OMP-1 device for detecting metal objects. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / metal detectors

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During urban and field geodetic surveys, topographers and surveyors often have to find geodetic marks. The latter are sometimes under a layer of soil, which makes it difficult to detect them. If the organization has an underground facility, it becomes necessary to determine the location of the well covers under a layer of soil or snow.

The device OMP-1, the description of which is given below, is designed to facilitate the solution of these problems. During testing, the device detected polygonometry points under the soil layer at a distance of 0,3-0,4 m, well covers - at a distance of 0,8-1 m.

Principle of operation OMP-1 device is based on the fact that the frequency of the generator changes if the search coil approaches a metal object. The closer the search coil is to a metal object, the more the oscillator frequency increases. Therefore, by registering in some way a change in the frequency of the generator, it is possible to find a metal object. In this case, the maximum frequency change corresponds to the minimum distance between the search coil and a metal object. The change in the frequency of the generator can be recorded by ear (using the beat method) or visually.

If an appropriately tuned FSS (concentrated selection filter) is connected between the generator with an external search coil and the DC amplifier, then the amplitude will change when the generator frequency changes, and hence the collector current of the transistor T3. A 3 µA device is included in the T200 collector circuit.

Schematic diagram instrument OMP-1 is shown in fig. 1, a. The generator of sinusoidal oscillations is made on the transistor T1 according to the three-point scheme. The operating point is determined by the voltage divider R1, R2 and the resistance R3. In addition to the relatively high stability of frequency, amplitude and good waveform, the generator has another advantage: it uses an unsectioned search coil. Variable capacitor C5 allows you to change the frequency of the generator from 430 kHz to 500 kHz. By changing the capacitance C5, you can choose the optimal location of the operating point on the frequency response of the FSS (in the section of the greatest steepness), this corresponds to the maximum sensitivity of the device.

The sinusoidal voltage of the generator through the resistance R4 is supplied to the FSS, tuned to a frequency of 445 kHz. Since the IF amplifiers in radio receivers are tuned to 465 kHz, the operating device does not interfere. The device uses the FSS used in the Atmosfera-2M radio receiver. With the help of tuned cores, its contours are tuned to the operating frequency of the device (445 kHz), without changing the winding data of the coils. The device can use FSS from other radio receivers. It is preferable to use high-quality loop coils, for example, the FSS of Topaz-2 and Sokol pocket radios.

The scheme shown in fig. 1b differs from the first circuit (Fig. 1a) in an additional second stage, which makes it possible to obtain a higher sensitivity of the device.

Rice. 1,a

Rice. 1b

Setting up the device. A correctly assembled generator starts generating immediately, and its adjustment consists only in selecting such a capacitance of the capacitor C4, at which the generation frequency is approximately equal to 445 kHz. In this case, the rotor of the variable capacitor C5 must be set to the middle position. The frequency was measured with a ChZ-7 device, which was connected through a resistance of several kiloohms to the emitter terminal of the transistor T1 and to the common positive terminal. To set up the FSS, GSS-6 and an output meter (a device with a sensitivity of 200 μA) are required. The setting of similar devices is described in the Radio magazine No. 8, 1960, p. 22.

The search coil, which is an oscillatory circuit, must be placed in an electrostatic shield. It is made of a duralumin tube with a diameter of 12 mm in the form of a ring with a diameter of 390 mm. A slot is cut along the outer circumference of the ring with a hacksaw and 14 turns of PELSHO 0,28 wire are laid.

After laying, the wire is impregnated with paraffin and the entire ring is wrapped with insulating tape or varnished cloth. The search coil is connected to the generator by a shielded coaxial cable that runs inside the tube. Both the ring itself and the tube are connected to the positive terminal of the power source (two KBS-0,5 batteries). They are located in the same housing with a microammeter. The tuning knob (variable capacitor C5) is brought out through the holes in the bottom and cover of the body of the device itself. The variable resistance R14, connected in series with the microammeter, serves to adjust the sensitivity. When carrying the device, the ring is pressed against the tube and fixed with a spring latch.

The main dimensions of the device are shown in fig. 2.

Fig. 2

The installation was carried out on a getinax board (Fig. 3) with dimensions of 100x75x2 mm.

Fig. 3

Authors: A.Zotov, V.Kharin

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