metal detector. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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metal detector. 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 original detector (Fig. 2.18) reacts to the approach of metal objects to the magnetic antenna WA1. The antenna itself is part of a high-frequency generator made on a transistor VT1. The frequency of the generator can be changed with a variable capacitor (a KPK-2 capacitor was used with a capacitance change from 25 to 150 pF).

Rice. 2.18. Schematic diagram of a metal detector

Schematic diagram

From the output of the generator, a high-frequency signal enters through the capacitor C4 to the rectifier (or detector) assembled on diodes VD1, VD2. The voltage released on the C5R6 chain opens transistors VT2, VT3. LED HL1 lights up. This state can be achieved by moving the slider of the variable resistor R3 from the bottom according to the output circuit.

Approaching a magnetic antenna, for example, scissors, will cause such a change in the frequency of the generator that the voltage at the base of the transistor VT2 will begin to decrease. The LED will turn off.

Setting

By changing the frequency of the generator with capacitor C1 and selecting the position of the variable resistor R3, it is necessary to achieve the highest sensitivity of the detector - it will react to a metal object from a distance of several centimeters to a magnetic antenna. Perhaps it will be possible to adjust the detector so that it can even respond to the approach of a hand.

In this embodiment, the generator frequency will change due to a change in the capacitance of the oscillatory circuit of the generator.

Design

The magnetic antenna can be made on a rod with a diameter of 8 mm and a length of 80 mm from 600NN ferrite. The winding should be wound in one layer with PEV-2 0,25 wire. It should contain 83 turns with a tap from the 9th turn, counting from pin 1.

Author: Bakomchev I.

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