With electronics behind treasures. Highly sensitive homemade metal detector. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / metal detectors

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Among amateur radio designs, developments that help detect metal objects hidden in the ground are of particular interest. Especially if the latter are small in size, lie at a considerable depth and are, moreover, non-ferromagnets.

A lot of high-quality electrical circuits of such devices, called metal detectors by analogy with well-known military developments, and descriptions of fully functional designs have been published in various technical publications. But they are often designed for trained, experienced do-it-yourselfers who have a good material base, scarce parts.

But the design we propose can be completely repeated, made even by a beginner. Moreover, the necessary parts (including a 1 MHz quartz resonator) will be quite affordable to purchase. Well, as for the sensitivity of the assembled metal detector... It can be judged at least by the fact that with the help of the proposed device it is easy to find, for example, a copper coin with a diameter of 20 mm and a thickness of 1,5 mm at a depth of 0,9 m.

Rice. 1. Schematic diagram of a highly sensitive homemade metal detector

Rice. 2. Printed circuit board indicating the location of radio elements on it

The principle of operation is based on the comparison of two frequencies. One of them is standard, and the other is changing. 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 generator operates at a frequency that differs by an order of magnitude from that which occurs in the field of the search coil. In our case, the reference generator (see circuit diagram) is implemented on two logic elements 3I-NOT integrated circuit DD2. Its frequency is stabilized and is determined by a ZQ1 quartz resonator (1 MHz). The generator with a variable frequency is made on the first two elements of the IC DD1. The oscillatory circuit here is formed by the search coil L1, capacitors C2 and C3, as well as the varicap VD1. And for tuning 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. And the capacitor C10 is used as a shunt for the high-frequency component coming from the mixer.

The printed circuit board configuration is shown in the corresponding illustration. And the layout of the radio elements on the side opposite to the printed conductors is given here in a different color.

The metal detector is powered by a 9 V direct current source. And since high stabilization is not necessary here, a Krona battery is used. Capacitors C8 and C9 successfully work as a filter.

The search coil requires special precision and attention in manufacturing. It is wound 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, it is wrapped with aluminum foil to create an electrostatic shield (reducing the effect of capacitance between the coil and ground). 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. And to protect the aluminum coating itself from mechanical damage, the coil is additionally wrapped with an insulating bandage tape.

The diameter of the coil may be different. But the smaller it is, 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.

Work with a metal detector as follows. Having placed the search coil in close proximity to the earth's surface, tune the generator with potentiometer R2. And so that the sound is not heard in the telephone capsule. When the coil moves above the surface of the earth (almost close to the latter), the cherished place is found - by the appearance of sound in the telephone capsule.

When using the device discussed above to find objects hidden in the ground of archaeological and national cultural value, prior permission is required from the relevant authorities.

Author: N.Kochetov

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