Metal detector on transistors with LED indication. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Metal detector on transistors with LED indication. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / metal detectors

Comments on the article

This device, unfortunately, does not allow determining the approximate dimensions and depth of the detected object, as well as the type of metal from which it is made.

Schematic diagram

Transistor metal detector with LED indication
Rice. 2.9. Schematic diagram of a metal detector with LED indication

The circuitry of the proposed metal detector is based on an RF generator, an RF oscillation detector, a DC amplifier with an LED indicator, and a supply voltage stabilizer. The high-frequency generator is made on a transistor T3, in the collector circuit of which an oscillatory circuit is included, consisting of a coil L1 and a capacitor C1, shunted by a resistor R4. The operating frequency of the RF generator is about 100 kHz and is determined by the inductance of the coil L1, which is also a search coil, and the capacitance of the capacitor C1.

If there are no metal objects in the coverage area of the coil L1, the RF signal excited in the coupling coil L2 is detected by a special detector, which is the emitter junction of the transistor T4. In this case, the transistor T4 opens. As a result, transistors T5 and T6, on which the DC amplifier is assembled, will be closed, and the LD1 LED will not light up.

After a metal object is near the search coil L1, its inductance will change. This will lead to the disruption of the oscillations of the RF generator, which will be immediately registered by the transistor T4, which will close. In this case, transistors T5 and T6 will open, and the LD1 LED will start to glow.

The design offered to the attention of readers is one of the variants of metal detectors of the FM (Frequency Meter) type, that is, it is a device based on the principle of analyzing the frequency deviation of the reference oscillator under the influence of metal objects that have fallen into the area of the search coil. In this case, the decision on the presence of a metal object is made by the disruption of the oscillations of the RF generator, which is recorded by a special receiver and fixed visually. The main distinguishing features of this device can be considered an interesting circuit design of the analyzer, as well as the use of an LED as an indicator (Fig. 2.9).

The metal detector with LED indication is powered from a source B1 with a voltage of 9 V. In this case, the supply voltage is stabilized by a special circuit made on transistors T1 and T2, which is a parallel voltage stabilizer.

Details and construction

As in the previous design, any breadboard can be used to make the metal detector in question. Therefore, the used parts are not subject to any restrictions related to overall dimensions. Installation can be both hinged and printed.

Coils L1 and L2 are wound coil by coil on a round ferrite core from the magnetic antenna of a transistor radio. In this case, the L1 coil contains 120 turns, and the L2 coil contains 45 turns of PEV-2 wire with a diameter of 0,3 mm. It should be noted that the sensitivity of the metal detector depends on the length of the ferrite rod used. The longer the ferrite core, the higher the sensitivity of the device.

Instead of transistors of the SF215 type indicated in the diagram (Fig. 2.9), in this design, almost any domestic low-power silicon transistors with a gain of at least 100 can be used. Instead of a GA100 diode, it is recommended to use any germanium diode of the D2 or D9 series, and a VQA13 type LED can be replaced without problems, for example, with an AL102 LED.

The scheme of the proposed metal detector can be greatly simplified if, instead of a parallel voltage regulator, made on the elements T1, T2 and R1-R3, a KS139 zener diode or any integrated regulator for a voltage of 4 V is installed.

As a power source V1, you can use, for example, a Krona battery or two 3336L batteries connected in series.

The board with the elements located on it and the power supply are placed in any suitable plastic or wooden case. The housing cover is equipped with LED LD1 and power switch S1. These elements are connected to the board with a flexible stranded wire. The body of the device can be placed at the end of any convenient handle.

A ferrite rod with a search coil L1 and a communication coil L2 is attached to the lower part of the case from the inside. In this case, the wires going from the coils to the board should be as short as possible. The ferrite core with coils can also be placed in a special case made of insulating material. Many years ago, the author used a plastic case for a toothbrush as such a cover, which was glued from the outside to the bottom of the metal detector body.

Establishment

The main condition that ensures the high-quality tuning of this device is the absence of large-sized metal objects at a distance of at least one meter from the search coil L1. The establishment of a metal detector should begin with setting such an operating mode of the RF generator in which the excited oscillations would be on the verge of collapse. To do this, first by adjusting the resistors R5 and R7, it is necessary to excite the RF oscillations, at which the LED will start to glow. Pre-engine tuning resistor R6 must be set to the middle position. Then, slowly rotating the slider of the resistor R6, it is necessary to ensure that the LED goes out.

If now a metal object is brought close to the ferrite rod, the LED will flash again. It is advisable to repeat the adjustment several times, trying to find such positions of the trimmer resistors R5 and R7, at which the maximum sensitivity of the device is achieved.

Operating procedure

The procedure for working with the device in question is simple and does not need further explanation. When the search coil L1 approaches a metal object, the LED should start to glow.

According to the data given in the original source, this metal detector should have the following sensitivity: large metal objects, such as central heating radiators, can be detected at a distance of 200 mm, small metal objects (scissors) - at a distance of 50 mm, and a copper power cable - at a distance of 40 mm. distance 30 mm. In addition, the device should begin to react to a small screwdriver from a distance of 20 mm, to a small nail driven into the wall from a distance of 10 mm, and to a copper telephone wire from a distance of XNUMX mm.

It should be noted that the parameters of the sample made according to the above scheme were less than those indicated by about 25-30%.

Author: Adamenko M.V.

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