A 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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When carrying out various works - earthworks, roads, construction of buildings, it often becomes necessary to find covers of fire and cable wells under a layer of earth, snow, ice, or various metal products hidden in the walls.

The device, which is described below, is used to determine the location of such objects in the thickness of non-magnetic masses with low electrical conductivity. For example, under a layer of soil, asphalt, snow, you can find a cast-iron manhole cover at a depth of up to 0,6-0,8 m, find under a wooden or concrete floor, as well as in walls, ceilings, the location of metal beams, pipes or electric cables with metal armor at a distance of up to 0,2-0,4 m (depending on the size and shape of the latter). You can also find out the direction of propagation of reinforcement beams in reinforced concrete structures at a depth of up to 0,1-0,15 m, etc.

This device is small in size, simple in design and construction; its manufacture is within the power of a radio amateur of average qualification.

The principle of operation of the device is based on a change in the natural frequency of the oscillatory circuit when its inductor approaches a metal object. If the circuit is included in the circuit of a tube generator, then when the inductance changes, the frequency of the generator will also change, and this change can be relatively easily registered. The magnetic field arising from the eddy currents induced in the searched object interacts with the inductance of the oscillatory circuit, reduces it and thereby increases the frequency of natural oscillations of the circuit. The maximum frequency change indicates the minimum distance of the frame from the metal body.

A schematic diagram of the device, which consists of two LC oscillators and a mixer, assembled on crystalline triodes, is shown in fig. 1.

Fig. 1

The first generator has an oscillatory circuit L1C1. Coil L1 is made in the form of a frame, with the help of which the location of a metal object is found.

The second oscillator with the L4C4 circuit is auxiliary, it serves as a reference frequency source to determine the moment of the change in the frequency of the first oscillator.

The AC voltage from both generators from the windings L3 and L6 is supplied to the base of the mixing triode PP3. In its collector circuit, currents arise with the frequency of the first and second generators, as well as currents of difference and sum frequencies and their harmonics.

The low-frequency components of these currents are heard on telephones included in the collector circuit of the PP3 triode.

The frequency of the second oscillator is preliminarily aligned with the frequency of the first, which is controlled with the help of telephones by ear according to "zero beats". When the frame of the device approaches a metal object, the frequency of the first generator increases, and the phones begin to

listen to a low frequency tone. The closer the frame is to the metal object, the higher the frequency of the first oscillator, the higher the difference frequency at the mixer output and the higher the pitch heard in telephones. When the frame is moved away from a metal object, the frequency of oscillations of the signal heard in the phones decreases. The highest tone determines the minimum distance from the frame to the object being searched for.

The frequency of the generators is selected within the range of 80-120 kHz, and the frequency listened to when an object is detected should not exceed 3-4 kHz.

The general view of the device is shown in fig. 2. All elements of the device circuit, including the power source, are placed in a common housing, fixed at the top of the frame handle. It is powered by a flashlight battery.

The oscillatory circuit of the first generator is made in the form of a separate frame connected to the device by a shielded cable. The frame is fixed on a wooden frame (Fig. 2), with which the handle is pivotally connected.

Fig. 2

In working condition, the frame is held in a predetermined position with respect to the handle with a cord. When carrying, the frame is applied to the handle and fixed with a latch.

The entire installation is carried out on a getinax board, the Vk1 switch and telephone jacks are fixed on the case - fig. 3.

Fig. 3

The tuning knob (the tuning core of the LA coil) is brought out through a hole in the side wall of the housing.

The template for winding the coil of the frame is made in the form of a rectangle measuring 300x400 mm from boards or a sheet of plywood. Nails are hammered in the corners of this rectangle, windings L1, L2 and L3 are wound on them.

To fasten the turns, the winding should be tied with a strong thread, after which the frame can be removed from the nails.

To protect the winding of the frame from moisture, it should be wrapped with a tape made of varnished cloth or thin polyethylene. Outside, the frame is wrapped with a cotton tape impregnated with bituminous varnish (The frame is varnished 6-8 times. After each coating, the varnish must be dried well.).

Capacitor C1 and the end of the connecting cable are also insulated against moisture, with particular attention to the termination of the cable exit.

After impregnation, the frame is fixed to the frame with plywood strips, as shown in fig. 4.

Fig. 4

The connecting cable is made of three flexible wires in PVC insulation, enclosed in a common screen, on which a PVC or rubber tube is put on from the outside. The screen simultaneously serves as a common wire for the windings L1, L2, L3 and is connected to the negative terminal of the device.

Inductors L4, L5 and L6 are wound on the SB-4 core. The data of their windings are given in the table.

As semiconductor triodes PP1, PP2 and PP3, any triodes of the type P-6, P-13, P-14, P-15 or the like can be used.

Setting up the device is reduced to selecting the value of the capacitance of the capacitor C4 or the number of turns of the inductance L4 so that in the middle position of the tuning core, the frequency of the second generator is equal to the oscillation frequency of the first generator.

Frequency alignment is performed using an electronic oscilloscope and an auxiliary generator according to the Lissajous figures.

If the frequency of the second generator is lower than the frequency of the first generator, then the capacitance C4 should be reduced or the number of turns of the L4 coil should be reduced, if the frequency of the second generator is higher, the capacitance of the capacitor C4 should be increased or a few turns should be added to the coil L4.

The final adjustment of the frequency of the second oscillator is performed by ear using "zero beats", or by observing a low-frequency signal from the mixer output on an oscilloscope.

In the absence of an oscilloscope and a generator, the device can be tuned using a conventional broadcasting receiver with a long-wave range. It is desirable that this receiver has an electro-optical tuning indicator.

First, the frequency of the first generator is measured. To do this, the PP2 emitter is disconnected from the power circuit, and the mixer output (when the telephones are turned on or a resistance of 4-5 kΩ replacing them) is connected through a capacitance of 15-20 pF to the "Antenna" socket of the receiver.

By changing the receiver setting, by the characteristic noise in the loudspeaker or by the tuning indicator, the frequency of one of the harmonics of the first generator is determined on the receiver scale. By changing the receiver tuning, the frequency of the next harmonic is determined. The frequency difference between two adjacent harmonics will be equal to the natural frequency of the first generator.

Similar measurements are also made for the frequency of the second oscillator (when the emitter PP1 is off).

By selecting the value of the capacitance of the capacitor C4 or changing the inductance L4, the frequency of the second generator is chosen such that its harmonics are heard at the same divisions of the receiver scale as that of the first generator.

The final frequency adjustment is done by ear using "zero beats" in telephones.

The tuning should be considered complete if "zero beats" can be achieved with the middle position of the tuning core in inductance L4.

Operation with the device is simple and does not require special training. Before starting the search, you should turn on the power of the device and, listening to its work on phones, by turning the screw of the tuning core, you need to achieve "zero beats". Then unscrew the core a little, slightly detun the second generator so that a low tone is heard in the phones. The device will have the highest sensitivity, since with fine tuning of both generators they are mutually synchronized even with small frequency deviations (when small or objects far away) the second generator is, as it were, "entrained" by the first one, and the object cannot be detected.

With a slight detuning, even a slight deviation in the frequency of the first generator when approaching a metal object causes a change in pitch, which is easily detected by ear.

The search is carried out by carrying the frame at a close distance from the ground surface, floor, ceiling, etc. in the area of ​​the supposed location of the metal object.

By swinging the frame from side to side, changes in tone easily determine the exact location of the item being searched for.

One battery for a flashlight is enough to operate the device for 100-150 hours, when powered by D-0,2 batteries, it can work 45-50 hours.

The device is not suitable for searching for small objects: bolts, washers, nails, etc.

Author: D.Ilyin

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