As practice shows, such a structure of devices for the selected method of forming the acoustic signal allows you to listen to both frequencies simultaneously, significantly facilitating the initial configuration of the device at a specific sensitivity. And the reliability is quite high. Even in an extreme situation, when, say, a search frame sensor approaches the massive metal object at the distance at which the difference frequency becomes almost critical (70 Hz), malfunction does not occur in the head phones is heard only by changing the frequency of the beating.
Now about the particulars reflected on the wiring diagram. Model generator executed on the elements DD1.1. Its frequency is stabilized by quartz resonator ZQ1 included in the chain of positive feedback. To ensure excitation of the generator when the power is resistor R1. Available here, the buffer element DD1.2 unloads the generator, and generates a signal with digital levels. Resistor R2 determines the degree of loading and maximum power dissipated in the quartz resonator.
This generator can operate with virtually any resonator with a current consumption of 500-800 mA. And behind him the frequency divider by two (element DD2.1) generates a signal with a symmetrical square wave required for normal operation of the mixer.
Measuring the generator is assembled according to the scheme of the asymmetric multivibrator (transistors VT1 and VT2). The output of the mode-excitation circuit provides a positive feedback capacitor C7. Customizability the elements of Sz — C5, VD1 and search-coil sensor L1. And generation is in the range from 500 kHz to 700 kHz, depending on crystal.
Fig. 1. Block diagram of metal detector
Fig. 2. Plots of voltages and currents at the test points of the device.
Fig. 3. Circuit diagram of metal detector.
Such an important parameter as short-term instability, this small generator. Frequency drift during the first 10 s immediately after turning on the power supply is not more than 0.7 Hz (and every 30 min up to 20 Hz), although for normal operation of the device is acceptable even 1 Hz for 1 min (without AFC).
Issued by measuring the generator sinusoidal signal, with amplitude 1-1. 2 V, is fed via a separation capacitor C9 to the trigger DD3.2, which generates rectangular pulses with digital levels and duty cycle 2. R5R6 — divider necessary for the normal operation of this part of the circuit. Well, a DD3.3 performs the role of a buffer cascade. The signal is fed to the mixer (T-flip-flop DD2.2). There also comes from the frequency divider model generator.
Features of work DD2.2 is such that if the inputs C and D of the logical element come two pulse sequences that are close in frequency, it outputs a signal of the difference frequency with a strictly symmetrical square wave. All taken from the output of the mixer 12 has the shape shown in figure 2A.
Direct, as well as the detainee (Fig. 26) proinvestirovany (because of the chain R8C11 and the element DD4.2) the signals are summed on the key DD5.1, performing the role of a logical AND/OR with the formation of short positive pulses of the recording (Fig. 2B) to operate the analog storage device (DD5.2, C13, VT3). But that’s not all. Remove from the output DD4.2, the signal arrives at the integrator, performed by the classical scheme using VD2, R10 — R11, DA1, C12. Resistor R11 limits the current recharging the capacitor C12, unloading the output of the element DD4.2.
Integrated signal (Fig. 2G) through the key DD5.2, which is controlled by pulses from DD5.1, is supplied to the storage capacitance C13, which is formed and until a new write cycle is held with high accuracy a voltage equal to the peak value that is supplied from the integrator (Fig. 2D). Capacitor C14 smooths out the effect of the type “step” that can occur when abrupt change of beat frequencies (Fig. 2E).
With the source follower signal is fed to a comparator DD4.3, VCO (controlled oscillator voltage) and in the loop circuit of the AFC. Divider R21R22 and R23 together with R24 feedback narrow the range of the control voltage to the amplitude of 1.2 V. the Operational amplifier DA2 compares with that given by R26R29 divider, and generates the voltage control aricopa VD1.
Resistor R26 is possible to set the initial point of capture AHR (sensitivity) roughly, a R27 — exactly. Moreover, when you move the engine of the R26 in the direction of the extreme (top or bottom) to position easily get out of the capture area aft (±300 Hz), carrying out a mode with the frequency of the beating is one-to-one, which makes the operation more flexible.
To understand the functioning of the node, slows the reaction aft an abrupt change in beat frequency, assume that the base of the transistor VT4 is available, for example, a steady-state Uб. Suppose also that at some point there is an abrupt change of the beat frequency and, accordingly, the voltage on C14. A working scheme of our IU-teledetection will respond to such “introductory” adequate deviation Uб transistor VT4 from the previous value (due to the large values of R19, R20 and C16). But the response to a smooth change of the beat frequencies will certainly be the reaction in the form of slow changes called the stress.
When in the detection zone search frame-the sensor gets a metal object and there is a relatively long time, on the basis of the set voltage VT4, which is usually enough to return at a specified frequency mode. But with a sharp withdrawal of the sensor in side the situation changes, U6 of the transistor VT4 will not be able to quickly return to the previous level. That is, the conditions for crossing the “0” (the emergence of positive feedback). To delete the last entered R19 shunting diode VD3, which occurs via a fast discharge of the capacitance C16 (U6 return for a fixed level).
In fact, aft has (depending on which way a change in the beat frequency) of two time constants. As well as a particular implementation of the sensor virtually eliminates the impact of ferromagnetic properties of items to increase fпоискового generator
, and the aft, and the instrument as a whole work in all modes very well. GONG (DD4.4, and R18, C15) converts the voltage varying with the frequency of beats, frequency. And configured using the divider R16R17 comparator DD4.3 allows him to do so in the area of maximum sensitivity, when fбиений= 0-70 Hz.
The frequency of the VCO is input to A mixer (key DD5.4). To log on to the come from the logical element DD4.1 and difference beats, and differentiating circuit formed C10R9 (for better sound of headphones, reduce power consumption) a short negative pulse. As a result, the output of the mixer is present or pratulananda fбиений the frequency of the VCO, or only the frequency of the beating. Moreover, the transition from one mode to another scheme performs automatically. Variable resistor R30 is the load and volume control, and combined it with SA1 switch power supply.
The use of chips of a series of CMOS operational amplifiers operating in microcurrent mode, reduced the current to 6 mA, making reasonable use of the battery “Krona” as a source of electrical power.
Like other counterparts (including published in the “Modeller-designer” № 8’4 89 and’96), almost all of
metallosalen in Ukraine
is mounted on the PCB of one-sided foil fiberglass. Search the generator is placed in a shielding box made of tin. The dimensions of the circuit Board contains only the adjustment of the resistance R26, R27, R30, slot connect power source and head phones and also frame sensor.
The technology and craftsmanship of the frame sensor is so important for the health of the entire detector, which require, apparently, a more detailed explanation. As the basis used here, the wiring composed of eleven 1100 mm pigtails ПЭВ2-1.2. Tightly wrapping a layer of duct tape, his wedge in an aluminum tube, having an inner diameter of 10 mm and a length of 960 mm. the workpiece shape of a rectangular frame 300×200 mm with rounded corners.
The end of the first of the wires is placed in an aluminum casing — electrostatic screen, consistently soldered to the second and so on till the formation of a kind 11-turn inductors. The spikes are isolated from each other paper tape and pour epoxy resin, except for the appearance of the closed loop at the expense of the bent tube frame.
It is desirable here to provide any closed high-frequency connector and a suitable (not metal) mount for rod-arm, which can be used one or two sections of a collapsible fishing rod. The cable that connects the frame with the block, better to use a coaxial television, for example, RK75.
The throttle 1_2 search generator (denoted hereinafter according to Fig. 1 and in accordance with the schematic electrical diagram of the detector, published in the previous issue of the journal) has 450 turns of wire PEL 1-0,01. Winding — bulk on the frame with a diameter of 4 and a length of 15 mm with a ferromagnetic core М600НН (you can use a suitable contour coil from an old radio). The inductance of this inductor 1 is 1.2 mH.
The device uses capacitors CSR or CTC (Sz, S4, S5), or KLS KM (C1, C2, C6 – C13, C15), K50-6, or K53-1 (C14, C16, C17). There is a choice and resistors. In particular, for the “trimmers” R26, R27 suitable SP5-2 or SP-3. The same can be said about AC R30, only it needs to be combined with a switch.
All other resistors — MLT-0,125 (sun-0,125).
The topology of the PCB.
Device, seeing through the earth.
Digital MS can be replaced by analogues of the well-proven series К176. DD1, DDЗ any of the same series, if only to contain the required number of inverters.
Allow replacement and transistors. As VT1 and VT2, for example, fit КПЗ0ЗБ (-W). On the spot /TK acceptable КПЗ0З or КП305 (alphabetic index at the end of the name in this case is not important), and КТ3102Г (VТ4)will replace КТ3102Е.
Quartz — one of those that is designed for a 1.0— 1.4 MHz. The choice of headphones is also not limited. As practice shows, it is quite suitable TONE-1 iliton-2. The varicap Д901 can be replaced by Д902. Diodes VD2 and VDЗ – KD522 (КД523) with any alphabetic index.
To customize the assembled device will need an oscilloscope and …accuracy in work. Carefully examined the whole installation, at the scheme serves the power supply. Then check the current consumption, which have correctly performed the functional design should be 5.5 — 6.5 mA. If the specified values to find and resolve bugs in rations and G. D.
In the operation of the exemplary generator convinced by the presence of at pin 1 of the chip DD2 frequency equal to 0.5 fкварцевого resonator
with a duty ratio of 2. Then go to “search” the control point on the PCB where R3 and C8, serves half the supply voltage by disconnecting the IC output DА2. And an oscilloscope connected to the drain of transistor VT2, check the amplitude of the output voltage. It must be from 1 V to 1.2 V. If the deviation exceeds 0.1 V. adjust the number of turns in the inductor L2.
And using capacitors Sz and S4 exhibit the optimal signal frequency of 0.5 fкварца . Moreover, the sensor must be located no closer than two meters from the metal. If necessary, selecting R5, seek to obtain a symmetrical output signal on pin 9 IC DD3 (the mixer should generate a signal of the difference frequency with a square wave, equal to 2). Then, setting the change in voltage on the varicap the frequency of the beating is equal to the 8 — 9 Hz, measure the signal at pin 6 of the integrator DA1 — it needs to be “on the verge of limitation from below”. The corresponding correction is carried out by selection of a nominal of the resistor R10.
By attaching the oscilloscope to the source of the transistor VT3, check the change in voltage level depending on the beat frequency. Resistors R16 and R17 make a logical zero at the output of the comparator (pin 10 of IC DD4) appeared only when fбиений will be above 70 Hz.
VCO adjust using a resistor R15 so that the generator started to work when the signal integrator “out of limits from below”. In the future it will significantly simplify the adjustment of the device before work, as the minimum frequency of the VCO and will correspond to the setting of the metal detector at maximum sensitivity.
Restoring on a printed circuit Board specially sealed previously, the connection of R3 and C8 with DA2, transition to the final phase of debugging of the device. Engine “podstroechnye” R26 turn in the extreme(“positive”)position that will correspond to the maximum frequency of the beating(and fпоискового generator> f model). Then, slowly rotate the engine in the opposite direction, begin to monitor the signal at pin 6 DA1. Notice how (at a certain position of the engine R26) on the oscilloscope screen emerges the moment of impact of the signal in the capture zone of the AFC.
Continuing the handle rotation trimmer 1327, achieve a beat frequency equal to 10 Hz, while checking the operation of the AFC (desire signal to return to its original state).
The engines of resistors 1326, 1327, you must move slowly, given the large inertia of the AFC. At the same time in the head phones will listen minimum frequency VCO and weak clicks and f beats. Some
cases, it may be the effect of “swimming” sound relative to some fixed state. In this case, it is necessary to select more accurately the ratio of resistors R23, R24 or decrease the values of 1319, R20.
As already mentioned, the electronic part of the detector (which is almost the whole device) can be mounted in any suitable housing mounted on the handle. Care must be taken to search frame sensor, and connecting wires were rigidly fixed relative to each other. Even a slight vibration of these parts occurring due to the movement of the operator can generate a false signal (especially at the maximum sensitivity of the method and lack of experience with the device). For the same reason, the shoulder should be worn behind the back with a bayonet up (away from the frame sensor). And the metal tips on the laces of the boot operator is unacceptable. Bring them interference threaten to nullify all the efforts of such an acute sense of the instrument unearth something with which she so unwillingly parted.
Working with metalldetektoren little different from the actions of modern manual metal detector. Of course, such precise devices need adjustment. In our particular case is the rotation of the engine podster-echnolo resistor R26 in the extreme (“positive”) position, and R27 in the middle. Filing for instrument power supply, rotate the knob R26 in the opposite direction until the head phones signal VCO. After this trimming resistor R27 set the required sensitivity. But with R26 arbitrarily put up (when the device operation mode beating “one to one”) f6иений in the range of 200 -300 Hz.
Aft and VCO, in fact, turned off, so the search are as usual. To better define the location of small items frame sensor brings to the search area either horizontally (rounded corner forward), or under an inclination of 45 — 90° to the sample surface (with a clear positional advantage of one of the sides of the frame).
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