Metal detector on transistors with a pointer indicator. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / metal detectors

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A feature of the proposed metal detector is an interesting circuit design of the analyzer and indicator. In this case, a pointer device is used as an indicator.

It should be noted that the considered metal detector has a relatively high sensitivity. In addition, according to the direction of deviation of the indicator arrow, it is possible to determine the type of metal (non-ferrous or black) from which the detected object is made.

Schematic diagram

The metal detector (Fig. 2.7) consists of two generators, an indication circuit and a supply voltage stabilizer.

Rice. 2.7. Schematic diagram of a metal detector with a dial indicator (click to enlarge)

A measuring oscillator is assembled on transistors T1 and T2, the oscillation frequency of which depends on the parameters of the circuit formed by the coil L2, as well as capacitors C1 and C2 connected in parallel. The reference oscillator is assembled on transistors T3 and T4 in a similar way. The oscillation frequency of this generator is determined by the parameters of the circuit made on the elements L3, C4 and C5.

The circuit that analyzes the occurrence of a deviation (deviation) in the frequency of the signal of the measuring oscillator compared to the frequency of the signal of the reference oscillator contains a measuring circuit, which consists of a pointer indicator PA1 with a zero mark in the middle of the scale, a capacitor C6 and diodes D1-D4. In the same circuit, the sign of the frequency deviation is estimated. The oscillations of the reference oscillator are fed into the measuring circuit through the coupling coil L4, and the signal from the measuring oscillator is fed through the coupling coil L1. In this case, the entire circuit is balanced so that if the oscillation frequencies of both generators coincide, the arrow of the PA1 indicator will be at zero division of the instrument scale.

When a metal object appears in the coverage area of ​​​​the search coil L2, the resonant frequency of the L2C1C2 circuit will change. This will lead to a change in the operating frequency of the measuring generator and, as a result, to the deviation of the indicator arrow PA1. The arrow deviation serves as a source of information about the detection of a metal object. The angle of deflection of the indicator arrow PA1 depends on the dimensions of the object and the distance from it to the measuring coil.

When an object made of ferrous metal is near the measuring coil L2, the operating frequency of the measuring generator, made on transistors T1 and T2, will decrease, and the indicator needle will deviate in one direction. If the object is made of non-ferrous metal (for example, brass), the frequency of the generator will increase, while the indicator needle will deviate in the other direction.

The direction of the arrow deviation depends on the polarity of the PA1 indicator connection, on the scale of which, after calibration, the corresponding inscriptions can be applied.

Rice. 2.8. Coil frame design L1 and L2

Coil L1 contains 20 turns, and L2 - 60 turns of PEV-2 wire with a diameter of 0,31 mm, wound round to round. The coils are protected by an electrostatic shield, which is an open brass tape wound on the surface of the frame. The gap between the beginning and end of the tape winding must be at least 10 mm.

In the manufacture of coils, it is especially necessary to ensure that the ends of the tape do not short circuit, since in this case a short-circuited coil is formed.

Coil L3 contains 160 turns, and L4 - 50 turns of PELSHO wire with a diameter of 0,12 mm, wound in bulk on a frame with a diameter of 7,5 mm. A tuning core with a diameter of 2,5 mm and a length of 12 mm, made of 600NN ferrite, is installed inside the frame. The frame with coils L3 and L4 is placed in an electrostatic screen with a hole opposite the tuning core. The screen must be grounded.

The supply voltage of 12 V is supplied to the measuring and reference generators from source B1 through a voltage regulator assembled on a zener diode D5 and a transistor T5.

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. Installation can be both hinged and printed.

Resistors can be, for example, type MLT-0,5, capacitors C1, C2, C4, C5, C7 - type KM or KLS. As containers C3 and C6, you can use any metal-paper capacitors, such as MBM or BMT. Capacitors C8, C9 can be replaced with any electrolytic, for example, type K50-6, transistors KT603G - with other transistors of this series or transistors of the KT315 series with a current transfer coefficient of at least 60, transistor MP42A - with any of the series MP39 - MP42 or KT361, diodes D9B - other diodes of this series. As an indicator PA1, it is recommended to use a pointer device of the M24 type with a total deflection current of the needle of 100 μA and zero in the middle of the scale.

Coils L1 and L2 are placed on a frame (Fig. 2.8) made of fiberglass or any other insulating material. The board with the elements located on it and the power supply are placed in any suitable plastic or wooden case. The PA1 indicator, the S1 switch, and the X1 connector for connecting the L1 and L2 coils are installed on the housing cover. These elements are connected to the board with a flexible stranded wire.

The frame with coils L1 and L2 is placed at the end of any convenient handle. In this case, the outputs of the coils are connected to the mating part of the X1 connector with a flexible stranded shielded wire. As a power source B1, you can use, for example, three 3336L batteries connected in series, or a rechargeable battery.

Establishment

Before tuning, the device should be positioned so that the search coil L2 is at least 1,5 m away from metal objects.

It is necessary to connect an oscilloscope to coil L1 and, by selecting the capacitances of capacitors C1, C2, set the frequency of the measuring generator, made on transistors T1 and T2, equal to 100 Hz. The shape of the oscillations is corrected by the selection of the resistances of the resistors R1-R3. Similarly, the reference oscillator is adjusted, while the oscilloscope is connected to the L4 coil, and the oscillation shape is corrected by selecting the resistances of the resistors R4-R6. Before starting tuning, the tuning core of the L4 coil should be set to the middle position.

Next, it is necessary to set equal oscillation amplitudes on coils L1 and L4, which should be in the range of 0,8-1 V. If necessary, the amplitude of the signals can be changed by selecting the number of turns of coils L1 and L4. After that, by rotating the tuning core of the L3 coil, you should set the indicator arrow PA1 to the zero mark of the scale.

Operating procedure

A feature of this metal detector is that no additional settings and adjustments are required during search operations.

When approaching the measuring coil L2 of an object made of ferrous metal, the operating frequency of the measuring generator decreases. In this case, the indicator arrow PA1 deviates in any direction. If the object is made of non-ferrous metal, such as brass, the frequency of oscillation of the measuring generator increases. In this case, the indicator arrow deviates in the opposite direction. Thus, it is possible to determine not only the presence of a metal object in the area of ​​​​action of the search coil, but also to assess what metal, non-ferrous or black, it is made of.

With the help of the considered metal detector, metal objects such as cans can be detected at a depth of up to 20-30 cm.

Author: Adamenko M.V.

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