Metal detector on three microcircuits. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Metal detector on three microcircuits. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / metal detectors

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In metal detectors of the BFO (Beat Frequency Oscillator) type, in which the reference and measuring oscillators are assembled on the elements of one microcircuit, there are some drawbacks. First of all, they include the occurrence of parasitic connections between individual elements inside the microcircuit crystal, which are almost impossible to eliminate. That is why in such metal detectors it is necessary to choose a beat frequency of more than 100-300 Hz, which inevitably leads to a decrease in the sensitivity of the device. Therefore, devices based on the analysis of the beat signal, in which the reference and measuring oscillators are assembled on separate microcircuits, are becoming more and more popular.

Schematic diagram

The proposed device is one of the variants of metal detectors of the BFO (Beat Frequency Oscillator) type, that is, it is a device based on the principle of analyzing the beats of two signals that are close in frequency. At the same time, in this design, the assessment of the change in the beat frequency is carried out by ear.

The basis of this device (Fig. 3.10) is the reference and measuring oscillators, matching stages, a mixer and an acoustic indication circuit.

Rice. 3.10. Schematic diagram of a metal detector on three microcircuits (click to enlarge)

In the design under consideration, two simple LC oscillators are used as reference and measuring oscillators. Circuit solutions of these generators are almost identical. In this case, the reference oscillator is assembled on the elements IC1.1 and IC1.2 of the IC1 chip, and the second, measuring or tunable, oscillator is made on the elements IC2.1 and IC2.2 of the IC2 chip. The operating frequency of the reference oscillator is determined by the parameters of the elements that form its oscillatory circuit, that is, the capacitances of the capacitors C1, C3, C5 and C6, as well as the inductance of the coil L1. The measuring oscillator circuit uses capacitors C2, C4, C7, C8 and search coil L2. In this case, both generators are tuned to an operating frequency of approximately 300 kHz.

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 through the capacitors C11 and C12 are fed to the mixer and then to the difference frequency oscillation amplifier, which are made on the IC3 chip. The beat signal is then sent to the BF1 headphones. In this case, the capacitor C15 provides filtering of the high-frequency component of the signal.

Power is supplied to the microcircuits from a source B1 with a voltage of 9 V through a filter formed by capacitors C16 and C17.

When approaching the search coil L2 of the oscillatory circuit of the tunable generator to a metal object, its inductance changes, which causes a change in the operating frequency of the generator. If a magnetic metal object is near coil L2, its inductance increases, which leads to a decrease in the frequency of the generator. Non-ferrous metal reduces the inductance of the coil L2, and the operating frequency of the generator increases. By changing the frequency of the beat signal in the head phones, one can conclude that a metal object has appeared in the search coil coverage area, and by increasing or decreasing the tone, what metal the detected object is made of.

Details and construction

All parts of the metal detector under consideration (with the exception of the search coil L2, connectors X1 and X2, as well as switch S1) are located on a printed circuit board 60x50 mm in size (Fig. 3.11), made of one-sided foil-coated getinax or textolite.

Three-chip metal detector
Rice. 3.11. The printed circuit board (a) and the location of the elements (b) of the metal detector on three microcircuits

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, C5-C7 and C8 can be of the KT-1 type, capacitors C3, C4, C9-C12, C15 and C16 - of the KM-4 or K10-7V type, and capacitors C13 and C17 - of the K50-6 type.

As a capacitor C1, it is recommended to use any variable capacitor from a small-sized radio receiver. You can also use tuning capacitors of the KPK-3 type with a capacity of 25-150 pF. The maximum capacitance of capacitor C1 must be at least 200 pF.

Coil L1 of the reference oscillator circuit is made on a frame of a 600NN K8x6x2 ring magnetic circuit and contains 50 turns of PELSHO wire with a diameter of 0,2 mm, which are evenly wound around the entire perimeter of the magnetic circuit.

Search coil L2 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 L2 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 L2, 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. On the housing cover are installed: connector X1 for connecting headphones BF1, connector X2 for connecting search coil L2 and switch S1.

Establishment

This metal detector should be adjusted in conditions when metal objects are removed from the L2 search coil at a distance of at least 1,5 m.

Direct adjustment of the device is to select the desired beat frequency. To do this, it is recommended to use an oscilloscope or a digital frequency meter. First of all, you should set the frequency of the reference oscillator, controlling its value at pin 10 of IC1. The frequency of the reference oscillator is set to approximately 300 kHz by selecting the capacitances of capacitors C5 and C6, and, if necessary, by adjusting the core of the coil L1. Previously, the rotor of the capacitor C1 should be set approximately in the middle position. Next, choosing the capacitance of capacitor C2, you should set the frequency of the measuring oscillator, controlling its value at pin 10 of IC2. In this case, the frequency of the measuring oscillator is chosen so that its value differs from the frequency of the reference oscillator by about 500-1000 Hz.

This completes the device setup process.

Operating procedure

The practical use of the considered metal detector does not differ significantly from the procedure for working with other BFO devices, in which the assessment of the presence of a metal object in the search coil coverage area is carried out by ear.

If, during operation, any metal object appears in the area of action of the search coil L2, then the frequency of the beat signal in the head phones will change. When approaching some metals, the frequency of the signal will increase, when approaching others, it will decrease. By changing the tone of the beat signal, having a certain experience, one can easily determine what metal, magnetic or non-magnetic, the detected object is made of.

The variable capacitor C1 maintains the necessary 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).

Using this device, small objects (for example, a medium-sized coin) can be detected at a depth of up to 60-70 mm, and a sewer manhole cover at a depth of up to 0,5 m.

Author: Adamenko M.V.

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