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Ultrasonic metal detector

Ultrasonic metal detector

A. Goshev , Rostov-on-Don

In the practice of treasure hunters, most cases of metal detection are “empty”, that is, the item found is a simple rusty piece of metal, and a lot of time is spent on work on its extraction. In order to free the searchers from unnecessary work and to give them the opportunity to determine its name immediately upon detection of the metal, 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 size of a metal object under the action of mechanical force on it in a constant magnetic field.

The emitter - BQ 1 piezoelectric element acts on the soil or building structures, which often hide the desired object, by ultrasound with a frequency of 6 MHz. The radiation wavelength is such that the particles of the crystal lattice of semiconductor-type substances, such as silicon, calcium, etc., or molecules of substances such as carbon, chlorine, etc., that make up the soil or walls of houses, come to a non-equilibrium state. It is characterized, on the one hand, by the transmission of the mechanical action of ultrasound from particle to particle, and on the other hand, by the appearance of the so-called "fluidity" state of substances in the ultrasonic field. As a result, metal objects appear to be suspended or freely floating in the area of ​​"fluidity".

The substance in the state of "fluidity" re - radiates the ultra - wide spectrum of ultrasonic vibrations (Fig. 2) with an average frequency of 6 MHz, and the bandwidth reaches 5 MHz. In the lower part of the band, there are frequencies of magnetostriction resonances of the most frequently sought for metals, which are indicated by spectral components (Fig. 2). Metallic objects are excited at frequencies of natural resonances that are different from others, and the amplitude of resonant radiation exceeds the noise background of the "fluidity", which allows them to be detected.

The metal detector design (Fig. 1) consists of a transmitter assembled on a domestic K174KHAZA microcircuit on a three-point capacitive with parametric frequency stabilization on the VT 1 transistor, a multichannel receiver in the form of a line of Al ... A 6 frequency detection indicator on a microcircuit type MC34119P ( DA 2). The transmitter, powered by one or two flat batteries, is loaded on a BQ 1 piezoelectric emitter, which, to increase the operating frequency, is mounted on a platform made 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 transmitting, and for decoupling at the frequency of radiation, their adjoining ribs are laid with rubber. From the piezoelectric element, the input signal goes to the receiving modules of channels A 1 ... A6, which differ only in the tuning frequency of the input electromechanical filters, which for aluminum (A1) is 3245 kHz, for copper (A 2 ) 3872 kHz, for iron (A3) 4731 kHz, 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 causes the K 1 relay to turn on , which turns on the light signaling, indicating the presence of a metal of a certain name. At the same time, an audible alarm is activated in the indicator circuit by supplying power to the AF2 DA2 generator through a decoupling VD 2 diode.

The design of the metal detector is simple, however, the oscillating system of the transmitter and the receiver filters require careful tuning by techniques well-known to amateurs.

Details and structural elements are intentionally available to facilitate repetition of the structure.