Metal detector with quartz stabilization on microcircuits. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / metal detectors

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Recently, on the shelves of radio markets, you can find all kinds of designers or sets of component parts, by purchasing which anyone can quickly assemble a simple metal detector without much effort. The author had the pleasure to get acquainted with several children's and youth designers, which can be recommended to adults.

The basis for one of these sets was the metal detector circuit, first published in the late 80s of the last century and after that, with various changes and additions, was published more than once in various domestic and foreign publications.

Schematic diagram

The metal detector under consideration is one of the numerous variants of the BFO (Beat Frequency Oscillator) type device, that is, it is a device based on the principle of analyzing the beats of two frequencies. At the same time, in this design, the assessment of the change in frequency is carried out by ear.

As you know, to a certain extent, you can increase the sensitivity of a BFO type metal detector if you select the frequency value of the reference oscillator 5-10 times higher than the frequency value of the measuring oscillator. In this case, the change in the beat frequency that occurs between the oscillations of the fundamental frequency of the reference oscillator and the nearest frequency harmonic of the measuring oscillator is estimated. As a result, a change in the frequency of the measuring generator under the influence of external influences by only 10 Hz leads to an increase in the frequency of difference oscillations by 50-100 Hz.

Thus, when choosing the frequency of the measuring generator in the range of 100-200 kHz, the frequency of the reference generator should be 500-2 kHz. It should be noted that the frequency of the reference oscillator must be stabilized.

The basis of the circuit of this device (Fig. 3.12) is the measuring and reference oscillators, buffer stages, a mixer and an acoustic indication circuit.

Rice. 3.12. Schematic diagram of a metal detector with quartz stabilization (click to enlarge)

The reference oscillator is made on the elements IC1.1 and IC1.2 of the IC1 chip, its operating frequency is stabilized by a Q1 quartz resonator (1 MHz). The measuring or tunable oscillator is made on the elements IC2.1 and IC2.2 of the IC2 chip. The operating frequency of this generator is determined by the parameters of the elements that form its oscillatory circuit, that is, the capacitances of the capacitors C2, C3 and the varicap D1, as well as the inductance of the coil L1. In this case, the change in the capacitance of the varicap D1 is carried out using a variable resistor R2. The operating frequency of the measuring generator is in the range of 200-500 kHz. Coil L1 of the oscillatory circuit of the tunable generator is a search coil. When approaching a metal object, the inductance of the coil changes, which leads to a change in the operating frequency of the generator and, accordingly, to a change in the beat frequency.

Cascades, made on the elements IC1.3 and IC2.3, provide decoupling between the generators by alternating voltage, and also weaken the influence of the mixer on the generators. From the outputs of the buffer stages, the RF signals are fed to the mixer, made on the element IC1.4. Next, the beat signal is sent to the BF1 headphones. In this case, the capacitor C10 provides filtering of the high-frequency component of the signal.

Power is supplied to the circuit from a source B1 with a voltage of 9 V through a filter formed by capacitors C8 and C9.

Details and construction

All parts of the device under consideration (with the exception of the search coil L1, resistor R2, connectors X1 and X2, as well as switch S1) are located on a printed circuit board measuring 50x50 mm (Fig. 3.13), made of one-sided foil getinax or textolite.

Rice. 3.13. Printed circuit board (a) and arrangement of elements (b) of a metal detector with quartz stabilization

There are no special requirements for the parts used in this device. It is recommended to use any small-sized capacitors and resistors that can be placed on a printed circuit board without any problems. At the same time, the board is designed to install fixed resistors of the MLT-0,125 type or other small-sized ones (for example, MLT-0,25 or VS-0,125). Capacitors C2, C3, C5 and C7 can be of the KT-1 type, capacitors C4, C7, C8 and C10 - of the KM-4 or K10-7V type, and capacitor C9 - of the K50-6 type.

The variable resistor R2 can be any small-sized one, however, it is not recommended to use resistors mechanically connected to the power switch S1 as such a regulator.

The Q1 quartz resonator is mounted on a separate board made of fiberglass, fixed parallel to the main one from the side of the parts. Its frequency can be any within 0,5-1,8 MHz. However, in the event that quartz with a resonance frequency greater than 1 MHz will be used, in some sources it is recommended to turn on a divider between the output of the buffer element IC2.3 (pin IC2 / 10) and the corresponding mixer input on the element IC1.4 (pin IC1 / 13). frequency, lowering the reference frequency to 0,5-1 MHz. Such a divider can be performed on a K176 or K561 series chip.

The search coil L1 contains 50 turns of PELSHO wire with a diameter of 0,27 mm and is made in the form of a ring with a diameter of 180-220 mm. This coil is easier to make on a rigid frame, but you can do without it. In this case, any suitable round object can be used as a temporary frame. The turns of the coil are wound in bulk, after which they are removed from the frame and impregnated with epoxy glue in order to increase mechanical strength. Then coil L1 is shielded with an electrostatic shield, which is an open strip of aluminum foil wound over a bundle of turns. The gap between the beginning and end of the tape winding (the gap between the ends of the screen) should be at least 15-20 mm. In the manufacture of coil L1, it is especially necessary to ensure that the ends of the shielding tape do not close, since in this case a short-circuited coil is formed. To protect against damage, the foil can be wrapped with one or two layers of electrical tape.

High-impedance headphones such as TON-2, TA-4 or similar can serve as a source of sound signals.

As a power source V1, you can use, for example, a Krona battery or two 3336L batteries connected in series.

The printed circuit board with the elements located on it and the power supply are placed in any suitable metal case. A variable resistor R2, a connector X1 for connecting headphones BF1, a connector X2 for connecting a search coil L1 and a switch S1 are installed on the housing cover.

Establishment

This device should be adjusted in conditions when metal objects are removed from the L1 search coil at a distance of at least 1,5 m.

The process of tuning the metal detector consists in tuning the measuring generator to a frequency of 100-200 kHz, which is carried out by selecting the value of the capacitance of capacitor C2. In this case, the slider of the variable resistor R2 should be in the middle position. The frequency of the measuring oscillator is controlled by a frequency meter at the output of element IC1.3 (output IC1/10). Control of the correctness of the selected value of the frequency of the measuring generator is carried out by listening to the difference frequency signal in the headphones. This signal should be loud enough at the largest possible frequency ratio of the reference and measuring oscillators. If necessary, an oscilloscope can be used to evaluate the amplitude of the beat signal.

Operating procedure

In practical use of this device, a variable resistor C1 should be used to maintain the required frequency of the beat signal, which can change under the influence of various factors (for example, when the magnetic properties of the soil change, the ambient temperature, or the battery is discharged).

If during operation any metal object appears in the coverage area of ​​the search coil L1, then the signal frequency in the phones will change. When approaching some metals, the frequency of the beat signal will increase, while approaching others, it will decrease. By changing the tone of the beat signal, having some experience, one can easily determine what metal, magnetic or non-magnetic, the detected object is made of.

Using this device, small objects (for example, a medium-sized coin) can be detected at a depth of up to 80-100 mm, and a sewer manhole cover at a depth of up to 55-65 cm.

Author: Adamenko M.V.

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