Unpretentious metal detector with increased sensitivity, working on the principle of beats. Scheme, description

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Unpretentious metal detector with increased sensitivity, working on the principle of beats. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / metal detectors

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Principle of operation and main characteristics

The metal detector (Fig. 2.28, a) works on the principle of beats, which are formed due to the difference between the oscillations of the reference and search oscillators (on the 5-10th harmonic, the closest in frequency is selected).

This allows you to bring the sensitivity of the device to a high level, which makes it possible to detect:

a five-kopeck coin in the ground at a depth of 10 cm;
steel manhole cover or pipe - at a depth of 65 cm.

Rice. 2.28. Schematic diagram of an unpretentious metal detector with increased sensitivity, working on the principle of beats (click to enlarge)

Performed on an affordable element base, the metal detector does not require careful tuning and is unpretentious in operation. Power supply - from the galvanic battery "Krona".

Schematic diagram

The search generator is assembled according to the so-called "capacitive three-point" scheme on logic elements DD1.1, DD1.2 of the K561LA7 IC. Its oscillatory circuit is formed by:

search coil L1;
capacitors C2-C4;
varicap VD1, the control voltage to which is supplied from the potentiometer R2, which acts as a tuning body for a low beat frequency.

The transistor VT1 is additionally introduced into the circuit. Its purpose is to provide thermal compensation for the VD1 varicap. If the manufactured metal detector is destined to work in favorable conditions, with small fluctuations in ambient temperature, then VT1 can be excluded from this device. The reference oscillator is implemented on two logic elements 3I-NOT of the DD2 (K561LA9) microcircuit. The frequency is stabilized by a ZQ1 quartz resonator (1 MHz).

Both the tunable and the reference oscillators have a buffer stage (logical element DD1.3 and, accordingly, DD2.3) that works on the mixer DD1.4. The difference frequency signal extracted in the mixer is fed to an amplifier (transistor VT2) with an emitter follower (VT3). The sound indicator of metal detection in the ground is the BF1 microtelephone capsule from the hearing aid.

The voltage regulator DA1 at 5 V provides the "electronics" with power, and the semiconductor diode VD2 protects against wrong polarity when the battery is connected.

Setting up the metal detector circuit

The search generator must be "brought" to the required frequency of 100-200 kHz by selecting capacitors C2, C3. With the middle position of the potentiometer R2 slider, it should be ensured that, with the greatest possible ratio of the frequencies of the reference and search oscillators, a beat signal reproduced loudly by the BF1 capsule should be obtained. An emitter-follower amplifier must be tuned by selecting resistors R10 and R12. The reference should be a voltage of 2,5 V on the VT2 collector and on the load resistor R14.

The adjustment of the thermal compensation performed on the transistor VT1 should be carried out by selecting the resistor R5. In this case, it is necessary to ensure that the voltage between the collector and emitter VT1 is within 2-2,5 V.

Search coil manufacturing

The search coil L1 is recommended to be wound on a blank with a diameter of 160 mm. It should contain 60 turns of PEL-0,2 wire. Then you need to make a single-layer winding with electrical tape. After that, the coil should be wrapped (with a slight vacuum between adjacent turns) with a strip of aluminum foil - for electrostatic shielding. Electrical contact between the ends of such a screen is unacceptable (otherwise a closed loop is formed).

The resulting frame-sensor still needs to be wrapped to protect against damage with two or three layers of insulating tape, glued with "epoxy" to the base of fiberglass (non-foiled!) 2-4 mm thick. Using the bracket, the device can be attached to a support rod, such as a fiberglass ski pole with a handle and block.

Compounds

In the case of the block, you need to place the Krona galvanic battery and all the "electronics" mounted on the printed circuit board. Connection of the search coil to the board - a coaxial cable passing inside the carrier rod.

Element base

Now about the radio components needed to assemble the metal detector. All of them, including semiconductor devices and microcircuits, can be selected from the category of inexpensive and widespread. In particular, fixed resistors of the MLT-0,125 type.

As a potentiometer R2, any small-sized potentiometer can be used, preferably with a switch. The latter is conventionally not shown on the circuit diagram. Capacitors of constant capacity C1, C9 and C11 can be any small-sized, but with the ratings indicated on the circuit diagram.

Capacitors C2, C4-C8 have more stringent requirements: for greater reliability and durability of their operation under different conditions, it is advisable to choose these capacitors from among the thermally stable ones. In particular, it is preferable to install a ceramic capacitor C3, which acts as a "trimmer", as the most resistant to significant sudden temperature changes (for example, of the KT4-23 type with a capacity of 4-20 pF). And as C10, C12-C15, you can use the capacitor K53-2, which guarantees stable operation of the circuit.

A sketch of the printed circuit board of a homemade metal detector is shown in fig. 2.29.

Rice. 2.29. The printed circuit board of an unpretentious metal detector with increased sensitivity, operating on the principle of beats

Author: Grichko V.

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