Metal detector for frequency comparison. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / metal detectors

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Metal detector features

Even a novice radio amateur can make this design. At the same time, the metal detector has a fairly high sensitivity.

Using the proposed device, it is possible to detect a copper coin with a diameter of 20 mm and a thickness of 1,5 mm at a depth of up to 0,9 m.

Operating principle

The principle of operation of the metal detector is simple, it is based on comparing two frequencies. One of them is reference (from the reference generator), and the other is changing (from the search generator). Moreover, its deviations depend on the appearance of metal objects in the field of a highly sensitive search coil.

In modern metal detectors, to which the design under consideration can be quite reasonably attributed, the reference oscillator operates at a frequency that differs by an order of magnitude from that which occurs in the field of the search coil.

Schematic diagram and PCB

Schematic diagram of the metal detector and printed circuit board is shown in fig. 2.35.

Rice. 2.35. A simple metal detector: a - a schematic diagram; b - printed circuit board

The reference oscillator is implemented on two logic elements 3I-NOT of the DD2 microcircuit. Its frequency is stabilized and is determined by a ZQ1 quartz resonator (1 MHz). The search generator is made on the first two elements of the DD1 chip. The oscillatory circuit here is formed by the search coil L1, capacitors C2 and C3, as well as the varicap VD1. To tune to a frequency of 100 kHz, the potentiometer R2 is used, which sets the required voltage to the varicap VD1.

As buffer signal amplifiers, logic elements DD1.3 and DD2.3 are used, working on a mixer DD1.4. The indicator is a high-resistance telephone capsule BF1, capacitor C10 is used as a shunt for the high-frequency component coming from the mixer.

Food

The metal detector is powered by a 9 V direct current source, a Krona battery is used. Capacitors C8 and C9 successfully work as a filter.

Search coil manufacturing

The search coil requires special precision and attention in manufacturing. It is desirable to wind the coil on a vinyl tube with an outer diameter of 15 mm and an inner diameter of 10 mm, bent in the shape of a circle with a diameter of 200 mm. The coil contains 100 turns of PEV-0,27 wire. When the winding is done, the coil is wrapped with aluminum foil to create an electrostatic shield (reducing the effect of capacitance between the coil and ground).

When winding and wrapping with foil, it is important to prevent electrical contact between the winding wire and the sharp edges of the foil. In particular, "twisting obliquely" will help here. To protect the aluminum coating itself from mechanical damage, the coil should additionally be wrapped with an insulating bandage tape.

The diameter of the coil may be different. But the following rule applies. The smaller the diameter of the search coil, the higher the sensitivity of the entire device, but the search area for hidden metal objects narrows. With an increase in the diameter of the coil, the effect is reversed.

Working with a metal detector

You need to work with a metal detector as follows. Having placed the search coil in close proximity to the earth's surface, adjust the generator with potentiometer R2, and so that no sound is heard in the telephone capsule.

When the coil moves above the surface of the earth (almost close to the latter), a metal object is found - by the appearance of sound in the telephone capsule.

Author: Kochetov N.

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