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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING A simple pulse metal detector on microcircuits. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / metal detectors Recently, pulsed metal detectors of the PI (Pulse Induction) type have become relatively widespread, in which, to assess the presence of metal objects in the search zone, the phenomenon of the occurrence of eddy surface currents in a metal object under the influence of an external electromagnetic field is used. In metal detectors of the PI type, a pulse signal is applied to a transmitting coil, in which an alternating electromagnetic field is initiated. When a metal object appears in the zone of action of this field, eddy currents periodically arise on its surface under the influence of a pulsed signal. These currents are the source of the secondary signal, which is received by the receiving coil. Due to the phenomenon of self-induction, the shape of the secondary signal will differ from the shape of the pulse emitted by the transmitting coil. In this case, the differences in the parameters of the secondary pulse signal are used for analysis with subsequent generation of data for the display unit. In all pulsed metal detectors known to the author, the change in the shape of the trailing edge of the secondary pulse is evaluated. The device in question uses a microprocessor with appropriate software. Unfortunately, by the time this book was published, it was not possible to publish a 100% workable version of his firmware. Therefore, interested and prepared readers have the opportunity to test their skills in creating firmware for the microcontroller. The author does not doubt for a second that Russian craftsmen will cope with this task with honor. Nevertheless, according to the author, the design of the proposed metal detector is quite complicated for repetition by novice radio amateurs. It should also be mentioned about the difficulties that arise when adjusting this device. It is necessary to pay special attention to the fact that errors during installation and incorrect setting of the device can lead to the failure of expensive elements. Schematic diagram The schematic diagram of the proposed simple pulse metal detector can be conditionally divided into two parts, namely: the transmitter unit and the receiver unit. Unfortunately, the limited volume of this book does not allow us to dwell in detail on all the features of the circuit solutions used to create this device. Therefore, the basics of functioning of only the most important nodes and cascades will be considered below. The transmitter unit (Fig. 3.14) includes a pulse shaping and synchronization module, the transmitter itself, and a voltage converter.
The main component of the entire design is the pulse shaping and synchronization module, made on the AT1C89 microprocessor IC2051 of the ATMEL company and providing the formation of pulses for the transmitter, as well as signals that control the operation of all other units. The operating frequency of the microcontroller IC1 is stabilized by a quartz resonator (3,5 MHz). At the specified value of the operating frequency, the microprocessor generates a periodic sequence of control pulses for various stages of the metal detector. This sequence consists of 250 cycles with a duration of 9 μs each. Initially, a control pulse for transistor T1 is generated at the output of IC14 / 6 of the microprocessor, after which a similar pulse is generated at the output of IC1 / 15 for transistor T7. This process is then repeated one more time. As a result, the voltage converter starts up. Further, sequentially on the conclusions of IC1/8, IC1/7, IC1/6, IC1/16, IC1/17, IC1/19 and IC1/18, transmitter trigger pulses are formed. In this case, these pulses have the same duration, but each subsequent pulse is delayed relative to the previous one by several cycles. The beginning of the first pulse generated at the IC1/8 pin coincides with the end of the second pulse at the IC1/15 pin. Using switch P1, you can select the delay time of the transmitter start pulse in relation to the start pulse. A few cycles after the end of the pulse at the IC1/18 pin, a short strobe pulse for one of the analyzer channels is generated at the IC1/3 pin. Then a similar pulse, intended for the second channel of the analyzer, is formed at the output of IC1 / 9. After that, at the output of IC1 / 11, a control signal is generated for the transistor T10 of the acoustic signaling circuit of the receiver unit. Then, after a short pause, the sequence of control pulses at the corresponding outputs of the microcontroller is formed again. The +5 V supply voltage, previously stabilized by IC2, is applied to the IC1/20 pin of the microcontroller. The voltage converter, made on transistors T6-T8 and stabilizer IC3, provides the formation of a bipolar supply voltage of 12 V, which is necessary to power the cascades of the receiving part. The control signals for transistors T7 and T8 are generated at the corresponding pins of the microcontroller IC1. At the same time, this signal is fed to transistor T8 through a level converter assembled on transistor T6. Further, the generated supply voltage is stabilized by the IC3 microcircuit, from the output of which the +12 V voltage is supplied to the cascades of the receiving part. The output stages of the transmitter are made on powerful transistors T1, T2 and T3, operating on a common load, which is the coil L1, shunted by a chain of resistors R1-R6. The operation of the transistors of the output stage is controlled by the transistor T4. The control signal to the base of the transistor T4 is supplied from the corresponding output of the processor IC1 through the transistor T5. The pulse generated by the microprocessor IC1 in accordance with the program stored in its memory is fed through the switch to the input of the transistor T5 and further, through the transistor T4, to the output stages of the transmitter, made on transistors T1-T3, and then to the transceiver coil L1. When a metal object appears in the coverage area of the L1 coil, eddy surface currents are excited on its surface under the influence of an external electromagnetic field initiated by the transmitter pulse. The lifetime of these currents depends on the duration of the pulse emitted by the coil L1. In turn, the surface currents are the source of a secondary pulse signal, which is received by the L1 coil with an appropriate delay, amplified, and fed to the analysis circuit. It should be noted that due to the phenomenon of self-induction, the duration of the secondary signal will be greater than the duration of the pulse emitted by the transmitting coil. In this case, the shape of the secondary pulse depends on the properties of the metal from which the detected object is made. The processing of information about the differences in the parameters of the pulses emitted and received by the coil L1 provides the formation of data for the indication unit about the presence of a metal object. In the considered metal detector, the parameters of the trailing edge of the secondary pulse signal are used for analysis. The receiver unit (Fig. 3.15) includes a two-stage input signal amplifier, an analyzer and a sound indication circuit.
The signal from a metal object is received by coil L1 and through a protection circuit made on diodes D1 and D2 is fed to an input two-stage capacitive feedback amplifier made on operational amplifiers IC4 and IC5. From the output of IC5 (output IC5 / 6), an amplified pulse signal is fed to the analyzer circuit, made on IC6-IC8 microcircuits. Amplifiers IC6 and IC7 are constantly turned off during the operation of the device, and the supply voltage is applied to them only when strobe pulses arrive at the corresponding inputs (outputs IC6/8 and IC7/8), the duration of each of which is 9 μs (one cycle). At the same time, a strobe pulse is applied to amplifier IC6, delayed in relation to the end of the selected transmitter trigger pulse by 30-100 μs, and to amplifier IC7 - delayed in relation to the end of the first strobe pulse by 200 μs. The need for such a delay is explained by the fact that the shape of the received signal depends on the influence of many extraneous factors, so the useful signal can be observed only in the interval of approximately 400 μs after the end of the pulse. In this case, a useful signal is an increase in positive voltage when the coil L1 approaches a metal object as a result of an increase in the duration of the trailing edge of the secondary pulse in comparison with the emitted pulse. At the end of the supply voltage at the outputs of each amplifier (microcircuits IC6 and IC7) for several seconds, the level of the received signal, fixed during the exposure to strobe pulses, is maintained. Thus, the received pulse signal is applied to one of the inputs of the corresponding amplifier (terminals IC6/3 and IC7/3), and the corresponding strobe pulse from pulse shaping and synchronization module (pins IC6/8 and IC7/8). The signals generated at the outputs of IC6 and IC7 (pins IC6 / 5 and IC7 / 5) are then fed to the corresponding inputs of the differential amplifier made on the IC8 chip. In this case, the signal from the output of amplifier IC6 passes through a variable resistor R45, with which the sensitivity of the device is adjusted. If there is a metal object in the coverage area of the metal detector, the signal levels at the corresponding inputs of the differential amplifier (pins IC8/2 and IC8/3) will be the same. As a result, the output of this amplifier (pin IC8/6) will be low. The voltage drop at the output of amplifier IC8 leads to the opening of the transistor T9 and the connection to the common wire of headphones BF1. When a control signal is received from the corresponding output of the microcontroller (pin IC1 / 11) to the transistor T10, an audio frequency signal will be heard in the phones. Resistor R44 limits the current flowing through the BF1 headphones. By selecting it, you can adjust the volume of the acoustic signal. The power supply of this metal detector is carried out from the source B1 with a voltage of 12 V. Details and construction All parts of the device under consideration (with the exception of the search coil L1, resistor R45, switch P1, and switch S1) are located on a printed circuit board measuring 105x65 mm (Fig. 3.16), made of double-sided foil getinax or textolite.
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 (Fig. 3.17).
The LF357 (IC4) type IC can be replaced with an LM318 or NE5534, however this may result in setup problems. As an amplifier IC5, in addition to the LF356 chip indicated in the diagram, you can use the CA3140 chip. Chips like LF398 (IC6, IC7) are easily replaced by MAC198. Instead of the CA3140 amplifier (IC8), you can use the TL071 chip. As transistors T1-T3, in addition to those indicated in the circuit diagram, you can use transistors such as BU2508, BU2515 or ST2408. The operating frequency of the quartz resonator should be 3,5 MHz. However, you can use any other quartz element with a resonance frequency of 2 to 6 MHz. To mount the microprocessor IC1, use a special socket. In this case, the microcontroller is installed on the board only after all installation work is completed. This condition must also be observed when carrying out adjustment work related to soldering when selecting the values of individual elements. Particular attention should be paid to the manufacture of coil L1, the inductance of which should be 500 μH. Coil L1 is made in the form of a ring with a diameter of 250 mm and contains 30 turns of wire with a diameter of not more than 0,5 mm. When using a wire of a larger diameter, the current in the coil will increase, but the parasitic eddy currents will increase even faster, which will lead to a deterioration in the sensitivity of the device. For the manufacture of the coil, it is not recommended to use varnished wire, since the potential difference between adjacent turns during the emission of a pulse reaches 20 V. If, during the winding of the coil turns, there are conductors nearby, for example, the first and fifth turns, insulation breakdown is practically guaranteed. This can lead to failure of the transmitter transistors and other elements. Therefore, the wire used in the manufacture of the L1 coil must be at least PVC insulated. The finished coil is also recommended to be well insulated. To do this, you can use epoxy resin or various foam fillers. Coil L1 should be connected to the board using a two-core well-insulated wire, the diameter of each core of which should not be less than the diameter of the wire from which the coil itself is made. It is not recommended to use coaxial cable due to its significant inherent capacitance. The source of sound signals can be either headphones with an impedance of 8 to 32 ohms, or a small loudspeaker with a similar coil impedance. It is recommended to use a rechargeable battery with a capacity of about 1 Ah as a power source for B2, since the amount of current consumed by this metal detector is at least 200 mA. The printed circuit board with the elements located on it and the power supply are placed in any suitable housing. A variable resistor R45, a switch P1, connectors for connecting headphones BF1 and a coil L1, as well as a switch S1 are installed on the housing cover. Establishment This device should be adjusted in conditions when any metal objects are removed from the search coil L1 at a distance of at least 1,5 m. The peculiarity of setting and adjusting the metal detector in question is that its individual blocks and cascades are connected gradually. In this case, each connection operation (soldering) is performed with the power supply turned off. First of all, it is required to check the presence and magnitude of the supply voltage at the corresponding pins of the socket of the IC1 microcircuit in the absence of a microcontroller. If the supply voltage is normal, then you should then install a microprocessor on the board and use a frequency meter or oscilloscope to check the signal at pins IC1/4 and IC1/5. The frequency of the pilot signal on these pins must match the operating frequency of the used quartz resonator. After connecting the transistors of the voltage converter (without load), the current consumption should increase by 50 mA. The voltage across the capacitor C10 in the absence of a load should be about 20 V. Then the transmitter stages should be connected. The operating modes of transistors T1-T4 must be the same and are set by selecting the values of resistors R13-R16. The resistance of the coil L1, shunted by resistors R1-R3, should be approximately 500 ohms. In this case, the conclusions of the coil and resistors must be well soldered, since a contact failure in this circuit entails the failure of the output transistors of the transmitter. To check the operation of the transmitter stages, you can hold the L1 coil near your ear and turn on the power to the metal detector. Approximately half a second later (after resetting the microcontroller), a low tone signal can be heard, the occurrence of which is due to the microvibration of individual turns of the coil. In this case, an unmodulated pointed pulse with a duration of about 1-3 μs will be formed on the collectors of transistors T10-T20, the shape of which can be controlled using an oscilloscope. An increase in the resistance of resistors R1-R3 leads to an increase in the amplitude of the output pulse with a decrease in its duration. To select the resistance value of the shunt of coil L1, it is not recommended to use a variable resistor, since even a short-term violation of the contact of the engine with the current-carrying track can lead to failure of the output transistors of the transmitter. Therefore, it is desirable to gradually change the value of the shunt in steps of 50 ohms. Before replacing parts, the power supply of the device must be switched off. Next, you can proceed to the establishment of the receiving part. If all the parts are in good order, and the installation is done correctly, then after turning on the metal detector (approximately 20 μs after the end of the start pulse), an exponentially increasing signal can be observed at the output of the IC4 chip (pin IC4 / 6) using an oscilloscope, turning into a constant level signal. The distortion of the front of this signal is eliminated by the selection of resistors R1-R3, shunting the coil L1. After that, you should check the shape and amplitude of the signal at the output of the IC5 chip (pin IC5 / 6). The maximum amplitude of this signal is set by selecting the value of the resistor R36. At the output of IC6 (pin IC6 / 5) a constant signal should be generated, depending on the pulse selected using switch P1, as well as on the presence of metal objects in the area of \u1b\u1bcoil LXNUMX. Ideally, this signal should be close to zero for all positions of switch PXNUMX. In conclusion, it remains to correctly establish the position of the exemplary measuring pulse with respect to the starting pulse. To do this, it is enough to select a suitable operating frequency by selecting a quartz resonator Q1. Operating procedure Before practical use of this metal detector, you should set the minimum pulse delay with switch P1, and the maximum sensitivity with resistor R45. If during operation a metal object appears in the coverage area of the search coil L1, then an acoustic signal will appear in the headphones. It should be noted that switching to the operating mode with a longer pulse delay will ensure the exclusion of the influence of not only the magnetic properties of the soil, but also eliminate the reaction of the device to all kinds of foreign objects (rusty nails, foil from cigarette packs, etc.) and subsequent futile searches. Author: Adamenko M.V.
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Comments on the article: What Language does not turn to call this device simple. Then Clone, Tracker... the simplest chtoli?? The simplest one is with the gene on NE555 and one K157UD2 at 20 cm per penny.
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