Metal detector on the beat method. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Metal detector on the beat method. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / metal detectors

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The metal detector works according to the beat method. It has two interchangeable searchcoils of different diameters 250mm and 500mm. With the help of the first coil, you can search for small, shallow metal objects (for example, a nail in a brick wall under a layer of plaster or wallpaper) and with the help of the second, deeper objects, for example, a 10 mm drill, the device notices from a distance of about 30 cm, and the cover hatch detects under a 1-meter layer of snow. A coin with a denomination of 5 rubles (modern "pyatak") is detected by the device from 12-15 cm.

(click to enlarge)

There are two generators in the metal detector circuit - stable and search. The stable oscillator is made on a D1 chip, its frequency of 100 kHz is stabilized by a Q1 quartz resonator. The search generator is made on the D2 chip, its generation frequency is about 100 kHz and is determined by the parameters of the L1C2C3C4 circuit. The frequency of this circuit is strongly affected by the change in the inductance of the bulk coil L1 when located near non-metal objects. Since the parts of the device are metallic, so that they do not affect its operation, the coil is fixed at one end of a dielectric (wooden) rod 80 cm long, and the device itself is at its other end.

The signals in the element D3.1 are summed and the sum and difference frequency signals are formed at its output. The sum frequency signal is suppressed by the R7C8R8 circuit. The difference frequency signal (beat signal) is fed to the Schmitt trigger on the elements D3.2-D3.3, which changes its state with each half-cycle of the input signal. At the output of the Schmitt trigger, the pulse shaper on the C9-R10 circuit and the VT1 transistor is turned on. As a result of the operation of the Schmitt trigger and the pulse shaper, we can hear beats with a frequency not only from a few kilohertz to hundreds of hertz, but also low-frequency beats with a frequency of 1-10 Hz, which are reproduced by the B1 speaker as crackles (the sound resembles the crackle of a radiation dosimeter). This allows you to hear a very small deviation in the frequency of the search generator.

As already mentioned, the device works with two replaceable coils. Both coils have the same design, but different diameter and number of turns. A coil with a diameter of 250 mm is wound on a mandrel of the same diameter. Only 30 turns of PEV-0,61 wire. Then the coil is removed from the mandrel and the resulting annular bundle is tightly wrapped with a rope coil to coil (the rope is laid like ferrite rings are wound). Then the coil is wrapped with a thin foil tape, but not over the entire surface, but in such a way that in the part opposite to the coil leads, an unshielded section of about 10 mm long remains. Then tightly wrapped with several layers of PVC electrical tape. Do not use metal parts in the coil mount.

The second coil has a similar design, but its diameter is 500 mm, and the number of turns is 21.

For rigidity, the coils are fixed on crosses connected from rails with the help of a rope glued with epoxy glue.

Capacitor C2 - three-section with an air dielectric 6-360 pF, all sections are connected in parallel. B1 - any speaker.

The adjustment consists in tuning the L1C3C4C2 circuit (using the selection of C3 and C4) so that, at an approximately average position of C2, a very low sound tone can be achieved, with further adjustment of C2, turning into rare crackles.

Author: Korotkov V.

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