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Ultrasonic metal detector

Ultrasonic metal detector

A. Goshev , Rostov-on-Don

In the practice of lovers of treasure hunts, most cases of metal detection are "empty", that is, the found object is a simple rusty piece of iron, and a lot of time is spent to extract it. In order to free the searchers from unnecessary work and to enable them to immediately determine the name of the metal when a metal is detected, and this scheme was developed.

The scheme of the ultrasonic metal detector is shown in Fig. 1. The principle of its action is based on the property of magnetostriction, which manifests itself as a change in the dimensions of a metal object when a mechanical force acts on it in a constant magnetic field.

Emitter - piezoelectric element BQ 1 affects the ground or building structures that most often hide the desired object, ultrasound at a frequency of 6 MHz. The wavelength of the radiation is such that the particles of the crystal lattice of substances of the semiconductor type, for example, silicon, calcium, etc., or molecules of substances such as carbon, chlorine, etc., of which the soil or the walls of the houses are composed, are in a nonequilibrium state. It is characterized, on the one hand, by the transmission of the mechanical action of ultrasound from the particle to the particle, and on the other hand by the appearance of the so-called "fluidity" state of the substances in the ultrasonic field. As a result, metal objects appear to be weighed or floating freely in the region of "fluidity".

The substance in a state of "fluidity" re - emits an ultra - wide spectrum of ultrasonic vibrations (Figure 2) with an average frequency of 6 MHz, with a bandwidth of 5 MHz. In the lower part of the band, frequencies of the magnetostrictive resonances of the most frequently found metals are found, which are indicated by the spectral components (Fig. 2). Metal objects are excited at the frequencies of their own resonances, different from the others, and the amplitude of the resonance radiation exceeds the noise background of the "fluidity" region, which makes it possible to detect them.

The construction of the metal detector (Figure 1) consists of a transmitter assembled on a domestic chip of type K174HAZA using a capacitive three-point with parametric frequency stabilization on a transistor VT 1, a multichannel receiver in the form of a line of frequency selective converters-indicators Al ... A 6 and a detection detector on a microcircuit of the type MS34119P (DA2). The transmitter, powered by one or two flat batteries, is loaded on a piezoelectric radiator BQ 1, which, for increasing the operating frequency, is mounted on a platform of electrical steel with a weight 2 kg .

The same piezoelectric element BQ 2 is used to receive ultrasonic vibrations, so it is placed on a similar platform next to the transmitter, and for decoupling by the frequency of radiation, their border ribs are laid with rubber. From the piezoelectric element, the input signal goes to the receiving modules of the channels A 1 ... A6, differing only in the tuning frequency of the input electromechanical filters, which is 3245 kHz for aluminum (A1), 3872 kHz for copper (A 2 ), 4731 kHz for iron (A3) for silver (A4) 5278 kHz, for gold (A5) 5621 kHz and for platinum (A6) 5722 kHz.

If there is a signal at the input of the channel amplifier that exceeds the noise level, the signal is detected, amplified and the relay K1 activated, which includes a light signal indicating the presence of a metal of a certain name. Simultaneously, in the indicator circuit, by activating the supply voltage to the DA2 DA2 generator, an audible alarm is activated via the decoupling diode VD 2.

The scheme of the metal detector is simple, however, the oscillator system of the transmitter and the receiver filters require careful tuning by techniques widely known to radio amateurs.

Details and structural elements are intentionally taken in order to facilitate repetition of the structure.