A simple transistor metal detector. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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A simple transistor metal detector. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / metal detectors

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The metal detector allows you to detect any metal object at a distance of up to 20 cm. The detection range depends only on the area of the metal object. For those for whom this distance is not enough, for example, treasure hunters, we can recommend increasing the size of the frame. This should also increase the depth of detection.

Schematic diagram

Schematic diagram of the metal detector is shown in fig. 2.50 a.

A simple transistor metal detector
Rice. 2.50. A simple metal detector on transistors: a - a circuit diagram; b - metal detector design

The circuit is assembled on transistors operating in microcurrent mode, and consists of an RF generator (100 kHz) on VT1, which is adjusted by resistor R1 for maximum sensitivity to metal objects. Two frames are used as coils L1 and L2 (Fig. 2.50, b). Transistors VT2, UTZ are included as diodes and provide stabilization of the modes of the oscillator - VT1 and the active detector on VT4 when the supply voltage and temperature change.

Resistor R6 sets the sensitivity of the metal detector. An audio oscillator is assembled on transistors VT5 and VT7, which is turned on by transistor VT6. In order to provide a loud sound of the HF1 piezo emitter, the L3 coil is connected in parallel with it. This increases the voltage across the piezo emitter due to the resonance between the internal capacitance HF1 and the inductance L3. When a metal object enters the field of coils L1-L2, the generator frequency changes, which leads to a decrease in the voltage amplitude at the detector input (VT4). It is locked, and the transistor VT6 opens, which allows the sound generator to work.

Scheme Benefits

This scheme, in comparison with similar devices using the principle of frequency beats, provides greater sensitivity and is easier to manufacture.

Power supply of the metal detector circuit

A battery of the "Korund" or "Krona" type (9 V) was used as a power source, but any stationary source with a voltage of 6-10 V can also be used. The current consumption in standby mode is not more than 1,5 mA, when the sound signal is working - ma.

PCB and placement of elements

All elements of the circuit can be placed on a printed circuit board made of one-sided fiberglass (Fig. 2.51). The case for the frame must be made of any dielectric materials, for example, glued from plexiglass.

Coil manufacturing

Coils L1 and L2 must be the same and contain 40 + 40 turns of PEL wire with a diameter of 0,25 mm (coil perimeter 340 mm).

A simple transistor metal detector
Rice. 2.51. PCB

The L3 coil is wound on two ferrite rings glued together, size K10 x 6 x 3 mm, brand 400-1000NM, and contains 250-300 turns of PEL wire with a diameter of OD mm.

Element base

The tuned resistors R1 and R6 must be of the SP5-16V type, the rest can be any small-sized ones. Capacitors C7 - type K50-35 for 16 V, the rest - type K10-17.

Diode VD1 can be replaced by any pulse. Microswitch SA1 type PD-9-2.

Setting up the metal detector

When setting up the device, if it is not possible to obtain generation on VT1 by adjusting the resistor R1 (you need to control the voltage across this resistor with an oscilloscope), you will need to change the phase of connecting the outputs of the coil L1.

When adjusting the circuit for maximum sensitivity to metal objects, it may be necessary to change the overlap distance of the coils "A" (Fig. 2.50, b), after which the coil frames must be fixed with glue.

Publication: lib.qrz.ru

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