Metal detectors FM (Frequency Meter), theory. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / metal detectors
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One of the first possible options for building a metal object detector, which can come to the mind of any person who is at least a little familiar with the basics of electrical engineering, can be considered a device based on a change in the generator frequency under the influence of a metal object.
It is known from school physics textbooks that the frequency of the signal generated in the circuit formed by the parallel-connected coil L and capacitor C depends on the inductance of the coil and the capacitance of the capacitor. If at least one of these parameters is changed, the resonant frequency of the circuit will change, which will lead to a corresponding change in the generation frequency. The easiest way to change the inductance of the coil. To do this, it is enough, for example, to place an object of the corresponding metal near it. This physical phenomenon underlies the design of metal object detectors operating on the principle of frequency change (Fig. 1.1). In specialized literature, such devices are often called FM (Frequency Meter) metal detectors.
Rice. 1.1. Simplified block diagram of a metal detector operating on the principle of a frequency counter
Despite the fact that the effect of metal objects placed in the zone of the field excited by the coil on the resonant frequency of the circuit has long been known, the practical implementation of such devices has been quite difficult until recently. The reason was that it was simply not possible to evaluate the change in the frequency of the contour by ear.
Currently, microprocessors controlled by special programs are used to analyze and evaluate changes in frequency deviation.
When a metal object appears in the zone of the electromagnetic field excited by the measuring coil, the resonant frequency of the reference oscillator circuit changes. This change is evaluated by a frequency meter, which is based on a microcontroller. The magnitude of the frequency deviation, as well as its sign, depend not only on the depth and size of the object, but also on the type of metal from which it is made. The processed data is sent to the display unit, which often includes a line of LEDs.
It should be noted that FM metal detectors are more sensitive than, for example, BFO metal detectors.
Author: Adamenko M.V.
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Comments on the article:
Valery
Foreign ones have good schemes, there are also Soviet ones, but little competent control of the tuning, it would be good to use foreign technology, but the simplest ones for beginners or as they used to produce circuits on cubes, it was possible to assemble both a receiver and a metal detector and a generator for tuning, etc., etc.
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