Cable break detector circuit. The simplest hidden wiring detector in a hurry. High-voltage pulse generator for searching for a break in a power transmission line

Often, before carrying out any excavation work or even for the purpose of servicing an underground cable, it is necessary to find this very cable. Agree, it would be very annoying to damage a cable laid underground, for example by catching it with an excavator bucket or accidentally drilling it.

In order to avoid such incidents, it is necessary to first obtain reliable information about the location of the cable underground, the same applies to underground communication pipelines.

If the information about the location of the cable laid underground is not reliable or is not accurate enough, then extra costs and mistakes, and such mistakes are sometimes fraught with disastrous consequences for the health and even life of people.

The condition of underground cables can be assessed by locators, but sometimes it is necessary to localize the cable underground in order to further conduct a thorough inspection and decide on the advisability of certain further actions. It is the methods of localizing cables underground that will be discussed in this article.

As you already understand, finding an underground cable is a responsible matter and requires great care and accuracy. Let's look at ways to find cables underground.

Find documentation

In principle, any facility on the territory of which there are underground cables has the appropriate documentation. You can request drawings and diagrams from the city administration or from utility service, in whose department this object is located.

These drawings should provide all the information about underground communications on the site: underground cables, pipes, channels, etc. This documentation will become a source of initial data for you to build on in order to know where to look. The data may turn out to be inaccurate, and then the operator's next steps will clarify the location of the cable underground.

One of the options is georadar, which can help probe the soil for the presence of a buried cable.

Ground penetrating radars are radars that can be used to examine the walls of buildings, water, land, but not air. These geophysical instruments are electronic devices whose operation can be described as follows.

The transmitting antenna emits radio frequency pulses into the medium under study, then the reflected signal arrives at the receiving antenna and is processed. The processes are synchronized so that the system allows, for example, the location of an underground cable to be seen on a laptop screen.

The use of ground penetrating radar, which operates on the principle of emitting and receiving electromagnetic waves, allows you to accurately determine the depth and size of an underground object. Using ground penetrating radar, it is easy to find plastic pipes and fiber optic cables underground. But only a professional can distinguish a plastic pipe with water from a compaction in the ground. However, it is possible to approximately determine the location of underground communications in various types of soils. Documentation will help the operator navigate and understand what he has discovered - a pipe with water or a pipe with cable.

Negative factors when working with GPR will be: high groundwater levels, clayey soil, sediments - due to their high conductivity, and, as a result, the capabilities of the device will be lower. Heterogeneous sedimentary rocks and rocky soil contribute to signal scattering.

To correctly interpret the information received, it is important to have sufficient experience in this field, and it is best if the operator is a qualified professional. The device itself is quite expensive, and the quality of its use, as you may have guessed, greatly depends on the conditions of the environment being studied.

In some cases, the temperature of an underground power cable can be very different from the temperature of the soil surrounding the cable. And sometimes the temperature difference may be enough to accurately locate the cable. But again, external conditions greatly influence, and for example wind or sunlight will significantly affect the result of the analysis.

Most the right way to search for a cable underground - use the electromagnetic location method. This is the most popular and truly universal way to search for any conducting communications underground, including cables. Based on the amount of information received, this method, perhaps the best.

The boundary of the cable zone is detected. The conductive material of the underground object is identified. The depth of the cable is measured by assessing the electromagnetic field from the center of the underground cable. Can work with any type of soil with equal efficiency. The locator is lightweight and does not require special skills from the operator when handling it.

During its operation, the electromagnetic cable line locator uses the well-known principle of electromagnetic induction: any metal conductor carrying current forms an electromagnetic field around itself. In the case of a power cable, this is the operating voltage current of the line; for a steel pipeline, this is the eddy pickup current. It is these currents that are captured by the device.

Andrey Povny

To prevent the search for wires hidden under a layer of plaster from becoming a real problem when renovating an apartment, it is enough to have an indicator in your arsenal hidden wiring.

Search for wiring

There are many different options for these factory-made devices (for example, the popular Woodpecker detector), but you can also assemble it yourself. To do this, we will consider options for design solutions to such a problem.

Types of hidden wiring finder designs

Depending on the principles of operation, such detectors are usually divided according to the physical characteristics of the electrical wiring:

• electrostatic - performing their functions by determining the electric field generated by voltage when connecting electricity. This is the most simple design, which is easiest to make with your own hands;
• electromagnetic - working by detecting the electromagnetic field created by electric shock in wires;
• inductive metal detectors – working like a metal detector. Detection of metal conductors of de-energized wiring occurs due to the appearance of changes in the electromagnetic field created by the detector itself;
• combined factory-made devices that have increased accuracy and sensitivity, but are more expensive than others. Used by professional builders for large scale work where high precision and productivity are required.

There are also finders that are included in the design of multifunctional devices (for example, a hidden wiring detector is included in the design of the Woodpecker multifunctional electrical network maintenance device).

Devices such as the Woodpecker allow you to combine several useful devices in one device.

Using a voltage indicator as a hidden wiring detector

Most in a simple way to find hidden electrical wiring, you will use an improved voltage indicator that has an autonomous power supply, an amplifier and an audible alert (the so-called sonic screwdriver).

In this case, you do not need to make anything with your own hands and no modifications are required in the tool itself, but only use its capabilities for another purpose. By touching the tip of a screwdriver with your hand, running it along the wall, you can detect hidden electrical wiring that is energized.

The electrical circuit in this case will respond to electromagnetic interference coming from the wiring.

Construction of a hidden wiring detector with your own hands using a circuit with a field-effect transistor

The simplest in design and easiest to manufacture indicator of hidden wiring is a detector that works on the principle of registering an electric field.

It is recommended to do it yourself if you do not have advanced skills in electrical engineering.
To make a simple hidden wiring detector, the circuit of which is based on the use of a field-effect transistor, you will need the following parts and tools:

• soldering iron, rosin, solder;
• stationery knife, tweezers, wire cutters;
• the field effect transistor itself (any of KP303 or KP103);
• speaker (can be from landline phone) with resistance from 1600 to 2200 Ohms;
• battery (battery from 1.5 to 9 V);
• switch;
• a small plastic container for mounting parts in it;
• wires.

Installation of a homemade finder

When working with a field-effect transistor that is vulnerable to electrostatic breakdown, it is necessary to ground the soldering iron and tweezers, and do not touch the leads with your fingers.

The principle of operation of the device is simple - the electric field changes the thickness n-p transition source-drain, as a result of which its conductivity changes.

Since the electric field changes with the frequency of the network, a characteristic hum (50 Hz) will be heard in the speaker, intensifying as it approaches the electrical wiring. It is important here not to confuse the terminals of the transistor, so you need to check the labeling of the terminals.

Since the control output, which responds to changes in the electric field, in this design is a gate, it is better to choose a field-effect transistor in a metal case that is connected to the gate.

Thus, the transistor body will serve as a receiving antenna for the electrical wiring signal. Assembling this finder is reminiscent of assembling the simplest electrical circuit at school, so it should not cause difficulties even for a novice master.

To visualize the process of detecting electrical wiring, you can connect a milliammeter or a dial indicator from an old tape recorder with a ballast resistor rated 1-10 kOhm (selected experimentally) in parallel to the source-drain circuit.

When the transistor closes (approaches the wiring), the indicator readings will increase, indicating the presence of an electric field and voltage in the hidden electrical wiring. Due to the simplicity of the design, installation is hinged, on single-core wires with the necessary elasticity.

Search for electromagnetic radiation in wiring

Another option for a homemade hidden wiring detector is to use a milliammeter connected to a high-resistance inductor.

The coil can be homemade, made in the form of an arc, or you can use primary winding from the transformer by removing part of the magnetic circuit.

This detector does not require power - due to inductance, the receiving coil will act as a current transformer winding in which an alternating current will be induced, to which the milliammeter will respond.

Many craftsmen use the head from an old tape recorder or player as a receiving antenna. In this case, if the amplification path remains in working condition, then it is used entirely, removing the head and connecting it with a shielded cable for ease of search.

As in the first case, a 50Hz hum will be heard in the speaker, and its intensity will depend not only on the distance, but also on the strength of the current flowing in the wires.

Advanced DIY Wiring Detectors

Greater sensitivity, selectivity and detection range are provided by hidden electrical wiring detectors made with several amplification stages based on bipolar transistors or operational amplifiers with elements of logic chips.

For self-made To use a device using these circuits, you need at least minimal experience in radio engineering with an understanding of the principles of interactions of the radio components used. Without going into the operating principles, we can distinguish two significantly different directions:

• amplification of the signal and its subsequent display in the form of a deflection of the indicator arrow or an increase in sound intensity. Here, circuits based on a field-effect transistor or a receiving antenna in the form of an inductor with the addition of amplification stages are improved;

• using the intensity of the electromagnetic field emitted by electrical wiring to change the frequency of visual signals and the tone of an audible warning. Here the receiving element (field-effect transistor or antenna) is included in the frequency control circuit of a pulse generator (monostable, multivibrator) based on bipolar transistors, a logical or operational microcircuit.

These detectors, although the simplest to manufacture, have significant drawbacks. This is a small detection range, as well as the need for voltage in hidden wiring.

Search metal for electrical wiring

To detect wiring in reinforced concrete structures or under significant thickness, without the possibility of applying voltage to the wires, it is necessary to use more complex and accurate designs of detectors that operate like metal detectors.

Independent production of such devices is economically unjustified, and also requires sufficiently deep knowledge of radio engineering, the availability of elemental base and measuring equipment. But an experienced craftsman, to test his strength and for his own pleasure, can use the metal detector circuits available on the network and make similar devices with his own hands.

For less experienced craftsmen, if it is necessary to detect hidden wiring without voltage, it will be easier and more profitable to purchase one of such tools as BOSCH, SKIL “Woodpecker”, Mastech and others.

Wiring Finder for Android

From the owners tablet computers and some smartphones on Android based, it is possible to use your devices as hidden wiring detectors.

To do this you need to download the corresponding software on GooglePlay. The operating principle is that the data mobile devices There is a module that performs the functions of a compass for navigation.

When using the appropriate programs, this module is used as a metal detector.

The sensitivity of this metal detector is not enough to search for treasures underground, but it should be enough to detect metal wires at a distance of several centimeters under a layer of plaster.

But it should be remembered that without the use of specialized instruments, or the use of a professional metal detector capable of distinguishing between metals, it will be impossible to detect electrical wiring hidden in reinforced concrete panels using an improvised Android-based detector.

This device has long been successfully used to search for breaks in multi-core telephone cables, car wiring, and, more recently, to search for hidden wiring.

The circuit contains only one microcircuit and a body kit for it.

The entire cable break detection device is assembled in any suitable housing - both ready-made and homemade. In my first version it was a pencil case for drawing supplies,

And now, glued from plastic using dichloroethane small size box. The circuit is soldered by surface mounting (in the first version it was a board torn out of a non-working player) and installed with heat shrink.

Over 9 years of operation, there have been no problems with the device for finding broken cables, except for replacing cords and batteries.

Since the current consumption is determined mainly by the sound emitter, when using headphones the batteries will last a very long time.

The probe is made from a bicycle spoke (a magnetic one is still in development). Although any shielded wire will do, I make my own (it’s more reliable and durable).

The central vein is MGTF. The braid was pulled off an old tape recorder. And all this is tightened into a PVC tube.

You will also need heat shrink of different diameters and, of course, a little knowledge of electronics. Best regards, UR5RNP.

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There are ways to detect hidden wiring using “folk” methods, without special instruments. For example, you can turn on a large load at the end of this wiring and search by compass deviation or using a coil of wire with a resistance of about 500 Ohms with an open magnetic circuit connected to the microphone input of any amplifier ( music Center, tape recorder, etc.), turning the volume to maximum. In the latter case, the wire in the wall will be detected by the sound of the 50 Hz pickup.

Device No. 1. It can be used to detect hidden electrical wiring, find a wire break in a bundle or cable, or identify a burnt-out lamp in an electric garland. This is the simplest device consisting of a field-effect transistor, a headphone and batteries. The schematic diagram of the device is shown in Fig. 1. The scheme was developed by V. Ognev from Perm.

Rice. 1. Schematic diagram of a simple finder

The principle of operation of the device is based on the property of the field-effect transistor channel to change its resistance under the influence of interference to the gate output. Transistor VT1 - KP103, KPZOZ with any letter index (in the latter, the housing terminal is connected to the gate terminal). The BF1 phone is a high-resistance phone, with a resistance of 1600-2200 Ohms. The polarity of connecting the GB1 battery does not matter.

When searching for hidden wiring, the housing of the transistor is moved along the wall and the maximum volume of sound with a frequency of 50 Hz (if it is electrical wiring) or radio transmissions (radio broadcast network) is used to determine the location of the wires.

The location of a broken wire in an unshielded cable (for example, the power cord of any electrical or radio device), or a burnt-out lamp of an electric garland is found in this way. All wires, including the broken one, are grounded, the other end of the broken wire is connected through a resistor with a resistance of 1-2 MOhm to the phase wire of the electrical network and, starting with the resistor, move the transistor along the bundle (garland) until the sound stops - this is the place where the wire breaks or a faulty lamp.

The indicator can be not only a headset, but also an ohmmeter (shown as dashed lines) or an avometer included in this operating mode. Power supply GB1 and telephone BF1 are not needed in this case.

Device No. 2. Now consider a device made with three transistors (see Fig. 2). A multivibrator is assembled on two bipolar transistors (VT1, VT3), and an electronic switch is assembled on a field-effect transistor (VT2).

Rice. 2. Schematic diagram of a three-transistor finder

The principle of operation of this finder, developed by A. Borisov, is based on the fact that an electric field is formed around an electric wire - this is what the finder picks up. If the SB1 switch button is pressed, but there is no electric field in the area of ​​the WA1 antenna probe, or the finder is located far from the network wires, the VT2 transistor is open, the multivibrator does not work, and the HL1 LED is off.

It is enough to bring the antenna probe connected to the gate circuit of the field-effect transistor closer to the conductor with current or simply to the network wire, transistor VT2 will close, the shunting of the base circuit of transistor VT3 will stop and the multivibrator will start working.

The LED will start flashing. By moving the antenna probe near the wall, it is easy to trace the route of network wires in it.

The field-effect transistor can be any other from the series indicated in the diagram, and bipolar transistors can be any from the KT312, KT315 series. All resistors - MLT-0.125, oxide capacitors - K50-16 or other small ones, LED - any of the AL307 series, power source - Corundum battery or accumulator battery voltage 6-9 V, push-button switch SB1 - KM-1 or similar.

The body of the finder can be a plastic pencil case for storing school counting sticks. The board is mounted in its upper compartment, and the battery is placed in the lower compartment.

You can regulate the oscillation frequency of the multivibrator, and therefore the frequency of LED flashes, by selecting resistors R3, R5, or capacitors CI, C2. To do this, you need to temporarily disconnect the source output of the field-effect transistor from resistors R3 and R4 and close the switch contacts.

Device No. 3. The finder can also be assembled using a generator based on bipolar transistors different structures(Fig. 3). The field-effect transistor (VT2) still controls the operation of the generator when the antenna probe WA1 enters the electric field of the network wire. The antenna must be made of wire 80-100 mm long.

Rice. 3. Schematic diagram of a finder with a generator on

Transistors of various structures

Device No. 4. This device for detecting damage to hidden electrical wiring is powered from an autonomous source with a voltage of 9 V. The circuit diagram of the finder is shown in Fig. 4.

Rice. 4. Schematic diagram of a finder with five transistors

The principle of operation is as follows: one of the wires of the hidden electrical wiring is supplied AC voltage 12 V from a step-down transformer. The remaining wires are grounded. The finder turns on and moves parallel to the wall surface at a distance of 5-40 mm. In places where the wire is broken or terminated, the LED goes out. The finder can also be used to detect core faults in flexible cables and hose cables.

Device No. 5. Hidden wiring detector, shown in Fig. 5, already made on the K561LA7 chip. The scheme is presented by G. Zhidovkin.

Fig.5. Schematic diagram of a hidden wiring finder on the K561LA7 chip

Note.

Resistor R1 is needed to protect it from increased voltage of static electricity, but, as practice has shown, it does not need to be installed.

The antenna is a piece of ordinary copper wire of any thickness. The main thing is that it does not bend under its own weight, that is, it is rigid enough. The length of the antenna determines the sensitivity of the device. The most optimal value is 5-15 cm.

This device is very convenient for determining the location of a burnt-out lamp in a Christmas tree garland - the crackling noise stops near it. And when the antenna approaches the electrical wiring, the detector emits a characteristic crackling sound.

Device No. 6. In Fig. 6 shows a more complex finder, which, in addition to sound, also has a light indication. The resistance of resistor R1 must be at least 50 MOhm.

Rice. 6. Schematic diagram of a finder with sound and light indication

Device No. 7. Finder, the diagram of which is shown in Fig. 7, consists of two nodes:

♦ voltage amplifier alternating current, which is based on the micropower operational amplifier DA1;

♦ oscillation generator audio frequency, assembled on an inverting Schmitt trigger DD1.1 of the K561TL1 microcircuit, a frequency-setting circuit R7C2 and a piezo emitter BF1.

Rice. 7. Schematic diagram of the finder on the K561TL1 chip

The principle of operation of the finder is as follows. When the WA1 antenna is located close to the current-carrying wire of the power supply network, the EMF pickup at a frequency of 50 Hz is amplified by the DA1 microcircuit, as a result of which the HL1 LED lights up. This same op-amp output voltage, pulsating at 50 Hz, drives the audio frequency oscillator.

The current consumed by the device microcircuits when powered from a 9 V source does not exceed 2 mA, and when the HL1 LED is turned on, it is 6-7 mA.

When the required electrical wiring is located high, it is difficult to observe the glow of the HL1 indicator and an audible alarm is sufficient. In this case, the LED can be turned off, which will increase the efficiency of the device. All fixed resistors- MLT-0.125, adjusted resistor R2 - type SPZ-E8B, capacitor CI - K50-6.

Note.

For a smoother adjustment of sensitivity, the resistance of resistor R2 should be reduced to 22 kOhm, and its lower terminal in the diagram should be connected to the common wire through a resistor with a resistance of 200 kOhm.

The WA1 antenna is a foil pad on a board measuring approximately 55x12 mm. The initial sensitivity of the device is set by trimming resistor R2. The faultlessly installed device, developed by S. Stakhov (Kazan), does not need adjustment.

Device No. 8. This universal indicator device combines two indicators, allowing you not only to identify hidden wiring, but also to detect any metal object located in the wall or floor (fittings, old wires, etc.). The finder circuit is shown in Fig. 8.

Rice. 8. Schematic diagram of a universal finder

The hidden wiring indicator is based on the DA2 micropower operational amplifier. When a wire connected to the input of the amplifier is located near the electrical wiring, a pickup frequency of 50 Hz is perceived by the WA2 antenna, amplified by a sensitive amplifier assembled on DA2, and switches the HL2 LED with this frequency.

The device consists of two independent devices:

♦ metal detector;

♦ hidden electrical wiring indicator.

Let's look at the operation of the device according to its schematic diagram. An RF generator is assembled on transistor VT1, which is put into excitation mode by adjusting the voltage based on VT1 using potentiometer R6. The RF voltage is rectified by the diode VD1 and moves the comparator assembled on the DA1 op-amp to a position in which the HL1 LED goes out and the periodic sound signal generator assembled on the DA1 chip is turned off.

By rotating the sensitivity regulator R6, the operating mode of VT1 is set at the generation threshold, which is controlled by turning off the HL1 LED and the periodic signal generator. When a metal object enters the inductance field L1/L2, the generation is interrupted, the comparator switches to a position in which the HL1 LED lights up. A periodic voltage with a frequency of about 1000 Hz with a period of about 0.2 s is applied to the piezoceramic emitter.

Resistor R2 is designed to set the lasing threshold mode at the middle position of potentiometer R6.

Advice.

The receiving antennas WA 7 and WA2 should be as far away from hand as possible and located in the head of the device. The part of the housing in which the antennas are located should not have an internal foil coating.

Device No. 9. Small-sized metal detector. A small-sized metal detector can detect nails, screws, and metal fittings hidden in walls at a distance of several centimeters.

Operating principle. The metal detector uses a traditional detection method based on the operation of two generators, the frequency of one of which changes as the device approaches a metal object. A distinctive feature of the design is the absence of homemade winding parts. The winding of an electromagnetic relay is used as an inductor.

The schematic diagram of the device is shown in Fig. 9, a.

Rice. 9. Small-sized metal detector: a - circuit diagram;

b - printed circuit board

The metal detector contains:

♦ LC generator on element DDL 1;

♦ RC generator based on elements DD2.1 and DD2.2;

♦ buffer stage on DD 1.2;

♦ mixer on DDI.3;

♦ voltage comparator on DD1.4, DD2.3;

♦ output stage on DD2.4.

This is how the device works. The frequency of the RC oscillator must be set close to the frequency of the LC oscillator. In this case, the output of the mixer will contain signals not only with the frequencies of both generators, but also with the difference frequency.

The R3C3 low-pass filter selects difference frequency signals that are fed to the input of the comparator. At its output, rectangular pulses of the same frequency are formed.

From the output of element DD2.4 they are supplied through capacitor C5 to connector XS1, into the socket of which a headphone plug with a resistance of about 100 Ohms is inserted.

The capacitor and the telephones form a differentiating chain, so clicks will be heard in the telephones with the appearance of each rising and falling pulse, i.e., with double the signal frequency. By changing the frequency of clicks, you can judge the appearance of metal objects near the device.

Element base. Instead of those indicated in the diagram, it is permissible to use the following microcircuits: K561LA7; K564LA7; K564LE5.

Polar capacitor - series K52, K53, others - K10-17, KLS. Variable resistor R1 - SP4, SPO, constant - MLT, S2-33. Connector - with contacts that close when the telephone plug is inserted into the socket.

The power source is a Krona, Corundum, Nika battery or a similar battery.

Preparing the coil. Coil L1 can be taken, for example, from an electromagnetic relay RES9, passport RS4.524.200 or RS4.524.201 with a winding resistance of about 500 Ohms. To do this, the relay needs to be disassembled and the moving elements with contacts removed.

Note.

The relay magnetic system contains two coils wound on separate magnetic circuits and connected in series.

The common terminals of the coils must be connected to capacitor C1, and the magnetic circuit, as well as the housing of the variable resistor, to the common wire of the metal detector.

Printed circuit board. Parts of the device, except the connector, should be placed on printed circuit board(Fig. 9, 6) made of double-sided foil fiberglass. One of its sides should be left metallized and connected to the common wire of the other side.

On the metallized side you need to attach the battery and the coil “extracted” from the relay.

The relay coil leads should be passed through the countersunk holes and connected to the corresponding printed conductors. The remaining parts are placed on the printing side.

Place the board in a case made of plastic or hard cardboard, and secure the connector to one of the walls.

Setting up a metal detector. Setting up the device should begin by setting the frequency of the LC generator within the range of 60-90 kHz by selecting capacitor C1.

Then you need to move the variable resistor slider to approximately the middle position and select capacitor C2 to make a sound signal appear in the phones. When moving the resistor slider in one direction or another, the frequency of the signal should change.

Note.

To detect metal objects with a variable resistor, you must first set the sound signal frequency as low as possible.

As you approach the object, the frequency will begin to change. Depending on the setting, above or below zero beats (equality of generator frequencies), or the type of metal, the frequency will change up or down.

Device No. 10. Indicator of metal objects.

When carrying out construction and repair work, it will be useful to have information about the presence and location of various metal objects (nails, pipes, fittings) in the wall, floor, etc. The device described in this section will help with this.

Detection parameters:

♦ large metal objects - 10 cm;

♦ pipe with a diameter of 15 mm - 8 cm;

♦ screw M5 x 25 - 4 cm;

♦ nut M5 - 3 cm;

♦ screw M2.5 x 10 -1.5 cm.

The operating principle of the metal detector is based on the property of metal objects to introduce attenuation into the frequency-setting LC circuit of a self-oscillator. The self-oscillator mode is set near the generation failure point, and the approach of metal objects (primarily ferromagnetic) to its contour significantly reduces the amplitude of oscillations or leads to generation failure.

If you indicate the presence or absence of generation, you can determine the location of these objects.

The schematic diagram of the device is shown in Fig. 10, a. It has sound and light indication of the detected object. An RF self-oscillator with inductive coupling is assembled on transistor VT1. The frequency-setting circuit L1C1 determines the generation frequency (about 100 kHz), and the coupling coil L2 provides the necessary conditions for self-excitation. Resistors R1 (RUB) and R2 (SOFT) can set the operating modes of the generator.

Fig. 10. Metal object indicator:

A - schematic diagram; b - design of the inductor;

B - printed circuit board and placement of elements

A source follower is assembled on transistor VT2, a rectifier is assembled on diodes VD1, VD2, a current amplifier is assembled on transistors VT3, VT5, and a sound alarm is assembled on transistor VT4 and piezo emitter BF1.

In the absence of generation, the current flowing through resistor R4 opens transistors VT3 and VT5, so LED HL1 will light and the piezo emitter will emit a tone at the resonant frequency of the piezo emitter (2-3 kHz).

If the RF self-oscillator is working, then its signal from the output of the source follower is rectified, and the negative voltage from the rectifier output will close transistors VT3, VT5. The LED will go out and the jamming alarm will stop sounding.

When the circuit approaches a metal object, the amplitude of vibrations in it will decrease, or the generation will fail. In this case, the negative voltage at the detector output will decrease and current will begin to flow through transistors VT3, VT5.

The LED will light up and sound sound signal, which will indicate the presence of a metal object near the contour.

Note.

With an audible alarm, the sensitivity of the device is higher, since it starts working at a current of a fraction of a milliampere, while a LED requires much more current.

Element base and recommended replacements. Instead of those indicated in the diagram, the device can use transistors KPZOSA (VT1), KPZZV, KPZZG, KPZOSE (VT2), KT315B, KT315D, KT312B, KT312V (VT3 - VT5) with a current transfer coefficient of at least 50.

LED - any with an operating current of up to 20 mA, diodes VD1, VD2 - any of the KD503, KD522 series.

Capacitors - KLS, K10-17 series, variable resistor - SP4, SPO, tuning - SPZ-19, constant - MLT, S2-33, R1-4.

The device is powered by a battery with a total voltage of 9 V. The current consumption is 3-4 mA when the LED is not lit and increases to approximately 20 mA when it is lit.

If the device is not used often, then switch SA1 can be omitted, supplying voltage to the device by connecting the battery.

Design of inductors. The design of the inductor coil of the self-oscillator is shown in Fig. 10, b - it is similar to the magnetic antenna of a radio receiver. Paper sleeves 2 (2-3 layers of thick paper) are put on a round rod 1 made of ferrite with a diameter of 8-10 mm and a permeability of 400-600; coils L1 (60 turns) and L2 ( 20 turns) - 3.

Note.

In this case, winding must be carried out in one direction and the terminals of the coils must be correctly connected to the autogenerator

In addition, coil L2 should move along the rod with little friction. The winding on the paper sleeve can be secured with tape.

Printed circuit board. Most of the parts are placed on a printed circuit board (Fig. 10, c) made of double-sided foil fiberglass. The second side is left metallized and is used as a common wire.

The piezo emitter is placed on back side board, but it must be isolated from metallization using electrical tape or adhesive tape.

The board and battery should be placed in a plastic case, and the coil should be installed as close to the side wall as possible.

Advice.

To increase the sensitivity of the device, the board and battery must be placed at a distance of several centimeters from the coil.

Maximum sensitivity will be on the side of the rod on which coil L1 is wound. It is more convenient to detect small metal objects from the end of the coil; this will allow you to more accurately determine their location.

♦ step 1 - select resistor R4 (to do this, temporarily unsolder one of the terminals of the diode VD2 and install resistor R4 of such a maximum possible resistance so that there is a voltage of 0.8-1 V at the collector of transistor VT5, while the LED should light up and the sound signal should sound.

♦ step 2 - set the resistor R3 slider to the bottom position according to the diagram and solder the VD2 diode, and unsolder the L2 coil, after which the transistors VT3, VT5 should close (the LED will go out);

♦ step 3 - carefully moving the slider of resistor R3 up the circuit, ensure that transistors VT3, VT5 open and the alarm turns on;

♦ step 4 - set the sliders of resistors Rl, R2 to the middle position and solder coil L2.

Note.

When L2 approaches close to L1, generation should occur and the alarm should turn off.

♦ step 5 - remove coil L2 from L1 and achieve the moment the generation fails, and use resistor R1 to restore it.

Advice.

When tuning, you should strive to ensure that coil L2 is removed to the maximum distance, and resistor R2 can be used to disrupt and restore generation.

♦ step 6 - set the generator to the brink of failure and check the sensitivity of the device.

At this point, setting up the metal detector is considered complete.

The acoustic method is almost universal and in many cable networks is the main method. They can detect damage of various types: single-phase and phase-to-phase short circuits with different transition resistances, breaks of one, two or all wires. In some cases, it is possible to detect several faults on one cable line. The method is used to determine locations of damage in power cable lines, which have the nature of a “floating” breakdown, and can also be used for short circuits with a transient resistance that provides stable spark discharges, and for broken cable cores.

The essence of the method is to create powerful electrical discharges at the site of damage and record sound vibrations on the surface of the earth using sensitive receiving devices. To create powerful discharges at the site of damage, electrical energy is preliminarily accumulated in high-voltage capacitors or in the capacitance of the cable itself by charging from a rectifier unit.

The stored energy is proportional to the capacitance (C) and the square of the voltage (U).

When the breakdown voltage is reached, this energy is consumed in a very short time (tens of microseconds) and a powerful shock occurs at the site of damage. The sound from this impact travels through the environment and can be heard on the surface of the earth. Typically, the frequency of discharges is 2-3 seconds.

Depending on the nature of the cable damage, an appropriate measurement circuit is assembled.

Drawing. Scheme for determining the location of damage during a short circuit between the conductor and the grounded shell (earth): 1 – cable conductors; 2 – cable sheath; 3 – location of damage.

The spark gap breakdown voltage should not exceed 70% of the cable test voltage of this type. In practice, for power cables with operating voltages up to 1, 6, 10 and 35 kV, the pulse voltage should not exceed 8, 25, 30 and 40 kV, respectively.

Drawing. Scheme for determining the location of damage during a short circuit between the residential and grounded shell (earth) when used as charging capacity cable cores: 1 – cable cores; 2 – cable sheath; 3 – location of damage.

In case of damage with an intermittent breakdown and wire breaks, the voltage is supplied to the cable directly from the rectifier unit, and the breakdown voltage at the point of damage can be brought to the test voltage.

Drawing. Scheme for determining the location of damage during a floating breakdown: 1 – cable cores; 2 – cable sheath; 3 – location of damage.

Drawing. Scheme for determining the location of damage when the cable cores break: 1 – cable cores; 2 – cable sheath; 3 – location of damage.

In practice, the occurrence of a stable spark discharge at the site of damage is ensured when the transition resistance is 40 Ohms or more. For lower values ​​of contact resistance and metal short circuits to the shell, the acoustic method cannot be applied. In these cases, the conductive bridge at the site of damage is destroyed by passing large discharge currents.

Currently, acoustic shock wave generators are used to create spark discharges at the site of cable damage. The generator has capacitors that are charged and then discharged into the defective cable through the working spark gap.

Drawing. Acoustic shock wave generator

The location of the cable damage is determined by the maximum audibility of the discharge sound. Typically, the hearing zone on the surface of the earth ranges from 2 to 15 meters, depending on the properties of the soil. The largest audibility zone is provided by dense and homogeneous soils, the smallest zone is provided by loose soils, slag, and construction waste.

If the damage zone is located at a distance of 10-50 m from a busy highway, it is recommended to search for damage at night, since the noise of cars will not allow the acoustic signal to be isolated.

The video below demonstrates acoustic discharges in cables.

The use of the acoustic method is most appropriate for cables laid in the ground and under water. When laying at least part of the cable route in cable ducts and collectors, it is not recommended to use the acoustic method due to the risk of fire. The latter is due to the fact that large pulse currents flowing at the moment of discharge cause sparking at points of contact with grounded structures and other cables, which can lead to fire of paint, cable coating, etc.

Additional material:

1. Receiver for searching for damage in power cables POISK 2006m. Manual.
2. Receiver for searching for damage in power cables P-806. Manual.
3. Acoustic shock wave generator GAUV-6-05-1. Passport.