First of all I want to immediately dispel preconceived notions about a person with a metal detector, as the Defiler of graves or the "black Ranger", relation mafia weapons during the first World War. Actually most "seekers hoards are seeking a variety of old metal objects, landfills, vacant lots that are for "treasure hunt" represent the same value as postage stamps for the philatelist. I assure You, found a teaspoon of the 18th century or copper nickle 19th century will be perhaps the most valuable discovery of all that "picks up" the metal detector.
Most homemade metal detectors constructed according to the schemes of zero beat, when there are two generator - reference with constant frequency and search, the frequency of which depends on the surrounding coil of metal objects. When this frequency generators are installed about 100 kHz and about the same. A small detuning of the search generator is required to locate the object, usually done with the help of a variable capacitor or a variable capacitor.
To increase the sensitivity of the detector based on this principle, if you raise the frequency of the reference oscillator so that it was 10 times more frequency
search generator. In this case there are the beating between the reference oscillation generator and 10th harmonic oscillations of the search generator. As a result the detuning of the search generator, at least 10 Hz leads to the increase in the frequency differential ripple at 100 Hz, and this is very noticeable.
Using this principle managed to get a metal detector, capable of detecting a penny coin to a depth of 100 mm or larger object (like a bucket) at a depth of 0.7 - 0.8 I.
Schematic diagram of the detector shown in the figure. In the scheme used two chips KLA, on a chip D1 is made search engine generator (elements D1.1-D1.3) and output amplifier (element D1.4). On the chip D2 - reference crystal oscillator (items D2 1-D2.3) and the mixer (element D2.4). Frequency this generator is determined by the resonance frequency of the quartz resonator Q1.
In search generator uses two element D1.1 and D1.2. Frequency generation specifies the circuit that consists of the search coil L1 and capacitors C1, C2, VD1. The varicap VD1 is used to tune the frequency of this oscillator by a small amount in the process of work. The adjustment is a variable resistor R3, which changes the voltage on the varactors. Compared to tuning by means of the variable capacitor electronic tuning has significant the advantage consisting in the fact that the body preferences (R3) can be arbitrarily removed from the circuit, and thus, placing the case with the generators in the middle search "sticks" of the metal detector, you can withdraw body adjustment (R3) at the end a "stick" that contains the handle of the device, and thus, removing the generator from hands it is possible to avoid the influence on setting up the external tanks (tanks of the human body).
Generator (PIC) is given by elements R2 C4 C5 C6. In fact, it known multivibrator circuit on two elements, but to the midpoint An RC circuit connected circuit, which sets the oscillation frequency.
From the outputs of both generators pulses to the element D2.4, which performed the mixer, and at its output a signal of the beating frequencies. This signal additionally enhanced by the power element D1.4 and is supplied to a sound projector B1, which are electromagnetic head phones type TC-47 resistance of 47 Ohms. You can use regular small the phones music player, including both the primer sequence. In this scheme phones Q1 connected between the output D1.4 and positive power bus, so that the current through phones is at a logical zero at the output of the element D1.4. It's not by chance, the fact that the elements of the circuits C, namely chips CLA develop the output much more current in the zero state (almost equal to 6 mA), and less current units (2-3 mA). As a result, in such inclusion, the maximum volume will be higher.
To adjust the volume serves as a variable resistor R6. Battery power composed of two flat galvanic batteries 4.5 V each (old marking "3336L"). We chose these batteries because when relatively small size, they provide long-lasting operation of the device, are relatively inexpensive, and due to the presence of strip tin-plated or brass contacts, they can be interconnected by soldering, which is not possible in the preparation of the battery separate six elements (requires special cassette with spring pins). Use of the popular "Crown" is also possible, but less preferred, because it has a lower capacity.
The electronic part is mounted in the housing, close foiled fiberglass. There are generators. The housing has a shielding the partition between chips located so that in one the compartment housed the D1 and the capacitors of the search generator and the other with D2 and the elements of the reference oscillator. The baffle serves to eliminate the influence of reference generator the search.
The search coil wound on a ring, made of rigid external cambrica with a diameter of 15 mm. you Can use any other tube of plastic or the pin such diameter of soft plastic. In any case, such a workpiece is bent in a ring outside diameter of 200 mm and is fixed either thermally or with the aid of glue. The resulting ring is winding 50 turns of wire PELSHO 0,27 ( worst can and PEL 0,27-0,35). After the winding is completed, the ring wrapped with one layer of tape, then the coil is shielded using aluminum foil. It is convenient to use a thin foil from old paper capacitors type MBM, BM or BMT for voltage more (foil will be thicker and stronger). The foil tape is wound so that the coils do not touch, that is winding are in increments of 1-5 mm. Then the foil is fixed with the help of another a layer of electrical tape. The role of frame stiffness coil performs a disk with a diameter of 250 mm from thin (1 mm) fiberglass without folgorante. Coil secured it threads, missed in pre-drilled in the disc hole. On one edge of the disc mounted connector military pattern, screwed to the outlet mounted on the end of aluminum tube with a diameter of 15-20 mm. At the other end tube handle with knobs, switch and connector for phones, and the middle the tube is attached to the housing of the electronic device and the battery (using clamps).
Setup is as simple as configuring circuit L1C1 at 100 kHz with an average position R3 (to control the frequency on the pin. 10 D1.3)
Author: Som P. M.