor C6. To reduce the influence of the ground background, the final selection of the beat frequency should be carried out when the search coil L1 approaches the ground. This completes the process of setting up a metal detector with increased sensitivity. Operating procedure In practical use of this metal detector, the required frequency of the beat signal should be maintained by a variable resistor R7, which changes when the battery is discharged, when the ambient temperature changes, or when the magnetic properties of the soil deviate. You also need to adjust the volume of clicks using the R16 knob. If, during operation, any metal object appears in the coverage area of the search coil L1, the signal frequency in the phones will change. When approaching some metals, the frequency of the beat signal will increase, and when approaching others, it will decrease. By changing the tone of the beat signal, having some experience, one can easily determine what metal, magnetic or non-magnetic, the detected object is made of. With the help of such a metal detector, small objects, such as nails, can be detected under a layer of soil at a depth of up to 10-15 cm, and large objects (for example, well covers) - at a depth of up to 50-60 cm. Author: Adamenko M.V.
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