Low voltage metal detector. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / metal detectors

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When developing this metal detector, the task was to create a small-sized, highly economical device with good repeatability and high performance, using widely used and inexpensive parts.

An analysis of most common circuits showed that they are all powered by a source with a voltage of at least 9 V (that is, "Krona"), and this is both expensive and uneconomical. So, the metal detector assembled on the K561LE5 chip works from one battery for no more than 6-8 hours.

Search coils for most devices are either tapped or have multiple windings. The sensitivity of simple metal detectors is low, while more complex ones require the use of quartz resonators or other scarce parts. As a result, A. Melnikov developed a circuit (Fig. 2.44, a) of a beating metal detector, as the easiest to set up and use.

Rice. 2.44. Metal detector with low-voltage power supply a - circuit diagram; b - a circuit diagram assembled entirely on silicon transistors of the KT3156 type; c - an example of hanging installation

Schematic diagram

The functionally developed scheme includes:

• two generators (the coil of the oscillatory circuit of one of them is the search one);
• balanced mixer;
• audio frequency amplifier loaded on headphones.

For such low-voltage devices, barrier oscillators are most suitable, which operate stably from a supply voltage of 0,8 V or more (for silicon transistors).

Another advantage is that at their output the constant component of the voltage (relative to the collector connected to the device case) is 0,65 V and is stabilized (the emitter-base junction of the transistor plays the role of a stabistor). This effect is used to stabilize the operating point of a balanced mixer.

The sound amplifier is single-transistor. For such a sound amplifier, it is desirable to use a transistor with a current transfer coefficient of at least 200.

Generators operate at different frequencies:

• search - approximately at a frequency of 100 kHz;
• reference - at a frequency of 200 or 300 kHz.

Thus, the balanced mixer extracts the beats between the frequency of the reference oscillator and the 2nd or 3rd harmonic of the search. This solution makes it possible to drastically reduce the phenomenon of "capture" of the frequency of the search generator, which in simple circuits does not allow setting the beat frequency below 200 Hz, and also increases the sensitivity - changing the frequency of the search generator by 10 Hz changes the beat frequency by 20 (or 30) Hz.

Of course, you can increase the frequency of the reference oscillator even more, but in this case, the beat level becomes very small, which means that:

• the sound volume is reduced;
• instability increases, making it difficult to work.

It should be noted that the temperature stability of the circuit is not high, but in practice this does not greatly affect the results:

• firstly, the same loop capacitors are used in oscillatory circuits, therefore their frequency changes in the same way, in the same direction, and as a result, the beat frequency does not change;
• secondly, some simple design measures can increase the thermal stability of the metal detector.

Namely, the following requirements are relevant:

• the search coil must be rigid;
• correct shielding must be applied;
• the board and case must be rigidly fixed to the rod.

The wire from the coil to the circuit must be shielded, it is advisable to use a thin television cable. The bar itself should be made of dry wood or fiberglass. It is advisable to fill the board in the case with paraffin. This will not only protect against moisture, but also from rapid temperature changes.

Search coil manufacturing

The search coil is made of a twisted pair cable, which is used for local networks. The cable must be shielded, category 5, preferably for outdoor installation (it has thicker insulation and the coil is stiffer).

Four turns of cable should be laid in a ring with an outer diameter of approximately 25 cm, and:

• lay first two outer turns on top of each other;
• then wrap in four places with electrical tape;
• then wind two turns inside.

All this then needs to be cut in the middle and wrapped with electrical tape. For such a winding, it is better to use cloth tape. From both ends of the cut, strip the insulation by about one and a half centimeters, irradiate the ends of the wires. The shielding foil must be cut off.

On the one hand, bite the wire that goes along with the foil, and on the other, connect it to one of the wires of the cable. This wire will be the output of the beginning of the winding. It should be noted that the screen must in no case form a short-circuited coil!

Further, the cable conclusions must be connected in accordance with-series, it is almost impossible to make a mistake, because all eight wires are of different colors. The result should be a coil of 32 turns with decent moisture resistance and rigidity.

Making another version of the coil

Another version of the coil is wound with a winding wire with a thickness of at least 0,3 mm. You can drive a few nails into the board at a distance of 40 cm and wind the wire (34 turns) around them, then carefully remove the coil and wrap it with electrical tape. Then the coil must be shielded. It is best to wrap it with foil taken from an old electrolytic capacitor.

It must be borne in mind that there is an alkaline electrolyte inside electrolytic capacitors, so it is advisable to unwind the foil from the capacitor under running water so that the electrolyte does not corrode your fingers. The foil must be wound so that it does not form a short-circuited coil; there should be a gap of about 1 cm between the beginning and end of the winding.

It is useless to try to solder the wire to the foil - it is aluminum and does not tin, so you need to wind several turns of bare tinned wire over the foil - this will be the screen output. It can already be connected to one of the ends of the coil. Next, connect this end to the braid of the shielded wire coming from the coil to the board, and on the board, with a common wire.

The second end of the coil must be connected to the central core wire and on the board with the base of the first transistor of the search generator. Rewind the tape over the foil.

Features of the application of various coils

The performance characteristics of the metal detector depend on the size of the coil. A coil with a diameter of 35 cm confidently catches a track from a tractor caterpillar at a depth of 80 cm, but does not detect coins, rings, nails and other trifles. This option is perfect for searching for ferrous metal, when massive pieces of iron (scrap metal) are of interest.

To search for rings, coins on the beach, you need a coil with a diameter of 15 cm. A small coil with a diameter of 15 cm consists of 6 turns of cable or 50 turns of wire. The depth of detection of coins is about 15 cm. The coil with a diameter of about 25 cm is a compromise solution, it has 40-45 turns.

Element base

Metal detector parts are the most affordable. Resistors and capacitors of almost any type, transistors in generators can be used KT315 (better with the letters B, G, E, some copies with the letters A and C refused to work - the current transfer coefficient is low). KT3102, KT368 work great.

Balanced mixer transistors must be germanium. Any transistor receiver from the 70s and 80s will provide you with plenty of them. Suitable P416 with any letter, P422, P423, P401, GT309, GT322, GT313. The selectors of SKM-24 TVs have GT346A transistors.

Since the operating frequencies of the circuit are not very high, even P27, P28, MP39B, MP42B, which were used in playback amplifiers for reel-to-reel tape recorders, will do. In amplifier 34, it is desirable to use a transistor with the highest current transfer ratio available.

The reference oscillator coil is wound on standard IF circuit fittings from Chinese radio tape recorders and receivers. Some coils have a built-in capacitor that needs to be removed. The coil is carefully unwound, and if it has more than 85 turns, then it is carefully wound with the same wire. If there are fewer turns, then 85 turns are wound with any winding wire. The wire must be thin enough, otherwise the required number of turns will not fit. In extreme cases, you can wind 75 turns.

The capacitance of the loop capacitors does not have to be exactly observed, it is only desirable to use capacitors of the same rating and type in both generators - for better thermal stability. The capacitance of 4700 pF can be from 3300 to 5100 pF, instead of 2200 pF, 1500 or 1800 pF can be used.

Device mounting

The printed circuit board was not developed, it turned out to be more reasonable to abandon the printed wiring and assemble the device on a thin (0,5 mm) piece of textolite, connecting the parts to each other with their own conclusions. An example of such an assembly, which occupies less than half the size of a matchbox, is shown in Fig. 2.44, c.

We used transistors KT3102 and GT322 in metal cases.

Scheme of an alternative metal detector

When manufacturing several metal detectors, the problem suddenly arose with the search for old germanium transistors. And in case radio amateurs did not have them at hand, a circuit was developed, assembled entirely on silicon transistors, such as KT315B. Despite some decrease in sensitivity, the circuit showed good performance. The scheme is shown in fig. 2.44b.

Author: Melnikov A.; Publication: mao-sim.nm.ru

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