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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING A metal detector underground will find. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / metal detectors The metal detector I developed has not yet been used either in peacekeeping operations to identify and neutralize minefields, or in large-scale geological or archaeological surveys. Designed not for professionals, but for amateurs, whose desire to "look under the ground" is able to satisfy the design with the parameters given in the table, it is an improved version of the "beating metal detector". The sensitivity of the device is increased due to the beneficial use (clear fixation) of the dependence of the duration of the probing pulse on the intensity of the parcels themselves with the introduction of automatic frequency control (AFC) into the search generator. Moreover, additional measures for voltage stabilization and temperature compensation of electronic units were not required. And the "irreconcilable contradictions" predicted by skeptics (they say that a change in the frequency of the search oscillatory circuit when metal enters the working area is incompatible with the normal functioning of the AFC system) was resolved by the practice itself. It turned out that when the sensor moves over the surface under study at a speed of 0,5-1 m/s, the device circuit does not conflict with auto-tuning of the frequency, which has a significant inertia (large time constant).
Already from the analysis of the block diagram it is clear that it is obviously more difficult to manufacture such a device than any of the previous less sensitive analogues. After all, the development I propose, in addition to the standard set of exemplary quartz (1) and measuring (2) generators, an external inductor L (search frame-sensor), a mixer (3) and a sound recorder BA (telephone capsule), there are new, significantly improving performance, devices. This is an integrator (4), which generates a sawtooth signal with an amplitude proportional to the control beat frequency, and a write pulse shaper (5), which, together with a key (6) and a source follower VT, is an analog storage device that fixes the peak voltage from the integrator. A metal detector cannot do without a comparator (7), which provides automatic transfer of electronics from the zone of maximum sensitivity to the one-to-one beat registration area (and vice versa), without a special VCO generator (8), which converts the voltage generated at the source follower into electrical oscillations frequency 200-8000 Hz. and also without the above-mentioned original auto-tuning system of the AFC (9) with a special unit that slows down the response of the device to an excessively sharp change in the control voltage. 10). Technical specifications
As practice shows, it is this composition of devices with the chosen method of generating an audio signal that allows you to listen to both frequencies simultaneously, greatly facilitating the initial tuning of the device to a certain sensitivity. And the reliability is quite high. Even in an extreme situation, when, say, a search frame-sensor approaches a massive metal object at a distance at which the difference frequency becomes almost critical (70 Hz), there are no malfunctions - only a changing beat frequency is heard in the headphones. Now about the particulars that are reflected in the circuit diagram. The exemplary generator is made on the element DD1.1. Its frequency is stabilized by a ZQ1 quartz resonator included in a positive feedback circuit. To ensure the excitation of the generator when the power is turned on, resistor R1 is used. The buffer element DD1.2 available here unloads the generator, and also generates a signal with digital levels. Resistor R2 determines the degree of load and the maximum power dissipated in the quartz resonator.
This generator can work with almost any resonator at a current consumption of 500-800 μA. And the frequency divider following it by two (element DD2.1) generates a signal with a symmetrical meander, which is necessary for the normal operation of the mixer. The measuring generator is assembled according to the scheme of an asymmetric multivibrator (transistors VT1 and VT2). The output to the self-excitation mode provides a positive feedback circuit on the capacitor C7. The frequency-setting elements are C3 - C5, VD1 and the search coil-sensor L1. Moreover, the generation is carried out in the range from 500 kHz to 700 kHz, depending on the available quartz resonator. Such an important parameter as short-term instability is small for this generator. The frequency drift for the first 10 s immediately after turning on the power is no more than 0,7 Hz (and every 30 minutes - up to 20 Hz), although even 1 Hz per 1 minute is considered acceptable for normal operation of the device (without AFC). The sinusoidal signal produced by the measuring generator, having an amplitude of 1 - 1,2 V, is fed through the separating capacitor C9 to the trigger DD3.2, which generates rectangular pulses with digital levels and a duty cycle of 2. R5R6 is a divider necessary for the normal operation of this section of the circuit. Well, a DD3.3 acts as a buffer stage. The signal from it is fed to the mixer (T-trigger DD2.2). The frequency from the divider of the exemplary generator also comes there.
The features of the DD2.2 operation are such that if two pulse sequences that are close in frequency come to the inputs C and D of this logic element, then a difference frequency signal with a strictly symmetrical meander is formed at the outputs. Moreover, everything removed from the output 12 of the mixer has the shape shown in Figure 2a. Direct, as well as delayed (Fig. 2b) inverted (due to the R8C11 circuit and element DD4.2) signals are summed on the DD5.1 key, which acts as a logical AND / OR with the formation of short positive recording pulses (Fig. 2c) for the operation of analog storage devices (DD5.2, C13. VT3). But that is not all. The signal taken from the output of DD4.2 comes to the integrator, made according to the classical scheme using VD2, R10 - R11, DA1, C12. Resistor R11 limits the recharge current of the capacitor C12, unloading the output of the element DD4.2. Integrated signal (Fig. 2d) through the DD5.2 key. which is controlled by pulses from DD5.1, is fed to the storage capacitance C13, where a voltage equal to the peak value of what comes from the integrator is formed and maintained with high accuracy until a new recording cycle (Fig. 2e). Capacitor C14 smooths out the "step" type effect, which can occur with a sharp change in beat frequencies (Fig. 2f). From the source follower, the signal goes to the DD4.3 comparator, VCO (voltage controlled generator) and to the AFC loop circuit. The divider R21R22 together with R23 and R24 feedback narrow the control voltage range to an amplitude of 1,2 V. The operational amplifier DA2 compares the received with that given by the divider R26R29, and generates the control voltage of the VD1 varicap. Resistor R26 can set the initial capture point of the AFC (sensitivity) roughly, and R27 - exactly. Moreover, when moving the R26 slider towards the extreme (upper or lower according to the scheme) position, it is easy to leave the AFC capture zone (± 300 Hz), implementing the one-to-one beat frequency mode, which makes working with the device more flexible. To understand the features of the functioning of the node, which slows down the response of the AFC to a sharp change in the beat frequency, we assume that on the basis of the transistor VT4 there is, for example, some steady Ub. We also assume that at some point there is a sharp change in the beat frequency and, accordingly, the voltage across C14. A working circuit of our metal detector will definitely respond to such an "introductory" "adequate deviation of the Ub transistor VT4 from its previous value (due to the large ratings of R19, R20 and C16). But the response to a smooth change in the beat frequency will certainly be a reaction in the form of a slow change in the named voltages. When a metal object enters the sensitivity zone of the search frame-sensor and stays there for a relatively long time, a voltage is set on the basis of VT4, which is usually enough to return to the specified frequency mode. But with a sharp removal of the sensor to the side, the situation changes, Ub of the VT4 transistor will not be able to quickly return to the previous level. That is, conditions are created for the transition through "0" (the occurrence of positive feedback). To exclude the latter, shunting of R19 with a VD3 diode was introduced, through which the capacitance C16 is quickly discharged (Ub returns to the set level). In fact, the AFC has (depending on which direction the beat frequency changes) two time constants. And since the special design of the sensor practically eliminates the influence of the ferromagnetic properties of the objects being detected on the increase in f of the search generator, then both the AFC and the device as a whole work very correctly in all modes. VCO (DD4.4, and R18, C15) converts the voltage, which changes with the beat frequency, into a frequency. And the DD16 comparator configured using the divider R17R4.3 allows him to do this in the zone of maximum sensitivity. The VCO frequency is fed to input A of the mixer (key DD5.4). The input CO comes from the logic element DD4.1 and the difference f beats, and a short negative pulse formed by the differentiating circuit C10R9 (for better sounding of headphones, reducing power consumption). As a result, either the modulated VCO frequency or only the beat frequency is present at the mixer output. Moreover, the scheme performs the transition from one mode to another automatically. The variable resistor R30 serves as a load and volume control, and SA1 combined with it serves as a power switch. The use of microcircuits of the CMOS series, operational amplifiers operating in the microcurrent mode, made it possible to reduce the current consumption to the level of 6 mA, making it acceptable to use the Krona battery as a power source. Like other analogues, almost the entire metal detector is mounted on a printed circuit board made of one-sided foil-coated fiberglass. The search generator is placed in a shielding box made of tin.
Only the control resistors R26, R27, R30, sockets for connecting the power supply and headphones, as well as the sensor frame are taken out of the dimensions of the board. DD1 K561LA8; DA1-DA2 KR140UD1208; DD2 K561TM2; VT1-VT3 KP303A; DD3 K176LP4; VT4 KT3102G; VD1 D902; VD2-VD3 KD522 The technology and care taken in the manufacture of the sensor frame are so important for the performance of the entire metal detector that they apparently require a more detailed presentation. A bundle made up of eleven 1100-mm pieces of wire PEV2-1,2 is used here as a basis. Wrapped tightly with a layer of electrical tape, it is squeezed into an aluminum tube having an inner diameter of 10 mm and a length of 960 mm. The resulting blank is shaped into a rectangular frame 300x200 mm with rounded corners.
The end of the first of the wires, placed in an aluminum case - an electrostatic screen, is successively soldered to the beginning of the second, and so on until a kind of 11-turn inductor is formed. Spikes are isolated from each other with paper tape and filled with epoxy resin, while excluding the appearance of a short-circuited coil due to the tube itself bent into a frame. It is advisable to provide here any closed high-frequency connector and a suitable (not metal) mount for the handle bar, which can be used as one or two sections from a collapsible rod. The cable connecting the frame to the block is better to use coaxial, television, for example, PK75. Choke L2 search generator (designation hereinafter - according to Fig. 1 and in accordance with the circuit diagram of the metal detector, published in the previous issue of the magazine) has 450 turns of wire PEL1-0,01. Winding - in bulk on a frame with a diameter of 4 and a length of 15 mm with a M600NN ferromagnetic core (you can use a suitable contour coil from an old radio). The inductance of such a choke is 1-1,2 mH. The device uses capacitors KSO or KTK (C3, C4, C5), KLS or KM (C1, C2, C6 - C13, C15), K50-6 or K53-1 (C14, C16, C17). There is also a choice of resistors. In particular, for "trimmers" R26, R27, SP5-2 or SP-3 are suitable. The same can be said about the variable R30, only it must be combined with the switch. All other resistors are MLT-0,125 (VS-0,125). Digital MS can be replaced with analogs from the well-established K176 series. DD1, DD3 - any of the same series, as long as they contain the required number of inverters. Transistors can also be replaced. As VT1 and VT2, for example, KP303B (-Zh) is suitable. In place of VT3, KP303 or KP305 is acceptable (the letter index at the end of the name does not play a role in this case), and KT3102G (VT4) will replace KT3102E. Quartz - one of those that are designed for 1,0-1,4 MHz. The choice of headphones is also unlimited. As practice shows, TON-1 or TON-2 are quite suitable. Varicap D901 can be replaced by D902. Diodes VD2 and VD3 KD522 (KD523) with any letter index. To set up the assembled device, you will need an oscilloscope and ... accuracy in work. Having carefully examined the entire installation, power is supplied to the circuit. Then they check the current consumption, which for a properly executed workable design should be 5.5 - 6,5 mA. When going beyond the specified values, they look for and eliminate errors in soldering, etc. The functioning of the exemplary generator is verified by the presence at pin 1 of the DD2 microcircuit of a frequency equal to 0,5 f of a quartz resonator with a duty cycle of 2. Then they go to the "search engine." Half the supply voltage is applied to the control point on the printed circuit board, where R3 and C8 converge, while disconnecting the output of the DA2 microcircuit. And with an oscilloscope connected to the drain of the transistor VT2, they check the amplitude of the output voltage. It should be from 1 V to 1,2 V. If the deviation exceeds 0,1 V, correct the number of turns in the inductor L2. With the help of capacitors C3 and C4, the optimal signal frequency is set equal to 0.5 fquartz. Moreover, the sensor itself should be located no closer than two meters from metal objects. If necessary, selecting R5, they seek to obtain a symmetrical output signal at pin 9 of the DD3 microcircuit (in this case, the mixer must output a difference frequency signal with a meander equal to 2). Then, having set the beat frequency equal to 8-9 Hz by changing the voltage on the varicap, the signal at pin 6 of the DA1 integrator is measured - it should be "on the verge of limiting from below." The corresponding adjustment is carried out by selecting the value of the resistor R10. By connecting an oscilloscope to the source of the transistor VT3, they check the change in the voltage level depending on the beat frequency. Resistors R16 and R17 ensure that a logical zero at the output of the comparator (pin 10 of the DD4 chip) appears only when f beats becomes higher than 70 Hz. The VCO is adjusted with resistor R15 so that the oscillator starts working when the integrator signal "leaves the limit from below". In the future, this will greatly simplify the adjustment of the device before operation, since the minimum frequency of the VCO will correspond to the setting of the metal detector for maximum sensitivity. Having restored on the printed circuit board the previously soldered connection R3 and C8 with DA2, they proceed to the final stage of debugging the device. The R26 "trimmer" engine is turned to the extreme ("plus") position, which will correspond to the maximum beat frequency (moreover, f search generator > f exemplary. Then, slowly rotating the engine in the opposite direction, they begin to control the signal at pin 6 of DA1. They notice how (at a certain position of the R26 slider) the moment the signal hits the AFC capture zone appears on the oscilloscope screen. By continuing to turn the knob of the tuning resistor R27, they achieve a beat frequency of 10 Hz, while simultaneously checking the operation of the AFC (as the signal tends to return to its original state). The engines of resistors R26, R27 must be moved slowly, given the large inertia of the AFC. In this case, the minimum VCO frequency and weak clicks from f beats will be heard in the headphones. In some cases, there may be an effect of "floating" of the sound relative to some fixed state. In this case, it is necessary to more accurately select the ratio of resistors R23, R24 or reduce the values of R19, R20. As already noted, the electronic part of the metal detector (and this is almost the entire device) can be mounted in any suitable case mounted on the handle. Care must be taken that the search frame-sensor, as well as the connecting wires, are rigidly fixed relative to each other. After all, even slight vibrations of these parts that occur when the operator moves can generate a false signal (especially with maximum sensitivity of the circuit and insufficient experience with the device). For the same reason, the spatula should be worn behind the back with the bayonet up (away from the sensor frame). And metal tips on the operator's boot laces are generally unacceptable. The interference they bring threatens to negate all the efforts of the ultra-sensitive device to find in the earth what it is so reluctant to part with. Working with a metal detector is not much different from working with a modern manual mine detector. Of course, such precise instruments need adjustment. In our particular case, this is the rotation of the trimmer resistor R26 to the extreme ("positive") position, and R27 to the middle one. After applying power to the equipment, rotate the R26 adjustment knob in the opposite direction until the VCO signal appears in the headphones. After that, the required sensitivity is set with a tuned resistor R27. And with the help of R26, they arbitrarily set (when working with the device in the one-to-one beat mode) f beats in the range of 200-300 Hz. AFC and VCO are essentially disabled, so the search is carried out as usual. To more clearly determine the location of small objects, the sensor frame is brought to the search area either horizontally (with a rounded corner forward) or at an inclination of 45-90 ° to the surface under study (with a clear positional advantage of one of the sidewalls of the frame). Author: Yu.Stafiychuk
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