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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Beat metal detector. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / metal detectors The proposed metal detector is designed for "near" search for objects. It is assembled according to the simplest scheme. The device is compact and easy to manufacture. The detection depth is:
Structural scheme The block diagram is shown in fig. 8. It consists of several functional blocks. The crystal oscillator is a source of rectangular pulses of stable frequency.
An oscillatory circuit is connected to the measuring generator, which includes a sensor - an inductor. The output signals of both generators are fed to the inputs of a synchronous detector, which generates a difference frequency signal at its output. This signal has an approximately sawtooth shape. For the convenience of further processing, the synchronous detector signal is converted using a Schmidt trigger into a rectangular signal. The display device is designed to generate a sound signal of the difference frequency using a piezoelectric emitter and to visually display the value of this frequency using an LED indicator. Schematic diagram The schematic diagram of the beat detector developed by the author is shown in fig. 9.
The crystal oscillator has a circuit similar to that of a metal detector generator based on the "transmit-receive" principle, but implemented 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 ("watch quartz"). The R1C2 circuit prevents the excitation of the generator at higher harmonics. Through the resistor R2, the PIC circuit is closed, and through the resonator Q, the PIC circuit is closed. The generator is characterized by simplicity, low current consumption from the power source, reliable operation at a supply voltage of 3..15 V, does not contain tuned elements and too high-resistance resistors. The output frequency of the generator is about 32 kHz. An additional counting trigger D2.1 is needed to generate a signal with a duty cycle exactly equal to 2, which is required for the subsequent synchronous detector circuit. measuring generator The generator itself is implemented on a differential stage on transistors VT1, VT2. The POS circuit is implemented galvanically, which simplifies the circuit. The load of the differential stage is the oscillatory circuit L1C1. The generation frequency depends on the resonant frequency of the oscillatory circuit and, to some extent, on the mode current of the differential stage. This current is set by resistors R3 and R3'. Adjusting the frequency of the measuring generator when setting up the device is carried out roughly - by selecting the capacitance C1 and smoothly - by adjusting the potentiometer R3'. To convert the low-voltage output signal of the differential stage to the standard logic levels of digital CMOS microcircuits, a cascade is used according to the scheme with a common emitter on the transistor VT3. The shaper with a Schmidt trigger at the input on the D3.1 element provides steep pulse edges for the normal operation of the subsequent counting trigger. An additional counting trigger D2.2 is needed to generate a signal with a duty cycle exactly equal to 2, which is required for the subsequent synchronous detector circuit. Synchronous detector The detector consists of a multiplier implemented on the D4.1 "XOR" element and an R6C4 integrating circuit. Its output signal is close in shape to a sawtooth, and the frequency of this signal is equal to the difference between the frequencies of the quartz oscillator and the measuring oscillator. Schmidt trigger The Schmidt trigger is implemented on the D3.2 element and generates rectangular pulses from the sawtooth voltage of the synchronous detector. Display device It is simply a powerful buffer inverter, implemented on the remaining three inverters D1.4-D1.6, connected in parallel to increase the load capacity. The load of the display device is the LED and the piezo emitter. Part types and design The types of microcircuits used are given in Table. four. Table 4. 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. The inputs of unused elements of digital circuits must not be left unconnected! They should be connected either to a common bus or to a power bus. Transistors VT1, VT2 are elements of an integrated transistor assembly of the K159NT1 type with any letter. They can be replaced by discrete transistors with npn conductivity of the types KT315, KT312, etc. Transistor VT3 - type KT361 with any letter or similar type with pnp conductivity. There are no special requirements for the resistors used in the metal detector circuit. They just need to be sturdy and easy to install. The rated power dissipation should be 0,125 ... 0,25 W. Compensation potentiometer R3' is 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 desirable to use two connected in series. One - for rough adjustment, with a nominal value of 1 kOhm. The other is for fine tuning, with a nominal value of 100 ohms. The inductor L1 has an internal winding diameter of 160 mm and contains 100 turns of wire. Wire type - PEL, PEV, PELSHO, etc. Wire diameter 0,2...0,5 mm. See below for coil design. Capacitor C3 is electrolytic. Recommended types - K50-29, K50-35, K53-1, K53-4 and other small ones. The remaining capacitors, with the exception of the capacitor of the oscillatory circuit of the coil of the measuring generator, are ceramic type K10-7, etc. The circuit capacitor C1 is special. High demands are placed on it in terms of accuracy and thermal stability. The capacitor consists of several (5 ... 10 pieces) individual capacitors connected in parallel. Rough tuning of the circuit to the frequency of the quartz oscillator 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, such as K71-7. In the end, you can try to use thermostable mica capacitors with silver plates such as KSO or polystyrene capacitors. LED VD1 type AL336 or similar with high efficiency. Any other LED in the visible range of radiation will do. Quartz resonator Q - any small-sized watch quartz (similar ones are also used in portable electronic games). 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). The design of the device can be quite arbitrary. When developing it, it is desirable to take into account the recommendations outlined in the sections on sensors and housing design. The printed circuit board of the electronic part of the metal detector can be made by any of the traditional methods; it is also convenient to use ready-made breadboard printed circuit boards for the DIP package of microcircuits (2,5 mm pitch). 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 the battery or 9V power supply, strictly observing the polarity. Turn on the device and measure the consumed current. It should be around 10mA. A sharp deviation from the specified value indicates incorrect installation or malfunction of the microcircuits. 3. Make sure that there is a pure meander with a frequency of about 3.1 kHz at the output of the crystal oscillator and at the output of element D32. 4. Make sure that there are signals with frequencies of about 2.1 kHz at the outputs of triggers D2.2 and D16. 5. Make sure that there is a sawtooth voltage of difference frequency at the input of element D3.2, and rectangular pulses at its output. 6. Make sure that the display device is working - visually and aurally. Possible modifications The scheme of the device is extremely simple and therefore we can only talk about further improvements. These include: 1. Adding an optional LED logarithmic frequency indicator. 2. Using a transformer sensor in a measuring generator. Let's take a closer look at these modifications. Logarithmic frequency indicator The logarithmic frequency indicator is an advanced LED indicator. Its scale consists of eight individual LEDs. When the measured frequency reaches a certain threshold, the corresponding LED lights up on the scale, the remaining seven do not light up. A feature of the indicator is that the frequency response thresholds for neighboring LEDs differ from each other by a factor of two. In other words, the indicator scale has a logarithmic graduation, which is very convenient for a device such as a beating metal detector. Schematic diagram of the logarithmic frequency indicator is shown in fig. 10. Despite the fact that the scheme of this indicator was developed by the author independently, it does not claim to be original, since a patent search has shown that such schemes are known. Nevertheless, both the indicator scheme itself and its implementation on the domestic element base are, in the author's opinion, of some interest.
The logarithmic indicator works as follows. The input of the indicator receives a signal from the output of the Schmidt trigger of the beating metal detector circuit (see Fig. 9). This signal is the input for binary counters D5.1-D5.2 (the numbering continues according to the scheme in Fig. 9). These counters are periodically reset by a high level signal from the auxiliary oscillator on the Schmidt trigger D3.3 with a frequency of about 10 Hz. On the rising edge of the auxiliary generator signal, the state of the counters is also written to the parallel four-bit registers D6 and D7. Thus, at the outputs of registers D6 and D7, there is a digital code for the frequency of the beat signal. It is quite easy to convert this code into a logarithmic scale (and this is the "highlight" of this scheme), if the corresponding LED on the scale is set to correspond to the appearance of one in a certain bit of the frequency code with all zeros in the higher bits of the code. Obviously, this task should be performed by a combinational circuit. The simplest implementation of such a scheme is a periodically repeating links of OR elements. In the practical circuit, elements OR-NOT D8, D9 are used together with powerful buffer inverters D10, D11. At the output of the circuit, a logical signal for controlling the scale LEDs is obtained in the form of a "wave of units". From the point of view of saving battery power, of course, it is more advisable to make the scale not in the form of a luminous column of LEDs (up to 8 pieces at a time), but in the form of a moving dot from one luminous LED. To do this, the LEDs of the indicator line are connected between the outputs of the combinational circuit. For very low frequencies, the indication in the form of a flashing LED is still more suitable. In the proposed scheme, it is combined with the beginning of the LED scale and goes out as soon as its next segment lights up. By selecting elements R8, C5, you can change the frequency value of the auxiliary generator, thus changing the frequency scale limit. Part types and design The types of microcircuits used are given in Table. four. Table 4. 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. The power supply wiring and pin numbering for the D8-D11 microcircuits are not conventionally shown for simplicity. LEDs VD2-VD9 type AJ1336 or similar with high efficiency. Their current-setting resistors R9-R17 have the same value of 1,0 ... 5,1 kOhm. The lower the resistance of these resistors, the brighter the LEDs will glow. However, in this case, the load capacity of the K561LN2 microcircuits may not be enough. In this case, it is recommended to use output inverters connected in parallel in the indicator circuit. It is most convenient to organize this parallel connection by simply soldering additional microcircuit cases of the same type (up to 4 pieces) on top of each of the K561LN2 microcircuits installed in the circuit. transformer sensor The idea of a transformer detector for metal detectors is simple and elegant. It has been known for a long time and arose from the desire to simplify the design of the metal detector sensor coil. A common disadvantage of a typical metal detector sensor of any design is a large (more than 100) number of coil turns. As a result, insufficient rigidity of the sensor design is obtained, which requires the adoption of special measures such as additional frames, epoxy casting, etc. In addition, the parasitic capacitance of such a coil is large, and to eliminate false signals due to the capacitive coupling of the coil (coils) with the ground and the operator's body, it is necessary to shield the windings. The way to eliminate the listed shortcomings is simple and obvious - it is necessary to use a coil consisting of a minimum number of turns - from one turn! Naturally, such a solution does not work "on the forehead", since the insignificant inductance of one turn would require gigantic capacitances of oscillatory circuit capacitors, signal generators with a huge output current and special tricks to ensure high quality factor. And here it's time to recall the existence of a device designed to match impedances, to convert alternating signals of high voltage with low current into low voltage signals with high current, and vice versa about the transformer. Indeed, let's take a transformer with a transformation ratio of about a hundred and connect its step-down winding to one turn, which is a metal detector sensor, and the step-up winding to the metal detector circuit instead of an inductor. Structurally, one turn of such a transformer sensor can be made in a variety of ways. For example, it can be a ring of copper or aluminum single-core wire with a cross section of 6 ... 10 mm2 for copper and 10...35 mm2 for aluminium. The inner conductors of power cables are convenient for use. It is possible to make a coil from a metal tube to reduce weight and increase rigidity. It is possible to manufacture a coil of foil by sticking to sheet material and even from ordinary foil fiberglass. In any convenient place, the coil is grounded by connecting to the common bus of the device, which provides compensation for parasitic capacitive couplings. The effect of these connections with a given design of the sensor is several orders of magnitude smaller due to the lower value of the total resistance modulus of one turn. The transformer sensor makes it possible to implement the folding design of a compact beat-out metal detector. Her sketch is shown in Fig. 11. The sensor transformer is made on a toroidal magnetic core installed directly on the metal detector board, placed in a plastic case. The step-down winding of the transformer and the coil of the sensor are structurally a single whole in the form of a rectangular frame made of copper insulated single-core wire with a cross section of 6 mm2, closed by soldering. The specified frame has the ability to rotate. In the folded position, the frame is located along the perimeter of the device body and does not take up extra space. In working position, it turns 180°. In order for the frame to be fixed in the installed position, sealing bushings made of rubber or other similar material are used. It is also possible to use any other suitable mechanical retainers for the frame.
The cross section of the conductor from which the coil of the transformer sensor is made must be no less than the total cross section of all the turns that make up the usual coil of the metal detector sensor. This is necessary not only to give the structure the necessary strength and rigidity, but also in order to obtain a not too low quality factor for an oscillatory circuit with such a transformer analog of an inductor (by the way, when using such a coil as a radiating coil, the current in it can reach tens of ampere!). For the same reason, proper selection of the wire size of the step-down winding of the transformer is necessary. It may have a smaller cross section than the cross section of the coil conductor, but its ohmic resistance should not be greater than the ohmic resistance of the coil. To reduce losses due to ohmic resistance, it is necessary to very carefully connect the turn with the step-down winding of the transformer. The recommended connection method is soldering (for a copper coil) and welding in an inert gas environment (for aluminum). The requirements for a transformer are: First, it must operate with low losses at the required frequency. In practice, this means that its magnetic circuit must be made of low-frequency ferrite. Secondly, its windings should not make a noticeable contribution to the impedance of the sensor. In practice, this means that the inductance of the step-down winding must be noticeably greater than the inductance of the coil. For toroidal ferrite cores with magnetic permeability μ\u2000d 30 and with a diameter of more than XNUMX mm, this is true even for one turn of the step-down winding. Thirdly, the transformation ratio must be such that the inductance of the step-up winding with the sensor turn connected to the step-down winding would be approximately the same as that of a conventional coil of a typical sensor. Unfortunately, the advantages of the transformer sensor far outweigh its disadvantages for beat detectors only. For more sensitive devices, such a sensor is not applicable due to the rather high sensitivity to mechanical deformations, which leads to false signals that appear during movement. This is why the transformer detector is only covered in the beat detector section. Author: Shchedrin A.I.
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