Metal detector on the K176LP2 chip. Scheme, description

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Metal detector on the K176LP2 chip. 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 simple and at the same time reliable and efficient BFO metal detector, operating on the principle of assessing changes in the frequency of the beat signal, can be assembled on just one K176LP2 chip.

Schematic diagram

A distinctive feature of the metal object detector under consideration is not only the use of a K176LP2 type microcircuit, but also the circuitry solutions used in the development of generators and analyzer (Fig. 3.8). At the same time, in this design, the change in the frequency of the beat signal is estimated by ear.

Rice. 3.8. Schematic diagram of a metal detector on a K176LP2 chip (click to enlarge)

This device is based on reference and measuring oscillators, a mixer and an acoustic indication circuit.

In the considered design, two simple LC oscillators are used, made on the elements of the IC1 chip. Circuit solutions of these generators are almost identical. In this case, the first oscillator, which is a reference one, is assembled on the IC1.1 element, and the second, measuring or tunable generator, is made on the IC1.2 element.

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 and C2, as well as the inductance of the coil L1. Capacitor C4 and search coil L2 are used in the measuring oscillator circuit. In this case, both generators are tuned to an operating frequency of approximately 100 kHz. 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. In this case, if there is an object made of ferrous metal near the 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.

From the outputs of the generators, RF oscillations are fed to the corresponding inputs of the mixer, made on the element IC1.3 (pins IC1 / 5,6). The load of the mixer is resistor R5, which also acts as a volume control. Then the low-frequency signal through the resistor R6 and the capacitor C8 goes to the bass amplifier, assembled on the IC1.4 element, and then to the BF1 headphones.

Power is supplied to IC1 from a 1 V source B9 through a filter formed by capacitors C10 and C11.

Details and construction

All parts of the metal detector under consideration (with the exception of the search coil L2, resistor R5, connectors X1 and X2, as well as switch S1) are placed on a printed circuit board 50x50 mm in size (Fig. 3.9), made of one-sided foil-coated getinax or textolite.

Rice. 3.9. The printed circuit board (a) and the location of the elements (b) of the metal detector on the K176LP2 chip

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 for the installation of fixed resistors of the MLT-0,125 type or other small-sized ones (for example, MLT-0,25 or VS-0,125). Capacitors C2-C7 can be of type KT-1, capacitors C8-C10 - type KM-4 or K10-7V, and capacitor C11 - type K50-6.

As a capacitor C1, it is recommended to use any variable capacitor from a small-sized radio receiver (for example, from a Mir transistor 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 150 pF.

The variable resistor R5 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.

Coil L1 of the reference oscillator circuit is made on a frame of a 600NN K8x6x2 ring magnetic circuit and contains 180 turns of PELSHO wire with a diameter of 0,14 mm, which are evenly wound around the entire perimeter of the magnetic circuit.

Search coil L2 contains 100 turns of PELSHO wire with a diameter of 0,27 mm and is made in the form of a ring with a diameter of 230-250 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. A variable resistor R5, a connector X1 for connecting headphones BF1, a connector X2 for connecting a search coil L2 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 L2 search coil at a distance of at least 1,5 m.

Using a frequency meter or oscilloscope, it is necessary to adjust the operating frequencies of the reference and measuring oscillators. The frequency of the reference oscillator is set to approximately 100 kHz by selecting the capacitance of the capacitor C2 and, if necessary, adjusting the core of the coil L1. Previously, the rotor of the capacitor C1 should be set approximately in the middle position. Further, by selecting the capacitance of the capacitor C4, the frequency of the measuring oscillator is selected 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

In practical use of this device, the necessary frequency of the beat signal should be maintained by the variable capacitor C1, 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 L2, 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.

With this device, small objects (for example, a medium-sized coin) can be detected at a depth of up to 50 mm, and a sewer manhole cover at a depth of up to 0,4 m.

Author: R. Sketeris

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