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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Metal detector from the available elements. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / metal detectors Features of the metal detector This metal detector is able to detect:
Operating principle The device is built on the basis of the most common parts that are available in the stocks of any radio amateur. The metal detector is made according to the principle of beats between the frequencies of two high-frequency generators, which is well-known and widely used in such devices. The frequency of one of them (reference) is constant, and the frequency of the second (search) changes under the influence of external metal objects that change the inductance of its coil when it enters the zone of its action. Schematic diagram of a metal detector from the available elements The schematic diagram of the metal detector is shown in fig. 2.37. The reference oscillator is made on the transistor VT1. The frequency of its oscillations is determined by the parameters of the L1C3 circuit and is about 1 MHz.
The search generator is made on the transistor VT2, it also generates a signal of approximately the same frequency. The difference is that the reference oscillator circuit uses a small coil with a ferrite core. Therefore, external metal objects practically do not have a significant effect on its inductance. The search generator circuit coil is wound on a larger frame in the form of a frame. She has no core. As a result, its inductance changes greatly when it approaches a metal object, which in this case begins to act as a moving core. The signals from both generators are fed to the diode mixer on the VD1 diode. As a result, the product of subtracting the frequencies of the generators is obtained on the capacitor C12. The closer the values of these frequencies, the lower the sound tone on this capacitor, and the more the frequencies of the generators differ, the higher the tone of the sound in the speaker, which receives the signal (the product of the diode mixer). The signal comes through a low-frequency amplifier on transistors VT3-VT6. With the help of a variable capacitor C7, the search generator can be adjusted in such a way that, in the absence of metal objects nearby, the sound tone in the speaker is the lowest. Then, when the coil L2 approaches the metal, the frequency of the generator on VT2 begins to change. The frequency difference of the generators increases, and therefore, the tone in the dynamics will rise. With the exact location of the metal, the sound will turn into a shrill squeak. Coil manufacturing Coil L1 should be wound on a ferrite rod with a diameter of 8 mm, for example, from a radio magnetic antenna. The rod length is reduced to 30 mm. First, a frame must be put on the rod - a sleeve glued from whatman paper, which moves along it with some friction. Coil L1 should contain 110 turns of PEV wire with a diameter of 0,2-0,3 mm. The tap must be made from the 16th turn, counting from the VT1 collector. Coil L2 - search. It must be wound on a frame, which is a frame measuring 120 x 220 mm, made of plexiglass, plastic or wood. Winding must be carried out with a PEV wire with a diameter of 0,4 x 0,6 mm. The coil should contain 45 turns with a tap from the 10th, counting from the VT2 collector. The coil must be connected to the main unit with a three-core shielded wire. The coil should be located at a distance of about 1 meter from the main unit (fixed on an aluminum tube or wooden rail). Design The device itself (the main unit containing a generator on VT1 and an ultrasonic frequency converter with a speaker and a battery) can be mounted in a housing from a radio receiver. From the same receiver it is advisable to use:
The design may be different, it all depends on the possibilities and desires. Element base and options for replacing elements Capacitor C7 can be with a minimum capacitance of not more than 10 pF, and a maximum capacitance of at least 150 pF. Transistors KT315 can be replaced with KT3102 or KT312, KT316. MP35 transistors can be replaced with MP35-MP38, and the MP39 transistor with MP39-MP42. Diodes D9 - with any letter, or D2, D18, GD507. Speaker - any resistance from 4 ohms to 100 ohms, for example, a speaker from a radio receiver or headphones. 9 V battery, you can use the "Krona" or a suitable battery. Power supply from a mains source of 220 V is not desirable, because in this case an alternating current background appears and the sensitivity of the device as a whole decreases. Setting up the metal detector The setting consists in adjusting the L1 coil in such a way that, with the rotor of the capacitor C7 in the middle position and in the absence of external metal objects, the sound of the lowest tone is heard in the speaker. In the future, during operation, adjustment before the start of the search will be performed by capacitor C7. In the absence of oscillations from the generator on VT1, you need to select the value of C4 or (and) adjust the operating mode of the cascade by selecting the value of R2. If the generator on VT2 is not excited, you need to adjust C8 and adjust the operation mode of the transistor by selecting the value of R6. use Features The device is highly sensitive, and working with it requires certain skills. So you need to practice. When working, it is important to take into account that when approaching ferrous metals (iron, steel, cast iron), the frequency of the generator on VT2 decreases, and when approaching non-ferrous metals, it increases. Author: Pavlov S.
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Comments on the article: a guest pasiba a guest The scheme works and distinguishes metals only from the author. Andrey Baryshev Normal working order. Simple and works well. And not only "from the author", as they say here, but also from me, for example ... Thank you very much for the diagram! Gennady Very interesting, must collect! Everything ingenious is simple - says wisdom ... Artem Well, in my storeroom there is almost the same scheme, a friend assembled it, he had to customize it a little, bring it to mind, but even after that he did not achieve the promised depths ... A bucket takes a maximum of 40-50 cm without taking into account the soil
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