Small metal detector. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Small 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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A small-sized metal detector can detect nails, screws, metal fittings hidden in the 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 when the device approaches a metal object. A distinctive feature of the design is the absence of home-made winding parts. The winding of an electromagnetic relay is used as an inductor.

Schematic diagram

The metal detector (Fig. 2.19, a) contains:

LC generator on the element DD1.1;
RC generator on the elements DD2.1 and DD2.2;
buffer stage on DD1.2;
mixer on DD1.3;
voltage comparator on DD1.4, DD2.3;
output stage on DD2.4.

The device works like this. The frequency of the RC oscillator should be set close to the frequency of the LC oscillator. In this case, at the output of the mixer there will be signals not only with the frequencies of both generators, but also with a difference frequency.

The R3C3 low-pass filter separates the difference frequency signals that are input to the comparator. At its output, rectangular pulses of the same frequency are formed. From the output of the DD2.4 element, they enter through the capacitor C5 to the XS1 connector, into the socket of which a headphone plug with a resistance of about 100 ohms is inserted.

Small metal detector
Rice. 2.19. Small-sized metal detector: a - schematic diagram; b - printed circuit board

The capacitor and the telephones form a differentiating chain, so the telephones will click with each rise and fall of the pulses, i.e. at twice the frequency of the signal. By changing the frequency of clicks, one can judge the appearance of metal objects near the device.

Element base

Instead of those indicated on the diagram, it is permissible to use microcircuits:

K561LA7;
K564LA7;
K564LE5.

Polar capacitor - series K52, K53, the rest - K.10-17, KLS. Variable resistor R1 - SP4, SPO, permanent - MLT, S2-33. Connector - with contacts that close when the phone plug is inserted into the socket. Power source - battery "Krona", "Korund", "Nika" or a similar battery.

Coil preparation

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

The magnetic system of the relay contains two coils wound on separate magnetic circuits and connected in series. The common conclusions of the coils must be connected to the capacitor C1, and the magnetic circuit, as well as the case of the variable resistor, to the common wire of the metal detector.

Printed circuit board

The details of the device, except for the connector, should be placed on a printed circuit board (Fig. 2.19, b) made of double-sided foil fiberglass. One of its sides must be left metallized and connected to the common wire of the other side. On the metallized side, you need to fix the battery and the coil "extracted" from the relay.

The outputs of the relay coil should be passed through the countersunk holes and connected to the corresponding printed conductors. The rest of the details are placed on the print side. Install the board in a plastic or hard cardboard case, on one of the walls of which fix the connector.

Setting up a metal detector

Setting up the device should begin with setting the frequency of the LC generator within 60-90 kHz by selecting capacitor C1. Then you need to move the variable resistor slider to approximately the middle position and, by selecting capacitor C2, achieve an audible signal in the phones. When moving the resistor slider in one direction or another, the signal frequency should change.

To detect metal objects with a variable resistor, you must first set the lowest possible frequency of the sound signal. As you approach the subject, the frequency will begin to change. Depending on the setting, above or below zero beats (equality of oscillator frequencies), or the type of metal, the frequency will change up or down.

Author: Nechaev I.

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