Metal detector on a chip series K176, K561, K564. Scheme, description

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Metal detector on a chip series K176, K561, K564. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / metal detectors

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Operating principle

The principle of operation of this metal detector is based on comparing the frequencies of two generators, one of which is a reference with a stable frequency, and the frequency of the other (search) changes under the influence of closely spaced metal objects.

Schematic diagram

The schematic diagram is shown in fig. 2.24 a. The reference oscillator is assembled on the element DD1.1. Through the resistor R1 and the inductor L1, a negative DC feedback is provided between the output and input of the element. Due to this, the element enters the linear section of the transfer characteristic. This creates conditions for excitation of the cascade at a frequency of about 100 kHz. This frequency is determined by the parameters of the L1C1C2C3 circuit.

Metal detector on a chip series K176, K561, K564
Rice. 2.24. Metal detector on a microcircuit of the K176, K561, K564 series: a - schematic diagram; b - printed circuit board; c - additional matching stage

The logical element of the microcircuit has a high input impedance, so the quality factor of the circuit and the stability of the generator frequency are relatively high. Resistor R1 weakens the shunt effect of the output resistance of the element on the circuit. The shape of the oscillations on the circuit is sinusoidal, and at the output of the element it is rectangular. The oscillation frequency can be changed within small limits by a variable capacitor C2.

The search generator is assembled on the DD1.2 element in a similar way, but the inductor L2 is remote, enclosed in a shielding metal tube. Rectangular oscillations from the reference and search generators are fed to the inputs of the DD1.3 element, which works as a signal mixer.

At the output of the element, there will be both signals of the fundamental frequencies of the generators, and of the difference and sum frequencies (including the frequencies of the harmonic components). One of the most powerful will be the difference frequency signal - they stand out on the resistor R4. The remaining signals are suppressed by the R3C6 filter. The amplitude of the output signal of the element DD1.3 is large enough, a few volts. Therefore, there is no need for an additional amplifier 34.

High-resistance headphones are connected to the XS1 output connector, for example, TON-2 with capsules connected in series. The sound volume is regulated by a variable resistor R4. When using low-resistance phones, the metal detector should be supplemented with a cascade on the transistor VT1 (Fig. 2.24, c), installing a resistor R3 with a resistance of 10 kOhm, and a capacitor C6 with a capacity of 1000 pF.

Element base and recommended replacements

In a metal detector, you can use microcircuits of the K176, K561, K564 series, containing at least three OR-NOT or NAND logic elements, for example, K561LE5, K561LA7, K561LA9, K561LE10. Variable capacitor - from the Yunost KP101 radio designer or another small-sized one with a maximum capacitance of at least 150 pF. The remaining capacitors are KLS, KM, KT, and capacitors C1, C3-C5 must be with TKE no worse than M750, M1500. This will increase the thermal stability of the device.

Variable resistor R4 - SP3-3v with a resistance of 68, 47, 33, 22 and even 10 kOhm, but mechanically connected to the SA1 power switch, the remaining resistors are MLT with a power of 0,125 W. Coil L1 is made on a three-section frame of the IF circuit of the Sokol-403 radio receiver, placed in an armored core with a diameter of 8,6 mm from 600NN ferrite with a trimmer with a diameter of 2,8 mm and a length of 12 mm from the same ferrite. It should contain 200 turns of PEV-2,0,09 wire.

Coil manufacturing

Coil L2 perform so. Thread 7 MGTF-950 conductors into an aluminum thin-walled tube with a diameter of about 18 mm and a length of about 0,07 mm. Then bend the tube on the mandrel, and connect the turns in series with each other.

The inductance of the coil should be approximately 350 uH. Leave the ends of the tube open, but connect a conductor connected to a common wire to one of them.

Design

Connector XS1 - socket for connecting headphones. Power source - battery "Krona" or battery. Details of the metal detector, except for the L2 coil, battery and connector, should be placed on a printed circuit board (Fig. 2.24, b) made of foil fiberglass with a thickness of 1-1,5 mm on the side of the printed conductors.

Unused input terminals of the fourth element of the microcircuit should be connected to a common wire. It is desirable to place the printed circuit board in a metal case (preferably aluminum). It needs to cut windows under the handles of the resistor R4 and the capacitor C2. The L2 coil must be attached to the upper part of the case, and the handle, inside of which the power source is located, is attached to the lower part, and the XS1 connector is installed outside.

Adjustment

With proper installation and serviceable parts, the adjustment comes down to setting the required frequency of the reference oscillator. To do this, the knob of the capacitor C2 should be set approximately to the middle position. It is desirable to achieve zero beats (loss of sound) in phones with the trimmer of the L1 coil.

If the setting is correct, a slight turn of the capacitor knob to either side will produce a low tone sound in the phones. This setting must be carried out at a distance of at least a meter from massive metal objects.

Using a metal detector

Use a metal detector like this. Capacitor C2 sets the possibly lower beat frequency. This will increase its sensitivity, since even small changes in the frequency of the tunable oscillator will be noticeable. Unfortunately, it will not be possible to set a very low frequency, because the sound volume in phones drops sharply at it.

When the coil L2 approaches a metal object, its inductance will change, and, therefore, the frequency of the search generator will also change. If the detected object is made of magnetic material (iron, ferrite, nickel), the inductance will increase and the frequency will decrease. If an object made of non-magnetic material (aluminum, copper, brass) is detected, then the inductance will decrease and the frequency will increase.

Following the above rule, when searching for magnetic materials, the frequency of the reference oscillator should be set higher than the frequency of the search oscillator. Then, when approaching such material, the frequency of the search generator will decrease, and the beat frequency will increase.

When searching for non-magnetic materials, the reference oscillator frequency should be set below the search frequency. If you immediately set the frequency of the reference oscillator higher than the search frequency by 400-500 Hz, then an increase in the beat frequency will indicate that the metal detector is approaching a magnetic metal object, and a decrease in it will indicate a non-magnetic one.

Author: Nechaev I.

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