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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Single coil induction metal detector. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / metal detectors The proposed induction type metal detector is universal. Its sensor is simple in design and can be manufactured with a diameter of 0,1 ... 1 m. Approximately in proportion to the diameter, the size of the detected objects and the distance at which the metal detector detects these objects will change. For a standard sensor with a diameter of 180 mm, the detection depth is:
The device is equipped with a simple discriminator that allows you to filter out signals from small iron objects if the latter are not of interest for the search. Structural scheme The block diagram is shown in fig. 14. It consists of several functional blocks.
The quartz oscillator is a source of rectangular pulses, from which a signal is subsequently formed that enters the sensor coil. The oscillator signal is divided by frequency by 4 using a ring counter on flip-flops. According to the ring circuit, the counter is designed so that two signals F1 and F2 can be generated at its outputs, shifted relative to each other in phase by 90 °, which is necessary to build a discriminator circuit. A rectangular signal (meander) is fed to the input of the first integrator, the output of which is a piecewise linear sawtooth voltage. The second integrator makes a signal out of the "saw", which is very close in shape to a sinusoidal one and consists of half-waves of a parabolic shape. This stable amplitude signal is fed to the power amplifier, which is a voltage-to-current converter loaded on the sensor coil. The sensor voltage is no longer stable in amplitude, as it depends on the signal reflected from metal objects. The absolute value of this instability is very small. To increase it, that is, to highlight the useful signal, the compensation circuit subtracts the output voltage of the second integrator from the voltage on the sensor coil. Here, many details of the construction of the power amplifier, the compensation circuit and the method of switching on the sensor coil are deliberately omitted, making this description easier to understand the principle of operation of the device, although not quite correct. For more details, see the description of the circuit diagram. From the compensation circuit, the useful signal is fed to the receiving amplifier, where it is amplified by voltage. Synchronous detectors convert the useful signal into slowly varying voltages, the values and polarity of which depend on the phase shift of the reflected signal relative to the voltage signal of the sensor coil. In other words, the output signals of synchronous detectors are nothing more than the components of the orthogonal expansion of the vector of the useful reflected signal in terms of the vector basis of the fundamental harmonics of the reference signals F1 and F2. A part of the useless signal, which is not compensated by the compensation circuit due to its imperfection, inevitably penetrates into the receiving amplifier. At the outputs of synchronous detectors, this part of the signal is converted into DC components. High-pass filters (HPF) cut off useless constant components, passing and amplifying only the changing components of the signals associated with the movement of the sensor relative to metal objects. The discriminator generates a control signal to start the sound signal shaper only with a certain combination of signal polarities at the filter output, which eliminates the sound indication from small iron objects, rust and some minerals Schematic diagram The schematic diagram of the induction metal detector developed by the author is shown in Fig. 15 - the input part, fig. 16 - synchronous detectors and filters, fig. 17 - discriminator and sound signal shaper, fig. 18 is a diagram of external connections. Crystal oscillator (Fig. 15) The crystal oscillator is assembled on D1.1-D1.3 inverters. The oscillator frequency is stabilized by a quartz or piezoceramic resonator Q with a resonant frequency of 215 Hz - 32 kHz ("clock quartz"). The R1C2 circuit prevents the excitation of the generator at higher harmonics. Through the resistor R2, the OOS circuit is closed, through the resonator Q, the POS circuit is closed. The generator is characterized by simplicity, low current consumption, reliable operation at a supply voltage of 3 ... 15 V, does not contain tuned elements and overly high-resistance resistors. The output frequency of the generator is about 32 kHz.
ring counter (Fig. 15) The ring counter has two functions. First, it divides the oscillator frequency by 4, up to a typical frequency of 8 kHz for such devices. Secondly, it generates two reference signals for synchronous detectors, shifted relative to each other by 90° in phase. The ring counter consists of two D-flip-flops D2.1 and D2.2, closed in a ring with signal inversion along the ring. The clock signal is common for both flip-flops. Any output signal of the first trigger D2.1 has a phase shift of plus or minus a quarter period (ie 90°) relative to any output signal of the second trigger D2.2. Integrators (Fig. 15) The integrators are made on the OS D3.1 and D3.2. Their time constants are determined by the circuits R3C6 and R5C9. DC mode is supported by resistors R4, R6. Separating capacitors C5, C8 prevent the accumulation of static error, which can bring the integrators out of the mode due to their high DC gain. The ratings of the elements included in the integrator circuits are chosen so that the total phase shift of both integrators at an operating frequency of 8 kHz is exactly 180°, taking into account both the main RC circuits and taking into account the influence of the separation circuits and the final speed of the op amp with the selected correction. The correction circuits of the op amps of the integrators are standard and consist of 33 pF capacitors. Amplifier (fig. 15) The power amplifier is assembled on a D4.2 op-amp with parallel voltage feedback. A thermally compensated current-setting element, consisting of resistors R72, R78 and thermistor R73 (see Fig. 18), is connected between the output of the second integrator and the inverting input of the op-amp D4.2. The amplifier load, which is also an element of the OOS, is an oscillatory circuit consisting of a sensor coil L1 and a capacitor C61. In the numbering of resistors and capacitors in the diagrams of fig. 15-18 some positions are omitted, that I is associated with numerous modifications to the circuit of the induction metal detector and this is not a mistake. The oscillatory circuit is tuned to resonance at a quarter of the frequency of the quartz resonator of the master oscillator, i.e. to the frequency of the signal applied to it. The impedance modulus of the oscillatory circuit at the resonant frequency is about 4 kOhm. The parameters of the sensor coil L1 are as follows: the number of turns is 100, the brand of wire is PEL, PEV, PELSHO 0,2 ... 0,5, the average diameter and the diameter of the winding mandrel are 165 mm. The coil has an aluminum foil screen connected to the instrument's common bus. To prevent the formation of a short-circuited turn, a small part, about 1 cm, of the circumference of the coil winding is free from the screen. The sensor elements R72, R73, R78, L1, C61 are selected so that: firstly, they are equal in value to the voltage at the input and output of the power amplifier. To do this, it is necessary that the resistance of the circuit R72, R73, R78 be equal to the impedance modulus of the oscillatory circuit L1, C61 at a resonant frequency of 8 kHz, or rather, 8192 Hz. This resistance module is, as already mentioned, about 4 kOhm and its value must be specified for a particular sensor. Secondly, the temperature coefficient of resistance (TCR) of the R71-R73 circuit must match in magnitude and sign with the TCR of the impedance module of the oscillating circuit L1, C61 at the resonant frequency, which is achieved: roughly - by choosing the value of the thermistor R73, and exactly - by choosing ratio R72-R78 and is achieved experimentally when tuning. The temperature instability of the oscillatory circuit is associated with the instability, first of all, of the ohmic resistance of the copper wire of the coil. With an increase in temperature, this resistance increases, which increases the losses in the circuit and reduces its quality factor. Therefore, the modulus of its impedance at the resonant frequency decreases. Resistor R18 does not play a fundamental role in the circuit and serves to maintain the D4.2 op-amp in the mode when the counterpart of the X1 connector is disabled. The D4.2 op amp correction circuit is standard and consists of a 33 pF capacitor. Compensation scheme (Fig. 15) The main elements of the compensation circuit that implement the subtraction of the output voltage of the second integrator from the sensor coil voltage are resistors R15, R17 with the same resistance value. From their common connection point, the useful signal is fed to the receiving amplifier. Additional elements, thanks to which manual adjustment and adjustment of the device is achieved, are potentiometers R74, R75 (Fig. 18). From these potentiometers it is possible to take a signal that lies in the range [-1, +1] from the voltage signal of the sensor (or the output signal of the second integrator, which is almost equal in amplitude to it). By adjusting these potentiometers, the minimum signal at the input of the receiving amplifier and zero signals at the outputs of synchronous detectors are achieved. Through the resistor R16, part of the output signal of one potentiometer is mixed into the compensation circuit directly, and using the elements R11-R14, C14-C16 - with a shift of 90 ° from the output of another potentiometer. Op-amp D4.1 is the basis of the compensator of higher harmonics of the compensation circuit. It implements a double integrator with inversion, the time constants of which are set by the R7C12 parallel voltage feedback circuit common to the integrator, as well as the capacitor C16 with all the resistors surrounding it. A meander with a frequency of 8 kHz is supplied to the input of the double integrator from the output of element D1.5. Through resistors R8, R10, the main harmonic is subtracted from the meander. The total resistance of these resistors is about 10 kOhm and is selected experimentally when setting the minimum signal at the output of the op-amp D4.1. The higher harmonics remaining at the output of the double integrator enter the compensation circuit in the same amplitude as the higher harmonics that penetrate through the main integrators. The phase relationship is such that at the input of the receiving amplifier, the higher harmonics from these two sources are practically compensated. The output of the power amplifier is not an additional source of higher harmonics, since the high quality factor of the oscillatory circuit (about 30) provides a high degree of suppression of higher harmonics. Higher harmonics, in the first approximation, do not affect the normal operation of the device, even if they are many times greater than the useful reflected signal. However, they must be reduced so that the receiving amplifier does not fall into clipping mode when the tops of the "cocktail" from the higher harmonics at its output, they begin to be cut off due to the finite value of the supply voltage of the op-amp. Such a transition of the amplifier to the nonlinear mode sharply reduces the gain of the useful signal. Elements D1.4 and D1.5 prevent the formation of a parasitic PIC ring through the resistor R7 due to the non-zero value of the output co- | trigger output resistance D2.1. An attempt to connect the resistor R7 directly to the flip-flop leads to self-excitation of the compensation circuit at a low frequency. The D4.2 op amp correction circuit is standard and consists of a 33 pF capacitor. Receiving amplifier (Fig. 15) The receiving amplifier is two-stage. Its first stage is made on the D5.1 op-amp with parallel voltage feedback. The useful signal gain is: Ku = - R19/R17 = -5. The second cascade is made on the D5.2 op amp with serial voltage feedback. Gain coefficient Ku = R21/R22 + 1 = 6. The time constants of the separating circuits are chosen such that at the operating frequency the phase shift created by them compensates for the signal delay due to the finite speed of the op-amp. Op-amp correction circuits D5.1 and D5.2 are standard and consist of 33 pF capacitors.
Synchronous detectors (Fig. 16) Synchronous detectors are of the same type and have identical circuits, so only one of them, the top one in the circuit, will be considered. The synchronous detector consists of a balanced modulator, an integrating circuit and a constant signal amplifier (CCA). The balanced modulator is implemented on the basis of an integrated assembly of analog switches D6.1 on field-effect transistors. With a frequency of 8 kHz, analog switches alternately close the outputs of the "triangle" of the integrating circuit, consisting of resistors R23 and R24 and capacitor C23, to a common bus. The reference frequency signal is fed to the balanced modulator from one of the ring counter outputs. This signal is the control signal for analog switches. The signal to the input of the "triangle" of the integrating circuit is fed through the decoupling capacitor C21 from the output of the receiving amplifier. Time constant of the integrating circuit t = -R23*C23 = R24*C23. More details about the synchronous detector scheme can be found in Sect. 2.1. OA UPS D7 has a standard correction circuit, consisting of a capacitor with a capacity of 33 pF for OA type K140UD1408. In the case of using an op-amp of the K140UD12 type (with internal correction), a correction capacitor is not needed, but an additional current-setting resistor R68 is required (shown in dotted line). filters (Fig. 16) The filters are of the same type and have identical circuits, so only one of them, the top one in the circuit, will be considered. As mentioned above, the type of filter refers to the HPF. In addition, the role of further amplification of the signal rectified by the synchronous detector is assigned to it in the circuit. When implementing this kind of filters in metal detectors, a specific problem arises. Its essence is as follows. Useful signals from the outputs of synchronous detectors are relatively slow, so the lower cutoff frequency of the HPF is usually in the range of 2...10 Hz. The dynamic range of signals in amplitude is very large, it can reach 60 dB at the filter input. This means that the filter will very often operate in a non-linear peak-to-peak mode. The exit from the nonlinear mode after exposure to such large amplitude overloads for a linear high-pass filter can take tens of seconds (as well as the device’s readiness time after turning on the power), which makes the simplest filter circuits unsuitable for practice. To solve this problem, they go to all sorts of tricks. Most often, the filter is divided into three or four stages with a relatively small gain and a more or less uniform distribution of the timing chains over the stages. This solution speeds up the output of the device to normal mode after overloads. However, its implementation requires a large number of OS. In the proposed scheme, the HPF is single-stage. To reduce the consequences of overloads, it is made non-linear. Its time constant for large signals is about 60 times less than for low amplitude signals. Schematically, the HPF is a voltage amplifier on the D9.1 op-amp, covered by the OOS circuit through the integrator on the D10 op-amp. For a small signal, the frequency and time properties of the HPF are determined by a divider of resistors R45, R47, the time constant of the integrator R43 C35 and the gain of the voltage amplifier on the op-amp D9.1. With an increase in the output voltage of the HPF after a certain threshold, the influence of the VD1-VD4 diode chain begins to affect, which are the main source of nonlinearity. The specified circuit shunts resistor R45 on large signals, thereby increasing the depth of the OOS in the HPF and reducing the time constant of the HPF. The useful signal gain is about 200. To suppress high-frequency interference, the filter circuit has a capacitor C31. The voltage amplifier op amp D9.1 has a standard correction circuit consisting of a 33 pF capacitor. The op amp of the D10 integrator has a correction circuit consisting of a 33 pF capacitor for the op amp of the K140UD1408 type. In the case of using K140UD12 type op-amp (with internal correction), a correction capacitor is not needed, but an additional current-setting resistor R70 is required (shown in dotted line).
Discriminator (Fig. 17) The discriminator consists of comparators on the op-amp D12.1, D12.2 and single vibrators on flip-flops D13.1, D13.2. When a metal detector sensor passes over a metal object, a useful signal appears at the filter outputs in the form of two voltage half-waves of opposite polarity, following one after another simultaneously at each output. For small iron objects, the signals at the outputs of both filters will be in phase: the output voltage will "swing" first to minus, and then to plus, and return to zero. For non-ferromagnetic metals and large iron objects, the response will be different: the output voltage of only the first (upper according to the filter circuit) will "swing" first to minus, and then to plus. The reaction at the output of the second filter will be the opposite: the output voltage will “swing” first into plus and then into minus. The output pulses of the comparators run one of the single vibrators on triggers D13.1, D13.2. The single vibrators cannot start at the same time - the cross feedback through the diodes VD9, VD11 blocks the start of the one vibrator if the other one is already running. The duration of pulses at the outputs of single vibrators is about 0,5 s, and this is several times longer than the duration of both bursts of the useful signal when the sensor moves quickly. Therefore, the second half-waves of the output signals of the filters no longer affect the decision of the discriminator - according to the first bursts of the useful signal, it triggers one of the single vibrators, while the other is blocked and this state is fixed for a time of 0,5 s. In order to exclude the operation of the comparators from interference, as well as to delay the output signal of the first filter relative to the second, integrating circuits R49, C41 and R50, C42 are installed at the inputs of the comparators. The time constant of the circuit R49, C41 is several times larger, therefore, with the simultaneous arrival of two negative half-waves from the filter outputs, the comparator D12.2 will be the first to work and the one-shot on the trigger D13.2 will start, giving out a control signal ("ferro" - iron). Sound Signal Conditioner (Fig. 17) The audio signal shaper consists of two identical controlled audio frequency generators on Schmidt triggers with AND logic at the input D14.1, D14.2. Each generator is started directly by the output signal of the corresponding discriminator single vibrator. The upper oscillator is triggered by the "metal" command from the output of the upper single vibrator - a non-ferromagnetic target or a large iron object - and produces a tone burst with a frequency of about 2 kHz. The lower oscillator is triggered by the "ferro" command from the output of the lower single vibrator - small iron objects - and produces a tonal message with a frequency of about 500 Hz. The durations of the messages are equal to the duration of the pulses at the outputs of the single vibrators. Element D14.3 mixes the signals of two tone generators. Element D14.4, connected according to the inverter circuit, is designed to implement a bridge circuit for switching on a piezoelectric emitter. Resistor R63 limits the bursts of current consumed by the D14 microcircuit, caused by the capacitive nature of the piezoelectric impedance. This is a preventive measure to reduce the effect of power interference and prevent possible self-excitation of the amplifying path. Diagram of external connections (Fig. 18)
The diagram of external connections shows elements that are not installed on the printed circuit board of the device and are connected to it using electrical connectors. These elements include:
The purpose of the listed elements is basically obvious and does not require additional explanation. Part types and design The types of microcircuits used are given in Table. four. Table 5. Types of microcircuits used
Instead of K561 series microcircuits, it is possible to use K1561 series microcircuits. You can try to use some chips of the K176 series. Dual operational amplifiers (op-amps) of the K157 series can be replaced by any single general-purpose op-amps of similar parameters (with corresponding changes in the pinout and correction circuits), although the use of dual op-amps is more convenient (the mounting density increases). It is desirable that the used types of OS are not inferior to the recommended types in terms of speed. This is especially true for D3-D5 microcircuits. Op-amps of synchronous detectors and high-pass filter integrators should approach precision op-amps in terms of their parameters. In addition to the type indicated in the table, K140UD14, 140UD14 are suitable. It is possible to use micropower op amps K140UD12, 140UD12, KR140UD1208 in the corresponding switching circuit. There are no special requirements for the resistors used in the metal detector circuit. They only need to be robust and miniature in design and easy to install. In order to obtain maximum thermal stability, only metal-film resistors should be used in the sensor circuits, integrators and in the compensation circuit. The power dissipation rating is 0,125 ... 0,25 W. Thermistor R73 must have a negative TKS and a value of about 4,7 kOhm. The recommended type of KMT is 17 W. Compensation potentiometers R74, R75 are desirable multi-turn type SP5-44 or with vernier adjustment type SP5-35. You can get by with conventional potentiometers of any type. In this case, it is advisable to use two of them. One - for rough adjustment, with a nominal value of 10 kOhm, included in accordance with the diagram. The other is for fine tuning, connected according to the rheostat circuit into the gap of one of the extreme terminals of the main potentiometer, with a nominal value of 0,5 ... 1 kOhm. Capacitors C45, C49, C51 are electrolytic. Recommended types - K50-29, K50-35, K53-1, K53-4 and other small ones. The remaining capacitors, except for the capacitors of the oscillatory circuit of the sensor, are ceramic type K10-7 (up to a nominal value of 68 nF) and metal-film type K73-17 (values above 68 nF). Circuit capacitor C61 is special. High demands are placed on it in terms of accuracy and thermal stability. Capacitor C61 consists of several (5 ... 10 pcs.) Capacitors connected in parallel. Tuning the circuit into resonance is carried out by selecting the number of capacitors and their rating. The recommended type of capacitors is K10-43. Their thermal stability group is MPO (i.e., approximately zero TKE). It is possible to use precision capacitors and other types, for example, K71-7. In the end, you can try to use the old thermostable mica capacitors with silver plated KSO type or some polystyrene capacitors. Diodes VD1-VD12 type KD521, KD522 or similar low-power silicon. It is also convenient to use integral bridge diode assemblies of the KD1 type as diodes VD4-VD5 and VD8-VD906. The conclusions (+) and (-) of the diode assembly are soldered together, and the conclusions (~) it is included in the circuit instead of four diodes. Protective diodes VD13-VD14 of types KD226, KD243, KD247 and other small ones for a current of 1 A. Microammeters - any type for a current of 50 μA with zero in the middle of the scale (-50 μA ... 0 ... + 50 μA). Small-sized microammeters are convenient, for example, type M4247. Quartz resonator Q - any small-sized watch quartz (similar ones are also used in portable electronic games). The switch of modes of operation - any type small-sized rotary biscuit or cam on 5 positions and 6 directions. Batteries of type 3R12 (according to the international designation) or "square" (according to ours). Piezo emitter Y1 - can be type ЗП1-ЗП18. Good results are obtained when using piezo emitters of imported telephones (they go in huge quantities "to waste" in the manufacture of telephones with caller ID). Connectors Х1-ХЗ - standard, for soldering on a printed circuit board, with a pin pitch of 2,5 mm. Such connectors are widely used at present in televisions and other household appliances. The X4 connector must be of external design, with metal external parts, preferably with silver-plated or gold-plated contacts and a sealed cable outlet. The recommended type is PC7 or PC10 with threaded or bayonet connection. Printed circuit board The design of the device can be quite arbitrary. When designing it, the recommendations outlined below in the paragraphs on sensors and housing design should be taken into account. The main part of the elements of the circuit diagram of the device is located on the printed circuit board.
The printed circuit board of the electronic part of the metal detector can be made on the basis of a ready-made universal breadboard printed circuit board for the DIP package of microcircuits with a pitch of 2,5 mm. In this case, the installation is carried out with a single-core tinned copper wire in insulation. This design is convenient for experimental work. A more accurate and reliable PCB design is obtained by routing tracks in the traditional way for a given circuit. Due to its complexity, in this case the printed circuit board must be double-sided metallized. The topology of printed tracks used by the author is shown in fig. 19 - side of the printed circuit board from the side of the installation of parts and in fig. 20 - side of the printed circuit board from the soldering side. The topology drawing is not actual size. For the convenience of making a photomask, the author gives the size of the printed circuit board along the outer frame of the picture - 130x144 (mm). PCB Features:
The density of the conductors on the printed circuit board is not too high, which makes it possible to make a drawing for etching at home. To do this, it is recommended to use a thin glass drawing pen or a sawn-off syringe needle complete with a plastic tube.
A common reagent for etching a standard printed circuit board made of fiberglass with copper foil 35 ... 50 microns is an aqueous solution of ferric chloride FeCl3. There are other ways to make printed circuit boards at home. The location of the parts on the printed circuit board is shown in fig. 21 (microcircuits, connectors, diodes "and a quartz resonator), in fig. 22 (resistors and jumpers) and in fig. 23 (capacitors).
Setting up the device It is recommended to set up the device in the following sequence. 1. Check the correct installation according to the circuit diagram. Make sure that there are no short circuits between adjacent PCB conductors, adjacent microcircuit legs, etc. 2. Connect batteries or a bipolar power supply, strictly observing the polarity. Turn on the device and measure the consumed current. It should be about 40 mA on each power rail. A sharp deviation of the measured values from the indicated value indicates incorrect installation or malfunction of the microcircuits. 3. Make sure that there is a pure meander at the output of the generator with a frequency of about 32 kHz. 4. Make sure that there is a meander with a frequency of about 2 kHz at the outputs of triggers D8. 5. Make sure that there is a sawtooth voltage at the output of the first integrator, and an almost sinusoidal voltage with zero constant components at the output of the second. Attention! Further adjustment of the device must be carried out in the absence of large metal objects near the metal detector sensor coil, including measuring instruments! Otherwise, if these objects are moved or the sensor is moved relative to them, the device will be out of tune, and if there are large metal objects near the sensor, tuning will not be possible. 6. Make sure that the power amplifier is working by the presence of a sinusoidal voltage at its output with a frequency of 8 kHz with a zero constant component (with the sensor connected). 7. Adjust the oscillatory circuit of the sensor to resonance by selecting the number of capacitors of the oscillatory circuit and their rating. Tuning is controlled roughly - by the maximum amplitude of the circuit voltage, exactly - by a phase shift of 180 ° between the input and output voltages of the power amplifier. 8. Replace the resistor element of the sensor (resistors R71-R73) with a fixed resistor. Choose its value so that the input and output voltages of the power amplifier are equal in amplitude. 9. Make sure that the receiving amplifier is working, for which check the mode of its op-amp and the signal flow. 10. Make sure that the higher harmonics compensation circuit is working. Adjustment potentiometers R74, R75 to achieve a minimum fundamental harmonic signal at the output of the receiving amplifier. By selecting an additional resistor R8, to achieve a minimum of higher harmonics at the output of the receiving amplifier. In this case, there will be some imbalance in the fundamental harmonic. Eliminate it by setting the potentiometers R74, R75 and again achieve a minimum of higher harmonics by selecting the resistor R8, and so on several times. 11. Make sure that the synchronous detectors are working. With a properly configured sensor and a properly configured compensation circuit, the output voltages of the synchronous detectors are set to zero approximately at the middle position of the potentiometer sliders R74, R75. If this does not happen (in the absence of installation errors), it is necessary to fine-tune the sensor circuit and select its resistor element more accurately. The criterion for the correct final adjustment of the sensor is the balancing of the device (ie setting zero at the outputs of synchronous detectors) in the middle position of the potentiometer sliders R74, R75. When adjusting, make sure that near the balancing state, only device W74 reacts to the movement of the handle of the potentiometer R1, and only the device W75 reacts to the movement of the handle of the potentiometer R2. If the movement of the handle of one of the potentiometers near the balancing state is reflected on two devices at the same time, then you should either put up with this situation (it will be somewhat more difficult to balance the device each time it is turned on), or more accurately select the value of the capacitor C14. 12. Make sure the filters are working. The constant component of the voltage at their outputs should not exceed 100 mV. If this is not the case, you should change the capacitors C35, C37 (even among the film type K73-17 there are units defective with leakage resistance - tens of megaohms). It may also be necessary to replace the OU D10 and D11. Make sure that the filters respond to a useful signal, which can be simulated by small turns of the R74, R75 knobs. It is convenient to observe the output signal of the filters directly using pointer devices W1 and W2. Make sure that the output voltage of the filters returns to zero after exposure to large amplitude signals (no later than a couple of seconds). It may turn out that an unfavorable electromagnetic environment will make it difficult to adjust the device. In this case, the arrows of the microammeters will make chaotic or periodic oscillations when the device is configured in the switch positions S1 "Mode 1" and w "Mode 2". The described undesirable phenomenon is explained by interference of the higher harmonics of the 50 Hz network on the sensor coil. At a considerable distance from the wires with electricity, the arrows should not fluctuate when the device is tuned. A similar phenomenon can also be observed in the case of self-excitation of the OA of integrators. 13. Make sure that the discriminator and the sound signal generation circuit are working. 14. Perform thermal compensation of the sensor. To do this, you first need to set up and balance the metal detector with a resistor instead of a resistive sensor element. Then slightly heat the sensor on the radiator or cool it in the refrigerator. Note in what position of the slider of the "metal" R74 potentiometer the device will be balanced when the temperature of the sensor changes. Measure the resistance of the resistor temporarily installed in the sensor and replace it with a circuit R72, R73, R78 with a thermistor and resistors of such ratings that the total resistance of the indicated circuit would be equal to the resistance of the constant resistor being replaced. Keep the sensor at room temperature for at least half an hour and repeat the experiment with a change in temperature. Compare the results. If the balancing point on the scale of the R74 slider shifts to one side, then the sensor is undercompensated and it is necessary to increase the influence of the thermistor, weakening the shunting effect of the resistor R72, for which increase its resistance, and reduce the resistance of the additional resistor R71 (to keep the resistance value of the entire chain constant) . If the balancing point for these two experiments is shifted in different directions, then the sensor is overcompensated and it is necessary to weaken the influence of the thermistor by increasing the shunting effect of the resistor R72, for which purpose reduce its resistance, and increase the resistance of the additional resistor R71 (to keep the resistance value of the entire chain constant) . Having carried out several experiments with the selection of resistors R71 and R72, it is necessary to ensure that the tuned and balanced device does not lose its ability to balance when the temperature changes by 40 ° C (cooling from room temperature to the temperature of the refrigerator freezer). If there are malfunctions and deviations in the behavior of individual components of the metal detector circuit, you should act according to the generally accepted method:
Author: Shchedrin A.I.
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