Interesting circuit solutions can be found when considering designs of metal detectors, the basis of which the principle of evaluation changes in the amplitude of the signal on the coil circuit, the resonant frequency of which close to the frequency applied to it by the signal of the reference oscillator. Main the advantage of these devices, sometimes denoted by the abbreviation OR (Off Resonance), are simplicity and reliability (Fig. 1.3).
Fig. 1.3. A simplified block diagram of the detector, operating at near-resonance frequency
The measuring coil is part of the oscillating circuit, resonant frequency which differs slightly from the frequency of the reference generator. With the appearance of a metal object in the zone of the electromagnetic field, excited by the measuring coil, the resonant frequency of this circuit changes. Depending on what kind of metal was in the area of validity of this device (color or black), the frequency of the circuit or increasing, or reduced. Thus there is a corresponding change in the amplitude of oscillations reference oscillator, which is measured by the analyzer. As a result analyzer generates a control signal to the display unit.
Author: M. V. Adamenko