Compact FET metal detector. Scheme, description

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Compact FET 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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Purpose and opportunities

A simple compact metal detector is needed to detect various metal objects in the walls under a layer of plaster (for example, pipes, wires, nails, fittings). This device is completely autonomous, powered by a 9-volt "Krona" battery, consuming 4-5 mA from it.

The metal detector has sufficient sensitivity to detect:

pipes at a distance of 10-15 cm;
wiring and nails at a distance of 5-10 cm.

Sensitivity can be significantly increased by increasing the dimensions of the device, but as practice has shown for domestic applications, this is not always advisable.

Schematic diagram

From a circuitry point of view, the device is an LC field-effect transistor oscillator coupled to a sound indication device.

Indication in the device is used precisely sound, although it is more expensive than light, but much more economical in relation to the battery. Since a conventional LED consumes a current of about 10-15 mA (for normal daytime visibility), and an intermittent sounding buzzer with a piezo emitter is about 2 mA. The device is quite simple, its schematic diagram is shown in Fig. 2.45 a.

Rice. 2.45. Compact FET metal detector: a - circuit diagram; b - printed circuit board; c - appearance of the device

The device works as follows: after applying power, you need to set the generator operation mode at the threshold of stall with a variable resistor R4. While the generator is running (at a frequency of the order of hundreds of kilohertz), the alternating voltage from its output is rectified by diodes VD1, VD2, charges the capacitor C5 to a voltage of 2-3 V.

This voltage is sufficient to block the operation of the intermittent sound generator assembled on the logic elements of the IC DD1 according to a typical circuit. As soon as a metal object appears next to the L1 coil, the generation breaks down, the capacitor C5 is discharged through R20 for 5 ms and now the generator is allowed to work on DD1.1, DD1.2, and already under its control the generator on DD1.3 starts to work intermittently, DD1.4.

The piezo emitter ZQ1 starts beeping intermittently. If the metal moves away from the coil, the generation in the L1C2 circuit is immediately restored and the sound emitter stops.

Setting up the metal detector

Depending on the degree of battery discharge, air temperature, objects located near the metal detector, it is required to frequently adjust it with resistor R4 during operation. The resistor slider must first be brought to the position of maximum resistance - the generation in the LC circuit should fail and the device should beep.

Then slowly reduce the resistance R4 until generation is restored - the device is ready for operation. In some cases, it may be necessary to select the resistor R3. To achieve the maximum sound volume of the emitter, it is desirable to select the value R9 or C7 in such a way that sound resonance occurs in the piezoelectric emitter.

Element base

The device does not contain scarce or expensive parts. The field effect transistor can be any of the KP303 series, ICs can be used cheaper, 176 series. The inductor is frameless, contains 50 + 50 turns of wire with a diameter of 0,1 mm, wound on a mandrel with a diameter of about 7 cm.

All fixed resistors are MLT-0,125 or similar. Capacitors - any small-sized ones, it is possible with a large TKE and an error, since you still have to constantly adjust the generator manually. Piezo emitter is also any. Electrolytic capacitors should not have large leakage currents, especially C1. This requirement is met by most new capacitors of the K50-35 type or similar imported ones.

Mounting a metal detector

All parts, except for the power battery, variable resistor, sound emitter and inductor, are mounted on a printed circuit board (Fig. 2.45, b). The board, together with the power battery, emitter and variable resistor, is placed in a suitably sized case.

The inductor is mounted on a dielectric stand at least 15 cm long, otherwise the device will detect its own battery.

Application of replacement coils

You can even try to make several interchangeable coils of different diameters, because the distance at which the device can detect metal will depend on this.

So, with a coil frame diameter of 12 mm, the detection range is several millimeters, but the metal detector reacts even to sawdust. With large coils, it can only find large objects at long distances.

Publication: qrz.ru

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