One of the first of the possible variants of a metal detector items that can come to mind to any person, at least a little familiar with electrical engineering, you can count the device, which is a change of the oscillator frequency under the influence of a metal object.
From school textbooks of physics it is known that the frequency of the signal produced the circuit formed by the parallel connected coil L and a capacitor C, depends on the coil inductance and the capacitance. The change of at least one of these options will change the resonant frequency of the circuit, leading to a corresponding change and generation frequency. Easier to change the inductance of the coil. It is sufficient, for example, is placed near it the subject of a suitable metal. This physical phenomenon it should be in the basis for the design of detectors metal objects, operating on the principle of changes in the frequency (Fig. 1.1). In the specialized literature such devices often referred to as metal detectors FM (Frequency Meter).
Fig. 1.1. A simplified block diagram of the detector working on the principle of frequency counter
Despite the fact that the impact of metal objects placed in the area excited by a field coil, a resonant frequency of the circuit has long been known, practical implementation of such devices until recently was quite difficult. The reason was that to assess the change in the frequency contour of the ear is just not possible.
At present time for analysis and assessment of change of frequency deviation are used microprocessors, operating under the control of special programs.
With the appearance of a metal object in the zone excited by the measuring coil of the electromagnetic field of the resonant frequency of the circuit of the reference oscillator changing. This change is measured by a frequency counter, which is based on a microcontroller. The magnitude of the frequency deviation and its sign depend not only on the depth and magnitude of the subject, but also on the type of metal from which it made. The processed information is supplied to the display unit, in which often used line of LEDs.
It should be noted that the FM detectors have higher sensitivity compared with BFO metal detectors.
Author: M. V. Adamenko