Metal detector circuit diagram analysis

This circuit is a bit like a “capacitor three-point” oscillating circuit. The alternating current flows through the collector of the triode. When the metal is encountered, the coil will have a certain suction force on the metal to detect the metal. This circuit uses an LC sine wave oscillator circuit (capacitor three-point) that can generate frequencies of tens of megahertz. The oscillation frequency of the circuit is determined by L, C2, C3, and C4. C1 is used as an AC ground. Resonate on the inductor. Here, only the detection portion of the metal detection is drawn in the circuit. There is no detection circuit. Assume that when L is close to the metal. The high frequency voltage induced by the detecting coil is detected by the high frequency oscillator to the amplifying portion. Finally, there is a speaker output indicating metal. Frequency F = 1/2 π root number LC. For the oscillation, in the transistor circuit analysis, there is an AC equivalent method. See if the three pins of the triode are connected with capacitors, and they are connected in a ring shape. Different capacitors charge and discharge satisfy the basic conditions of oscillation. If the phase shift drawn in the plot is greater than 135 degrees, the oscillation can continue. At the moment of power-on, C1, C2, and C3 are turned on and start charging. The C1 capacity is large, and the voltage is established. When the voltage reaches a certain value, the transistor is turned on, the voltage on C3 drops, the voltage on C4 rises, the transistor cuts off, and C1 starts again. During the charging and induction coils, a current flows, and C3 is recharged. The voltage on C4 is discharged through the parallel resistors, and the voltage of the e-pole begins to drop. When the voltage of the triode is positively biased and the voltage on C1 rises, the triode is turned on again. This cycle.