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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Metal detector with increased sensitivity on microcircuits. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / metal detectors One of the features of all metal detectors of the BFO type is that the reference and exemplary generators of these devices are structurally made on the elements of one microcircuit. It should be recognized that in addition to certain advantages (for example, simplicity of the circuit, temperature stabilization), such designs also have a number of disadvantages. The main one is the occurrence of parasitic connections between individual elements inside the microcircuit crystal, which are almost impossible to eliminate. That is why in such metal detectors it is necessary to choose a beat frequency of more than 100-300 Hz, which inevitably leads to a decrease in its sensitivity. An attempt to rid metal object detectors based on the analysis of the beat signal, at least from the indicated shortcomings, was made when creating a device based on a scheme published in domestic and foreign publications in the mid-90s of the last century. Schematic diagram The proposed design is one of the many options for BFO (Beat Frequency Oscillator) metal detectors, that is, it is a device based on the principle of analyzing the beats of two signals that are close in frequency. At the same time, in this design, the assessment of the change in the beat frequency is carried out by ear. The basis of the circuit of this device (Fig. 3.6) is the measuring and reference oscillators, mixer, low-pass filter, analyzer and acoustic indication circuit.
The measuring and reference oscillators are two simple LC oscillators based on IC1 and IC2 microcircuit elements. In this case, the reference oscillator is assembled on the IC1.1 element, and the measuring or tunable generator is assembled on the IC2.1 element. The oscillation frequency of the reference oscillator is determined by the parameters of the elements of its circuit, that is, the inductance of the coil L1 and the capacitances of the capacitors C1, C2. The values of these parameters are chosen so that the operating frequency of the reference oscillator is about 100 kHz. The oscillatory circuit of the measuring generator is formed by the search coil L2 and capacitors C3-C5. The operating frequency of this generator is close to the frequency of the reference generator and can be slightly changed by adjusting the variable capacitor C3. Elements IC1.2 and IC2.2 perform the function of cascades that provide isolation between generators by alternating voltage. From the outputs of both generators, the RF signals are fed to the mixer, made on the IC3.1 element, at the output of which oscillations are formed with the total and difference frequencies of the generators and their harmonics, which are fed to the low-pass filter circuit. Unlike many other metal detectors of the BFO type, in the proposed device, a low-pass filter is used to isolate the signals of the difference (sound) frequency, which is assembled on the elements R3 and C6. Next, the low-frequency signal is fed to the analyzer. As is known, the sensitivity of metal object detectors that estimate the frequency of the beat signal depends to a large extent on the signal of the lowest frequency that can be registered by this device. The metal detectors that provide the analysis of beats with a frequency of several hertz have the best sensitivity. However, it is impossible to listen to such a signal directly on headphones due to the limited operating frequency range of telephone capsules. Quite often, developers resort to the simplest solution to this problem, namely: they simply increase the frequency of the beat signal using various multipliers. One of the options for the frequency doubling circuit (more precisely, converting a sinusoidal signal into a sequence of doubled frequency pulses) was already considered in the previous chapter when describing a transistor metal detector with increased sensitivity. In the analyzer of the considered metal detector, to increase the frequency of the beat signal, a circuit is used that converts a sinusoidal (almost triangular) signal into short pulses with a doubled repetition rate. For this, a voltage comparator is used, made on the elements IC3.2-IC3.4. For one period of the beat frequency, the comparator switches twice from one logical state to another, after which the rectangular pulses generated by it are differentiated by the C7R8 circuit and then fed through the capacitor C7 to the volume control R8. As a result, the BF1 headphones connected to the X2 connector receive short voltage pulses of double the frequency. The device is powered from a source B1 with a voltage of 9 V. At the same time, the IC1 and IC2 microcircuits of the metal detector are powered from a DC source through decoupling filters R6C8 and R7C9. Details and construction All parts of the metal detector under consideration (with the exception of the search coil L2, resistor R8, capacitor C3, connectors X1 and X2, as well as switch S1) are located on a printed circuit board 80x60 mm in size, made of double-sided foiled getinax or textolite (Fig. 3.7). In this case, the elements are mounted from the side of the conductors, and the foil on the other side plays the role of a screen.
There are no special requirements for the parts used in this device. It is recommended to use any small-sized capacitors and resistors that can be placed on a printed circuit board without any problems. Capacitor C3 should have a maximum capacitance of 180-240 pF. You can use any tuning capacitor from a small-sized radio receiver (for example, type KP-180). To improve thermal stability, it is desirable that capacitors C1, C2, C4 and C5 have a TKE no worse than M1500. Fixed resistors can be, for example, type MLT-0,125. Microcircuits of the K561LE5 type can be replaced with K176LE5, K176LA7 or K561LA7 microcircuits. Coil L1 contains 30 turns of PEV-2 wire with a diameter of 0,08 mm. To wind it, it is recommended to use a frame from the coil of the IF circuit of a transistor radio receiver (for example, "Alpinist-407" or similar). Search coil L2 contains 100 turns of PEV-2 wire with a diameter of 0,6 mm and is made in the form of a torus with an inner diameter of 240-250 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, for the manufacture of which an aluminum foil tape is wound over the bundle of turns. The gap between the beginning and the end of the tape winding (the gap between the ends of the screen) should be about 10 mm. In the manufacture of coil L2, it is especially necessary 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 adhesive before shielding. 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 metal detector with increased sensitivity is powered from a source B1 with a voltage of 9 V. As such a source, you can use, for example, a Krona battery or two 3336L batteries connected in series. The printed circuit board with the elements located on it and the power supply are placed in any suitable metal case. Capacitor C3, variable resistor R8, connector X1 for connecting the search coil L2, switch S1 and connector X2 for connecting headphones BF1 are installed on the housing cover. Establishment The metal detector in question should be tuned in conditions where metal objects are removed from the search coil L2 at a distance of at least 1,5 m. The direct setup of the device must begin by selecting 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 input of the low-pass filter (pin IC3 / 3). The waveform at this point resembles the waveform of a modulated RF signal. Further, by adjusting the coil L1 and, if necessary, selecting the capacitances of the capacitors C1 and C2, it is necessary to ensure that the modulation frequency (beat frequency) is approximately 5-10 Hz. When using a digital frequency meter to set up a metal detector, the frequency meter should be connected first to pin 1 of IC3, and then to pin 2 of the same chip. By changing the parameters of the previously mentioned elements (the inductance of the coil L1, the capacitances of the capacitors C1 and C2), it is necessary to ensure that the difference in the frequencies of the signals at the indicated points is also approximately 5-10 Hz. You can select the desired beat frequency without an oscilloscope and a frequency meter. In this case, it is usually sufficient to adjust the operating frequency of the reference oscillator. To do this, high-resistance telephones (for example, TON-3.1) must be connected to the output of the IC3 element (pin IC3 / 2), and then, by adjusting the tuning core of the L1 coil, an audio signal should appear in the headphones. In this case, the rotor of the capacitor C3 must be set to the middle position. Then, by rotating the tuning core of the L1 coil, it is necessary to set the mode in which clicks will be heard in the phones, following at a frequency of several hertz. After tuning the generator, it is advisable to fix the tuning core of the L1 coil with a drop of glue. Next, you need to set up the voltage comparator. To do this, you need to choose the value of the resistor R9 shown in Fig. 3.6 dashed lines. Its resistance can be in the range from 300 kΩ to 1 MΩ. It should be noted that the resistor R9 should be connected between pins 5, 6 of IC3.2 and the common wire if there is a high level voltage at the output of the comparator (pins IC3/10,11). Operating procedure In practical use of this device, the necessary frequency of the beat signal should be maintained by the variable capacitor C3, which can change under the influence of various factors (for example, when the magnetic properties of the soil change, the ambient temperature, or the battery is discharged). If, during operation, any metal object appears in the coverage area of the search coil L2, then the frequency of clicks in the headphones will change. When approaching some metals, it will increase, and when approaching others, it will decrease. By changing the frequency of clicks, having some experience, you can easily determine what metal, magnetic or non-magnetic, the detected object is made of. The volume of clicks is regulated by a variable resistor R8. Author: Adamenko M.V.
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