High-frequency oscillation metal detector production, High-Frequency Oscillatory Metal Detectors
Keywords: metal detector, detector circuit, metal detector circuit
Talking about the metal detector, people will think of the diarrhea, and the engineers use it to detect the buried mines. The metal detector is a instrument specifically used to detect the metal, in addition to the mine with metal housing or metal parts, can also be used to detect wires covered in the wall, burying underground water pipes and cables, even It is possible to explore the treasure underground and found a metal object buried underground. Metal detectors can also be used as utensils to carry out adolescent defense education and science activities, of course, is not a fun entertainment toy.
working principle
As can be seen from the circuit block diagram of the metal detector, the metal detector consists of a high frequency oscillator, an oscillator detector, an audio oscillator, and a power amplifier.
High frequency oscillator
Composed of triode VT1 and high frequency transformer T1, is a transformer feedback type LC oscillator. The primary coil L1 and the capacitor C1 of T1 form an LC parallel oscillation loop whose oscillation frequency is about 200 kHz, which is determined by the electrical capacity of L1 and the capacitance of C1. The secondary coil L2 of T1 serves as a feedback coil of the oscillator, and its "C" ends the base of the oscillating tube VT1, "D" terminates VD2. Since the VD2 is in a forward conduction state, the "D" end can be considered grounding. In the high-frequency transformer T1, if "a" and "D" are the first end, the secondary coil winding direction, respectively, from the "C" terminal input to the feedback signal of the oscillating tube VT1 base, the circuit can be made Form positive feedback to produce self-excited high frequency oscillation. The size of the oscillator feedback voltage is related to the number of turns of the coils L1, L2. The number of turns is too small. Since the feedback is too weak, it is not easy to start, and the oscillation waveform distortion will be caused, and the metal detector sensitivity is reduced. . The bias circuit of the oscillating tube VT1 is composed of R2 and diode VD2, and R2 is a current limiting resistor of VD2. Since the diode is in the forward threshold voltage constant (about 0.7V), the base of the VT1 is applied to the base of the VT1 through the secondary coil L2 to obtain a stable bias voltage. Obviously, this stabilizing bias circuit can greatly enhance the stability of the VT1 high frequency oscillator. In order to further improve the reliability and sensitivity of the metal detector, the high-frequency oscillator is powered by a regulated circuit, and its circuit is composed of a regulator diode VD1, a current limiting resistor R6, and a decoupling capacitor C5. The oscillating tube VT1 emitter and the ground therewith have two tandem potentiometers, and has a negative feedback effect of the emitter current. The larger the resistance value, the stronger the negative feedback effect, the lower the magnification of VT1, and even make the circuit Stop. RP1 is a coarse tunator of the oscillator gain, and RP2 is a fine adjustment potentiometer.
The principle of high frequency oscillator detection metal
Adjust the gain potentiometer of the high-frequency oscillator, just make the oscillator in a critical oscillation state, that is, just cause the oscillator to vibrate. When the detecting coil L1 is approached to the metal object, the eddy current is generated in the metal conductor due to the electromagnetic induction, which increases the energy loss in the oscillation circuit, and is weakened, and the oscillator in the critical state is reduced, and even the oscillation cannot be maintained. The minimum energy required is stopped. If this change can be detected, converted into a sound signal, depending on the sound, it can be determined whether or not the probe coil has a metal object.
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