Compact beating metal detector. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / metal detectors
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Principle of operation and possibilities
This metal detector also works according to the beat method. It has two interchangeable search coils of different diameters:
With the first coil (250 mm) you can search for small, shallow metal objects (for example, a nail in a brick wall under a layer of plaster or wallpaper).
With the help of the second coil (500 mm) you can search for deeper objects.
For example, the device notices a 10 mm drill from a distance of about 30 cm, and detects a manhole cover under a 1-meter layer of snow. A coin with a denomination of 5 rubles (modern "pyatak") is detected by a device with a coil of 500 mm at a depth of 12-15 cm.
Schematic diagram of a small-sized metal detector
In the circuit of this metal detector (Fig. 2.36) there are two generators - a reference and a search one. The reference oscillator is made on a D1 chip, its frequency is 100 kHz, stabilized by a Q1 quartz resonator.
The search generator is made on a D2 chip, its generation frequency is about 100 kHz and is determined by the parameters of the L1C2C3C4 circuit. The frequency of this circuit is strongly affected by the change in the inductance of the bulk coil L1 when located near non-metal objects.
Rice. 2.36. Schematic diagram of a small-sized metal detector
Since the parts of the device are metal, so that they do not affect its operation, the coil should be fixed at one end of a dielectric (wooden) rod 80 cm long, and the device itself at its other end.
The signals of the generators are summarized at the element D3.1. At its output, signals of the sum and difference frequencies are formed:
- the sum frequency signal is suppressed by the R7C8R8 circuit;
- the difference frequency signal (beat signal) is fed to the Schmidt trigger, made on the elements D3.2, D3.3, which changes its state with each half-cycle of the input signal.
At the output of the Schmidt trigger, a pulse shaper on the elements C9R10 and transistor VT1 is turned on. As a result of the operation of the Schmidt trigger and the pulse shaper, you can hear:
- beats with a frequency of several kilohertz;
- low-frequency beats with a frequency of 1-10 Hz, which are reproduced by the B1 speaker as crackles (the sound resembles the crackling of a radiation dosimeter).
Such a circuit implementation allows you to hear a very small deviation in the frequency of the search generator.
Interchangeable search coils
The device works with two replaceable coils. Both coils have the same design, but different diameter and number of turns. A coil with a diameter of 250 mm must be wound on a mandrel of the same diameter. In total, 30 turns of PEV 0,61 wire are wound. Then the coil should be removed from the mandrel. The resulting annular tourniquet must be tightly wrapped with a rope coil to coil.
Then the coil must be wrapped with a thin foil tape. It should be wrapped with tape, but not over the entire surface, but in such a way that in the part opposite to the coil leads, an unshielded section of approximately 10 mm long remains. Then the coil must be tightly wrapped with several layers of electrical tape.
Do not use metal parts in the coil mount.
The second coil has a similar design, but its diameter is 500 mm, and the number of turns is 21, the wire is the same. For rigidity, the coils can be fixed with a rope glued with epoxy glue on crosses connected from rails.
Element base
Capacitor C2 - three-section, with an air dielectric for 6-360 pF, all sections are connected in parallel.
As B1, any speaker will do.
Adjustment
Adjustment consists in tuning the L1C3C4C2 circuit (using the selection of C3 and C4) so that with the middle position of C2 it is possible to achieve a very low tone of sound, turning into rare crackles.
Author: Korotkov V.
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