Electronic metal detector with low operating frequency. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / metal detectors

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Operating principle

The metal detector is a relatively simple device, the electronic circuit of which provides good sensitivity and stability.

A distinctive feature of such a device is its low operating frequency. The inductors of the metal detector operate at a frequency of 3 kHz. This provides:

• on the one hand, a weak response to unwanted signals (for example, signals that occur in the presence of wet sand, small pieces of metal, etc.);
• on the other hand, good sensitivity when searching for hidden water pipes and central heating lines, coins and other metal objects.

The metal detector generator excites oscillations in the transmitting coil at a frequency of about 3 kHz, creating an alternating magnetic field in it. The receiving coil is located perpendicular to the transmitting coil in such a way that the magnetic lines of force passing through it will create a small EMF. At the output of the receiving coil, the signal is either absent or very small.

A metal object, falling into the field of the coil, changes the value of the inductance. In this case, an electrical signal appears at the output, which is then amplified, rectified and filtered. Thus, at the output of the system there is a constant voltage signal, the value of which increases slightly as the coil approaches a metal object.

This signal is fed to one of the inputs of the comparison circuit, 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 voltage leads to a change in the state at the output of the comparison circuit. This in turn actuates the electronic switch. As a result of this process, an audio signal is sent to the output amplifying stages, notifying the operator of the presence of a metal object.

Schematic diagram

The circuit diagram of the metal detector is shown in fig. 2.38.

Rice. 2.38. Schematic diagram of an electronic metal detector with a low operating frequency (click to enlarge)

The transmitter, consisting of a transistor VT1 and associated elements, excites oscillations in the coil L1. The signals arriving at the L2 coil are then amplified by the D1 chip and rectified by the D2 chip, which is included in the amplitude detector circuit. The signal from the detector goes to capacitor C9 and is smoothed out by a low-pass filter, which consists of resistors R14, R15 and capacitors SYU and C11.

Then the signal is fed to the input of the comparison circuit D3, where it is compared with the reference voltage set by the variable resistors RP3 and RP4. The generator, assembled on a transistor with one VT2 junction, operates in continuous mode. However, the signal generated by it enters the base of the transistor VT4 only when the transistor VT3 closes. After all, being in the open state, this transistor shunts the output of the generator.

When a signal is received at the input of the D3 microcircuit, the voltage at its output decreases, the transistor VT3 closes, and the signal from the transistor VT2 through the transistor VT4 and the volume control RP5 enters the output stage and loudspeaker.

Circuit power supply

The circuit uses two power supplies, which eliminates the possibility of any feedback from the circuit's output to its sensitive input. The main circuit is powered by an 18 V battery, which is lowered to a stable voltage of 4 V using the D12 chip. At the same time, a decrease in battery voltage during circuit operation does not change the device settings.

The output stages are powered by a separate 9V power supply.

Power consumption requirements are quite low, so three rechargeable batteries can be used to power the device. The output stage battery does not require a special switch, since the output stage draws practically no current in the absence of a signal.

Circuit mounting

It is recommended to mount the metal detector circuit on a breadboard. Such a board is shown in Fig. 2.39. The board has 24 copper strips with 50 holes each with a 2,5 mm pitch. First of all, 64 cuts must be made in the strips and three mounting holes must be drilled.

Rice. 2.39. Printed circuit board and layout of elements

Then on the back of the board you need to install:

• 20 jumpers;
• pins for external connections;
• two pins for capacitor C5.

Then you can install capacitors C16, C17 and chip D4. These elements form a power source with a voltage of 12 V. This stage is checked by temporarily connecting a battery with a voltage of 18 V. In this case, the voltage across capacitor C16 should be 12 ± 0,5 V.

After that, you can proceed to the installation of the elements of the output stage:

• resistors R23-R26;
• capacitors C14 and C15;
• transistors VT4-VT6.

The body of the transistor VT6 is connected to its collector, so the contact of the body with neighboring elements and jumpers is unacceptable. Since the output stage draws no current in the absence of a signal, it is enough to check it by temporarily connecting a loudspeaker, a variable resistor RP5 and a 9 V battery.

Then you need to install resistors R20-R22 and transistor VT2, which form a sound signal generator. When two power sources are connected, the sound background is heard in the speaker, which changes with the position of the volume control knob. After that, it is necessary to mount resistors R16-R19, capacitor C12, transistor VT3 and chip D3 on the board.

Checking the operation of the comparison scheme

The operation of the comparison circuit is checked as follows. Variable resistors RP3 and RP3 must be connected to the measuring input D4. This input is formed using two 10 kΩ resistors, one of which is connected to the positive +12 V supply rail, and the other to the zero rail. Connect the second terminals of the resistors to pin 2 of the D3 chip. The jumper from this pin serves as a temporary connection point.

With coarse tuning (both batteries are on), which is carried out by a variable resistor RP4, in a certain position, the sound signal is interrupted, while fine tuning with a variable resistor RP3 should be a smooth change in the signal near this position.

Preliminary check of cascade performance

When these conditions are met, you can proceed to install resistors R6-R15, capacitors C6-C11, diode VD3 and microcircuits D1 and D2. Turning on the power source, you first need to check for a signal at the output of the D1 chip (pin 6). It should not exceed half the value of the power supply (approximately 6V).

The voltage across capacitor C9 should not differ from the output voltage of this chip, although interference from the AC mains may cause a slight increase in this voltage. Touching the input of the microcircuit (the base of the capacitor C6) with your finger causes an increase in voltage due to an increase in the noise level.

If the tuning knobs are in a position in which there is no sound signal, touching capacitor C6 with your finger leads to the appearance and disappearance of the sound signal. This concludes the preliminary check of the performance of the cascades.

Final check and adjustment

The final check and adjustment of the metal detector are carried out after the manufacture of the inductors. After a preliminary check of the cascades of the circuit, the rest of the elements can be installed on the board, with the exception of the capacitor C5. Variable resistor RP2 temporarily set to the middle position. Attach the board to the L-shaped aluminum chassis through plastic washers (to eliminate the possibility of a short circuit) using three screws.

The chassis is fixed in the control console body with two bolts that hold two clamps that are designed to fasten the console body to the search bar. The side of the chassis secures the power supplies to the chassis. When assembling the remote control, make sure that the switch leads on the reverse side of the variable resistor RP5 do not touch the board elements.

After drilling a rectangular hole, glue the speaker. The stem and connecting parts that form the finder head holder can be made from plastic tubes with a diameter of 19 mm. The finder head itself is a plate with a diameter of 25 cm, made of durable plastic. The interior must be carefully sanded to ensure good adhesion to the epoxy.

Manufacturing of the transmitting coil

The main characteristics of a metal detector largely depend on the coils used, so their manufacture requires special attention. Coils having the same shape and dimensions should be wound on a D-shaped loop, which is created from pins fixed on a suitable piece of the board.

Each coil shall consist of 180 turns of 0,27 mm enameled copper wire, tapped from the 90th turn.

Rice. 2.40. Coils of the metal detector: a - method of winding coils; b - installation diagram of finished coils

Before removing the coils from the pins, they must be tied in several places, as shown in fig. 2.40 a. Then each coil must be wrapped with a strong thread so that the turns fit snugly together. This completes the production of the transmitting coil.

Making a take-up coil

The receiving coil must be equipped with a screen. The shielding of the coil is provided as follows. First, it must be wrapped with wire, and then wrapped with a layer of aluminum foil, which again needs to be wrapped with wire.

This double winding guarantees good contact with the aluminum foil. There should be a small gap or gap in the wire windings and in the foil, as shown in fig. 2.40, 6, which prevents the formation of a closed loop around the circumference of the coil.

Assembling a metal detector

Coils made in this way must be fixed with clamps at the edges of a plastic plate and connected to the control unit using a four-core shielded cable.

Connect the two central taps and the screen of the receiving coil to the neutral bus through shielding wires.

Functional Testing

If you turn on the metal detector and the radio located near the coil, you can hear a high-pitched whistle (at the frequency of the metal detector), due to the pickup of the audio signal in the radio. This indicates the health of the metal detector generator.

In this case, it does not matter what band the radio is tuned to, so any cassette recorder can be used instead to check it.

The place of the working position of the coils is determined:

• or by the output signal of the metal detector, which should be minimal;
• or according to the readings of a search device (voltmeter) connected directly to the capacitor C9.

The second option for fitting coils is much simpler.

The voltage across the capacitor should be approximately 6 volts. After that, the outer parts of the coils can be glued with epoxy, and the inner parts, passing through the center, must be left loose, allowing for final adjustment.

Final setup

The final adjustment consists in placing the loose parts of the coils in such a position that non-ferrous objects such as coins cause a rapid increase in the output signal, and other objects cause a slight decrease in it. If the desired result is not achieved, it is necessary to swap the ends of one of the coils.

It should be remembered that the final adjustment or adjustment of the coils should be carried out in the absence of metal objects. After installing and firmly fixing the coils, you need to cover them with a layer of epoxy, then put fiberglass on them and seal all this with epoxy.

After manufacturing the search head, the following actions should be carried out:

• embed capacitor C5 in the circuit;
• set the variable resistor RP1 to the middle position;
• adjust the variable resistor RP2 to the minimum output signal.

At the same time, on one side of the middle position, the variable resistor RP1 provides recognition of steel objects, and on the other side - objects made of non-ferrous metal. With each change in the nominal value of the resistance of the variable resistor RP1, it is necessary to re-configure the device.

In practice, the metal detector is a light, well-balanced, sensitive device. During the first few minutes after turning on the device, there may be a zero level imbalance, but after a while it disappears or becomes insignificant.

Publication: loktek.ru

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