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The principle of high-frequency oscillator for detecting metals:

Adjust the gain potentiometer of the high-frequency oscillator to precisely bring it to the critical oscillation state, where the oscillator is on the verge of starting. When the detection coil L1 approaches a metallic object, due to the electromagnetic induction phenomenon, eddy currents are generated in the metal conductor, increasing energy loss in the oscillation circuit and weakening positive feedback, causing the oscillator to weaken or even fail to maintain oscillation. The required minimum energy has ceased. If this change can be detected and converted into an audio signal, the presence of a metallic object below the detection coil can be determined based on the volume of the sound.