Sensitive metal detector. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Sensitive metal detector. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / metal detectors

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First of all, I want to immediately dispel the preconceived notion of a man with a metal detector as a grave defiler or a "black tracker" supplying the mafia with weapons from the First World War. In fact, most "treasure hunters" are looking for various old metal objects in landfills, in wastelands, which for the "treasure hunter" are of the same value as postage stamps for a philatelist. I assure you that the found teaspoon of the 18th century or the copper nickel of the 19th century will be perhaps the most valuable find of all that "detects" the metal detector.

Most homemade metal detectors are built according to zero beat circuits, when there are two generators - a reference one with a constant frequency and a search one, the frequency of which depends on the metal objects surrounding it. In this case, the generator frequencies are set to about 100 kHz and are approximately the same. The slight detuning of the search generator required to search for an item is usually done with a varicap or variable capacitor.

You can increase the sensitivity of a metal detector built on this principle by raising the frequency of the reference oscillator so that it is 10 times higher than the frequency

search generator. In this case, beats occur between the oscillations of the reference oscillator and the 10th harmonic of the oscillations of the search oscillator. As a result, the detuning of the search generator, at least by 10 Hz, leads to an increase in the frequency of difference pulsations by 100 Hz, and this is already very noticeable.

Using this principle, it was possible to obtain a metal detector capable of detecting a penny coin at a depth of up to 100 mm or a larger object (for example, a bucket) at a depth of up to 0,7 - 0,8 m.

The schematic diagram of the metal detector is shown in the figure. The circuit uses two K561LA7 microcircuits, a search generator (elements D1-D1.1) and an output amplifier (element D1.3) are made on the D1.4 microcircuit. On the D2 chip - a reference crystal oscillator (elements D2 1-D2.3) and a mixer (element D2.4). The frequency of this oscillator is determined by the resonance frequency of the quartz resonator Q1.

Sensitive metal detector

The search generator uses two elements D1.1 and D1.2. The generation frequency is set by a circuit consisting of a search coil L1 and capacitances C1, C2, VD1. The VD1 varicap serves to adjust the frequency of this generator within small limits during operation. The adjustment itself is made by a variable resistor R3, which changes the voltage on this varicap. Compared to tuning with a variable capacitor, electronic tuning has a significant advantage, which consists in the fact that the tuning element (R3) can be arbitrarily removed from the circuit, and thus, by placing the case with generators in the middle of the search "stick" of the metal detector, you can display adjustment organ (R3) to the end of the "stick", on which the instrument's handle is located, and thus, by removing the generator from the hands, it is possible to avoid affecting the tuning of external capacitances (human body capacitance).

The generator mode (POS) is set by the elements R2 C4 C5 C6. In fact, this is a well-known two-element multivibrator circuit, but a circuit is connected to its midpoint of the RC circuit, which sets the generation frequency.

From the outputs of both generators, the pulses arrive at the element D2.4, on which the mixer is made, and a frequency beat signal appears at its output. This signal is additionally amplified in power by the element D1.4 and goes to the sound emitter B1, the role of which is performed by electromagnetic headphones of the TK-47 type with a resistance of 47 ohms. You can also use ordinary small-sized phones from an audio player by turning on both capsules in series. In this circuit, telephones B1 are connected between the output D1.4 and the positive power bus, so that the current flows through the telephones at a logic zero at the output of element D1.4. This is not accidental, the fact is that the elements of the K561 microcircuits, namely the K561LA7 microcircuits, develop at the output a much higher current in the zero state (almost equal to 6 mA), and a lower unit current (2-3 mA). As a result, in this inclusion, the maximum volume will be higher.

To adjust the volume is a variable resistor R6. The power battery is composed of two "flat" galvanic batteries of 4,5V each (old marking "3336L"). The choice fell on these batteries because, with relatively small dimensions, they provide long-term operation of the device, are relatively inexpensive, and due to the presence of strip tinned or brass contacts, they can be connected to each other by soldering, which cannot be done when compiling a battery of separate six elements (requires special cassettes with spring contacts). The use of the popular "Krona" is also possible, but less preferable, since it has a smaller capacity.

The electronic part is mounted in a case soldered from foil fiberglass. It contains generators. The housing has a shielding partition between the microcircuits, located in such a way that D1 and the capacitors of the search oscillator are placed in one compartment, and D2 and the elements of the reference oscillator are placed in the other. The partition serves to exclude the influence of the reference generator on the search one.

The search coil is wound on a ring made of hard cambric with an outer diameter of 15 mm. Any other plastic tube or soft plastic pin of the same diameter can be used. In any case, such a blank is bent into a ring with an outer diameter of 200 mm and fixed either thermally or with glue. On the resulting ring, 50 turns of PELSHO 0,27 wire are wound (at worst, PEL 0,27-0,35 is also possible). After the winding is completed, the ring is wrapped with one layer of electrical tape, then the coil is shielded with aluminum foil. It is convenient to use thin foil from old paper capacitors such as MBM, BM or BMT for a higher voltage (the foil will be thicker and stronger). The foil tape is wound so that its turns do not touch, that is, the winding is carried out in increments of 1-5 mm. Then the foil is fixed with another layer of electrical tape. The role of the rigidity frame of the coil is performed by a disk with a diameter of 250 mm made of thin (1 mm) fiberglass without foil. The coil is fixed on it with threads passed through holes previously drilled in the disk. A military-style connector is attached to one edge of the disc, screwed to a socket attached to the end of an aluminum tube with a diameter of 15-20 mm. At the other end of the tube is a handle with regulators, a switch and a telephone jack, and in the middle of the tube the body of the electronic device and batteries are attached (using clamps).

The setting comes down to setting the L1C1 circuit to 100 kHz with the middle position of R3 (control the frequency on pin 10 D1.3)

Author: Somov M.P.

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