Magnetic field indicator. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Indicators, detectors

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Around the conductors through which an alternating current flows, an alternating not only electric, but also a magnetic field is created. Therefore, to detect hidden wiring, you can register an alternating magnetic field.

The proposed magnetic field indicator contains a magnetic field sensor B1, an AC amplifier assembled on the op-amp DA1, and a voltage comparator on the op-amp DA2. An alternating magnetic field excites an alternating voltage in the sensor coil, which, after amplification, is supplied to one of the inputs of the comparator, and a constant regulated voltage from the variable resistor R3 is connected to its second input.

If the sensor is located outside the magnetic field, the voltage amplitude at the output of the op-amp DA2 is small (noise and interference), the comparator output will have a constant voltage of 1 ... 1,5 V. Therefore, the HL1 LED either does not light up or glows weakly - it depends on the properties a specific instance of the DA2 op-amp and the HL1 LED When the sensor is brought closer to the current-carrying conductor, an alternating voltage appears at the output of the DA1 amplifier, sufficient to switch the comparator

Voltage pulses appear at the output of the comparator, and the HL1 LED turns on, signaling that current flows through the conductor under test. To increase the sensitivity of the sensor and the noise immunity of the device, a capacitor C1 is connected in parallel with the winding of the sensor B2. Together with the winding, this capacitor forms a circuit tuned to a frequency equal to the mains frequency. The threshold of the comparator, and hence the sensitivity of the indicator can be adjusted by a variable resistor R3.

Almost all parts of the device are placed on a printed circuit board made of one-sided foil fiberglass 1 mm thick. The board is placed in a separate metal shielding case. The size of the board is chosen so that it can be mounted in rectangular clips from spent batteries "Krona" or "Korund". A probe is attached to the indicator case, at the end of which a magnetic field sensor is mounted.

As sensor B1, you can use a ready-made universal head from a cassette recorder or player. It is easy to make a sensor yourself. The head is based on an annular magnetic circuit with a diameter of 7 mm made of 1500NM ferrite. The ring is carefully broken in half and glued again with epoxy glue, having previously inserted a non-magnetic gasket (for example, made of paper or textolite) about 0,5 mm thick into one of the gaps. This gap is working, it will serve as a sensitive area of ​​\u400b\u2bthe head. Then 0,1 turns of PEV-2 wire XNUMX mm are wound around the ring. The edges of the ring should be blunted. The wire is wound so that the entire winding is located on the half of the ring opposite to the working gap. The winding is impregnated with the same glue, the sensor is fixed on the probe and wrapped in a thin layer of glue to protect it from mechanical damage. Capacitor CXNUMX is placed in the probe next to the sensor. Connect the sensor to the board with a shielded wire.

In the device, in addition to those indicated in the diagram, it is possible to use op-amps K140UD6B, K140UD7A. K140UD7B; LED - AL102A-AL102D, AL307A-AL307N, AL316A, AL316B, AL341A-AL341E, AL360A, AL360B. Resistor R2 - SPO or SP4-1, the rest - VS, MLT; capacitors C1, C5 - K50-6, K53-1, K52-1, the rest - KM, KLS. Establishment comes down to setting the R1C2 circuit to the generator frequency. A capacitor can be made up of several connected in parallel. Generally speaking, the circuit can not be adjusted, and even completely abandon the capacitor C2, but the sensitivity of the indicator will be two to three times less. It is necessary to power the device from a stabilized voltage source with an output current of 60 ... 70 MA, but it is also possible that it can be powered independently from Korund batteries or rechargeable batteries - 7D-0,125.

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