SENSITIVE METAL DETECTOR

I have designed the metal detector works on the principle of beating is generated due to the difference of the oscillation model and tunable oscillators (5-10-th harmonic, selects the nearest frequency). This allows to increase the sensitivity of the instrument to the point that it becomes possible to detect, say, a five-kopeck coin in the soil at 10 cm, and a steel manhole or pipe — to 65-cm depth. Performed on the available element base, a metal detector does not require careful tuning and maintenance. Power supply — galvanic battery “Crown”.

Tunable generator is assembled by the so-called scheme “capacitive Trento key” on the logic elements DD1.1 — DD1.2 domestic IC K561LA7. His oscillating circuit formed by the search coil L1, condensers C2 — C4 and the varicap VD1, supply the required voltage which is provided by a potentiometer R2, which functions as an on setting to a low frequency beating.

The scheme introduced additional transistor VT1. Its purpose is to provide temperature compensation to the varicap VT1. If the manufactured detector was destined to work in favorable conditions with small fluctuations of the ambient temperature, then VT1 can be removed from the device.

Model generator implemented on two logical elements of Z, NOT the chip DD2 (К561ЛА9). The frequency is stabilized by quartz resonator ZQ1 (1 MHz).

And tunable, and model generators available on the buffer cascade (logic element DD1.3 and, accordingly, DD2.3) working on the mixer DD1.4. Allocated in the last signal of the difference frequency supplied to the amplifier (transistor VT2) with emitter follower (VТЗ). Sound indicator of metal detection in soil serves as the capsule Communicator ВF1 from a hearing aid. The voltage regulator provides DА1 “electronics” unchanged In 5, and semiconductor diode VD2 protects it from incorrect polarity when connecting the battery power supply.

Tunable oscillator “output” to the required 100-200 kHz, selecting the capacitor C2 and changing the capacity “of podstroechnye” Sz at the middle position of the slider of the potentiometer R2. Achieve, to the greatest possible frequency ratio model and tunable generators to loud ВF1 reproduced output level signal of the beating.

Amplifier with emitter follower set up by the selection of resistors R10 and R12. The benchmark can serve as a control for 2.5 V on collector VT2 and the load resistor R14. Adjustment of the temperature compensation performed on the transistor VT1, carry out the selection of R5. Thus to achieve that the voltage between the collector and emitter /T1 was in the range of 2-2. 5 V.

The coil L1 is wound onto the disc with a diameter of 160 mm. It contains 60 turns of wire PEL of 0.2. Then there is a single layer of winding tape. After that, the coil is wrapped (with a small vacuum between adjacent turns) streamers of aluminum foil for electrostatic shielding. Electrical contact between the ends of the screen is invalid (otherwise it is a vicious spiral).

The resulting frame sensor wrapped to protect from damage two or three layers of adhesive tape, glue “epoxy” type EDP to the base (a circle made of fiberglass with a thickness of 2-4 mm) and a bracket attached to the carrier rod — fiberglass ski pole with a handle and a block. The unit casing is placed a galvanic battery “Krona” and the entire “electronics”, mounted on a printed circuit Board 1.5 mm foil getinaks. The connection of the search coil with cost — coaxial cable extending inside the support rod.

Circuit diagram for a homemade metal detector n its embodiment in reality:

1 — the handle;

2 — supporting rod (fiberglass ski pole, L900— 1000);

3 — metal casing of the electronic unit;

4— galvanic battery “Krona”;

5 — printed circuit Board with mounted thereon the electronic components;

6— mounting bracket housing (2 PCs.);

7 — controller “settings”;

8 — coaxial cable;

9 — bracket;

10 search coil (d160 min, 60 turns of PEL-0,2);

11 — insulating wrapping (tape, layer);

12—electrostatic screen (obuhivka thin streamers of aluminum foil with the ends open);

13 —the protective winding (tape, 2-3 layers);

14— the base (a circle of fiberglass, s2—4)

Now, the radio components needed to assemble a metal detector. All of them, including semiconductor devices and circuits, choose from the category of inexpensive and widespread. In particular, fixed resistors type MLT-0,125. As potentiometer R2 will fit any small, preferably with a switch (last on the circuit diagram is not shown).

The capacitors C1, C9 and C11 can be arbitrary small, but with the values specified on the wiring diagram.

More stringent requirements have C2, C4 — C8 for greater reliability and durability of their work in different conditions, these capacitors it is advisable to choose from among thermostable. In particular, performing the role of “podstroechnye” the Sz condenser is preferably ceramic, such as most resistant to considerable sudden temperature changes (e.g., type КТ4-23 with a capacity of 4…20 pF). And as bolsaescola C10,C12 — C15 can be used “electrolytes” K50-6, to ensure stable operation of the scheme.

With such circuitry, the author has produced on the above the development of metal detectors. It is gratifying to note that any difficulties with their set-up and operation have arisen.