A simple metal detector with a 4,6 kHz circuit. Scheme, description

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A simple metal detector with a 4,6 kHz circuit. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / metal detectors

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Operating principle This metal detector consists in registering a signal reflected by a metal object. This signal arises due to the effect on the metal of an alternating magnetic field of the transmitting (radiating) coil.

The receiving coil is located in the same plane as the transmitting one in such a way that the magnetic lines of force passing through it create a small EMF. There is no signal at the terminals of the receiving coil or it is very small.

An additional reduction of this signal is provided by the compensation node. But if a metal object enters the field of the coils, the inductive coupling between the coils changes, an electrical signal appears at the terminals of the receiving coil, which is amplified, rectified, and then filtered.

As a result, a certain constant voltage appears at the output of the filter, which increases as the coils approach a metal object. This voltage is fed to one of the inputs of the comparison unit, where it is compared with the reference voltage applied to its second input. The reference voltage level is adjusted in such a way that even a small increase in the signal leads to a significant change in the signal level at the output of the comparison node.

This, in turn, activates the electronic key that controls the metal detector buzzer.

Schematic diagram

The scheme of the metal detector is shown in fig. 2.2. The generator, made on the VT2 transistor and the L1C3 circuit, operates at a frequency of approximately 4,6 kHz.

Rice. 2.2. Schematic diagram (click to enlarge)

The low frequency generator provides:

on the one hand, the weak response of the metal detector to unwanted signals (for example, those that occur in the presence of wet sand, small pieces of metal, etc.);
on the other hand, good sensitivity.

The depth of detection of objects by any metal detector depends on the frequency of the working signal, its power, the size of the inductors, as well as the size and shape of the object and its position.

The higher the frequency of the generator, the smaller the depth of detection of small objects. The larger the inductors, the greater will be the depth of detection. The generator is assembled on a transistor assembly 2TC3103A. Transistor VT2 works directly in the generator, and transistor VT1, together with a divider of parts R2 ... R4, works in a temperature stabilizer that provides temperature compensation.

The signals arriving at the receiving coil L2 are limited in amplitude (in case a large metal object is detected) by diodes VD1, VD2, and then amplified by the operational amplifier DA1.1. The input of this microcircuit through capacitor C5, resistors R7-R10 and capacitor C8 receives a compensation signal from the generator. It attenuates the signal coming to the L3 coil from the L1 coil in the absence of metal objects nearby.

After amplification, the signal passes through the R16C11 filter at the op-amp DA1.2. With a positive input voltage supplied to the non-inverted input of the microcircuit, the VD3 diode is open and provides negative feedback. Capacitor C12 is charged, and the indicator arrow PA1 deviates.

With a negative input voltage, the diode is closed, there is no feedback, and there is zero voltage at the cathode of the diode.

A simple metal detector with a 4,6 kHz loop

Rice. 2.3. Appearance of three printed circuit boards of a metal detector with a 4,6 kHz circuit

The signal from the detector is smoothed by the R21C14R22C15 filter and fed to the DA2.1 comparator, where it is compared with the reference voltage regulated by the variable resistors R23 (rough) and R25 (fine).

When the comparator is triggered, the voltage at its output decreases, the transistor VT3 closes, and the tone generator assembled on the op-amp DA2.2 starts working.

Its output signal is fed to a power amplifier made on a VT4 transistor, the load of which is a headphone from a hearing aid. The sound volume is regulated by a variable resistor R38. The output stage is powered from a separate source, which eliminates the possibility of excitation of the device. The main part of the metal detector circuit is powered by a 12 V source, which is additionally stabilized by the DA3 chip at 9 V.

The details of the metal detector are mounted on three printed circuit boards (Fig. 2.3, a-c) made of one-sided foil fiberglass. They are designed for the use of resistors MLT-0,125, resistor SP4-1 (R10) capacitors K71-7 (C3, C6). Arrow indicator RA1 - recording level indicator from any tape recorder.

Coil manufacturing

Particular attention should be paid to the manufacture of coils. This will determine the quality of the metal detector in the future. Coils are optimally wound on a mandrel with a diameter of 140 mm, for which it is good to use a glass jar.

Each coil should consist of 200 turns of 0,27 mm diameter enameled copper wire with a lead from the middle thread. Before removing the mandrel coil, it should be tied up in three or four places, and after removal, wrap it with a strong thread so that the turns fit snugly together.

Next, the coils must be shaped as shown in Fig. 2.4, and attach them to a plastic plate 1 with threads 2. The transmitting coil 3 is placed at the bottom, and the receiving coil 4 is at the top. The receiving coil must be equipped with an aluminum (from a strip of foil) screen with a gap that prevents the formation of a closed loop.

The coil leads must be connected to the rest of the instrument with a shielded braided cable. The distance between the vertical (according to Fig. 2.4) turns of the coils should be approximately 25 mm. It is finally specified after setting up the metal detector according to the minimum reading of the PA1 indicator in the absence of metal objects near the coils.

A simple metal detector with a 4,6 kHz loop

Rice. 2.4. coil shape

After the final fixing of the coils with glue or sealant, they can be covered from above with a decorative casing and a rod can be fixed to it.

Setup and operation

Setting up the metal detector consists in setting the slider of the resistor R10 (with the middle position of the slider of the resistor R48) to such a position that the arrow of the indicator PA1 is at the "zero" mark. For ease of operation of the device, it is advisable to set the arrow to the middle of the scale by selecting the resistor R19. To do this, in some cases it will be necessary to change the connection of the terminals of one of the generator coils.

When operating the metal detector, after a 20-minute setting of the operating mode with resistor R8, it is necessary to achieve a "zero" reading of the dial indicator. After that, variable resistors R25 and R23 should set the reference voltage close to the operation of the comparator and the appearance of a tonal sound.

Author: Grichko V.

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