A simple metal detector based on the K561LE5 chip. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / metal detectors

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Schematic diagram

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

Rice. 3.4. Schematic diagram of a metal detector on a K561LE5 chip (click to enlarge)

The basis of the circuit of this device is the measuring and reference oscillators, the mixer and the acoustic indication circuit (Fig. 3.4). The reference and measuring oscillators are made on the elements of the IC1 chip.

The reference oscillator is assembled on the element IC1.1. Negative DC feedback between the output (pin 3) and the input (pins 1, 2) of this element is carried out through the resistor R1 and the inductor L1. The parameters of the coil L1 and the resistor R1 are chosen so that the element operates in the linear section of the transfer characteristic. Thus, conditions are created for excitation of the cascade at a frequency of approximately 100 kHz, which is determined by the parameters of the L1C1C2C3 circuit elements. The IC1.1 element has a high input impedance, so the quality factor of the circuit and the stability of the oscillator frequency are relatively high. Resistor R3 weakens the shunting effect of the output resistance of the element on the circuit. If necessary, the oscillation frequency of the reference oscillator can be changed within small limits by a variable capacitor C2.

The measuring generator is made in a similar way on the element IC1.2. In this case, the operating frequency of this generator is determined by the parameters of the L2C4C5 circuit elements. Coil L2 is a search coil. 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.

Oscillations from the reference and measuring oscillators are fed to the inputs of the IC1.3 element, which acts as a signal mixer. As a result, the output of the IC1.3 element will contain not only the signals of the fundamental frequencies of the generators, but also the signals of the harmonic components of the difference and sum frequencies. One of the most powerful will be the difference frequency signal, which is allocated to the resistor R4. The remaining signals are suppressed by the filter, which includes resistor R3 and capacitor C6.

The output signal via the R4 volume control is fed directly to the BF1 headphones. An additional low-frequency amplifier is not required, since the output signal of IC1.3 is a few volts.

IC1 is powered by a 1V source B9.

Details and construction

For the manufacture of the considered metal detector, you can use any prototyping board. Therefore, the used parts are not subject to any restrictions related to overall dimensions.

Rice. 3.5. The printed circuit board (a) and the location of the elements (b) of the metal detector on the K561LE5 chip

The details of this metal detector are recommended to be placed (with the exception of the search coil L2, resistor R4, connector X1 and switch S1) on a printed circuit board measuring 60x55 mm (Fig. 3.5), made of one-sided foil-coated getinax or textolite. The unused input terminals of the fourth element of the IC1 chip must be connected to a common wire. In this device, you can use microcircuits of the K176, K561, K564 series, containing at least three logical elements "or - not" or "and - not", for example, type K561LE5, K561LA7, K561LA9 or K561LE10.

As a capacitor C2, it is recommended to use any variable capacitor from a small-sized radio receiver. The maximum capacitance of this capacitor must be at least 150 pF. The remaining capacitors can be any small-sized ceramic ones, such as KLS, KM or KT. It should be noted that in order to increase the thermal stability of the device, capacitors C1, C3-C5 must have a TKE no worse than M750 or M1500. Fixed resistors can be any small-sized, for example, type MLT-0,125. Variable resistor R4 can have a resistance of 10 to 68 kOhm. At the same time, 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 can be made on a frame from the IF circuit coil of any small-sized transistor receiver. This coil is wound on a three-section frame of the IF circuit of the Sokol-403 radio receiver. In this case, the L1 coil is placed in an armored core with a diameter of 8,6 mm from 600NN ferrite with a trimmer with a diameter of 2,8 and a length of 12 mm from the same ferrite. Coil L1 contains 200 turns of PEV-2 wire with a diameter of 0,09 mm.

For the manufacture of the L2 search coil, it is recommended to use a piece of copper or aluminum tube with an inner diameter of 6-8 mm and a length of about 950 mm. Inside the tube, stretch a bundle of 18 pieces of MGTF wire with a diameter of 0,07 mm, previously stretched into a PVC tube. The duralumin tube with the wires in it must be bent according to the template into a ring with a diameter of about 300 mm. The end of the wire, which is the beginning of the first turn, should be soldered to the corresponding terminal of the capacitor C4, the beginning of the second turn - to the end of the first turn, and so on. The end of the last turn is soldered to the corresponding terminal of the capacitor C5. The result is a coil containing 18 turns and having an inductance of approximately 350 μH.

In the manufacture of coil L2, special care must be taken to ensure that the ends of the shielding tube do not close, since in this case a short-circuited coil is formed.

Instead of a thin-walled tube for the manufacture of the screen, you can also use ordinary aluminum foil. In this case, additional rigidity of the design of the coil L2 can be given if it is placed between two disks of plywood or getinaks of appropriate sizes.

As a source of sound signals, high-impedance headphones with as high a resistance as possible (about 2000 ohms) should be used. Suitable, for example, well-known phones TA-4 or TON-2. When using low-resistance phones, the metal detector should be supplemented with a cascade on a KT315B transistor by installing a resistor R3 with a resistance of 10 kOhm, and a capacitor C6 with a capacity of 1000 pF.

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 R4, 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

As with the adjustment of other metal detectors, the adjustment of this device should be carried out in conditions when metal objects are removed from the L2 search coil at a distance of at least one meter.

First you need to set the operating frequency of the reference oscillator. To do this, the frequency of the reference oscillator is initially set equal to the operating frequency of the measuring oscillator by adjusting the position of the tuning core of the coil L1 until the sound signal in the headphones disappears completely, that is, until zero beats are set. Previously, the rotor of the capacitor C2 should be set approximately in the middle position. As a result, with a slight turn of the knob of the capacitor C2 in any direction, a low-pitched sound should appear in the phones. If necessary, you can use a frequency counter or an oscilloscope to adjust the frequency of the reference oscillator.

The recommended frequency difference between the reference and measuring generators should be 400-500 Hz. In this case, the frequency of the reference oscillator must be higher than the frequency of the measuring oscillator. The choice of such a high value of the difference frequency is explained by the fact that both oscillators, the reference and the measuring ones, are made on the elements of one common chip of the microcircuit, and therefore parasitic connections inevitably arise between them, which are almost impossible to eliminate. This fact makes it necessary to use beats with a frequency of more than 100-300 Hz in this metal detector, which inevitably leads to a decrease in its sensitivity.

Operating procedure

With error-free installation, serviceable parts and proper adjustment, the metal detector in question is ready for operation immediately after the setup is completed. Before starting search work, it is desirable to set the beat frequency as low as possible with capacitor C2. This will increase the sensitivity of the device, since it will ensure the registration of even small changes in the frequency of the measuring generator. However, it will not be possible to choose a very low beat frequency, because the sound volume in phones will drop sharply at it.

If the frequency of the signal in the headphones changes during operation, this indicates the presence of a metal object in the area of ​​​​the search coil L2. When the coil approaches objects made of magnetic metals (for example, iron, ferrite or nickel), the frequency of the beat signal will increase, and when approaching objects made of non-magnetic metals (for example, aluminum, copper or brass), 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 volume level of the signal in the headphones is regulated by the resistor R4.

Author: I. Nechaev

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