The detection principle of the handheld metal detector
The detection principle of the handheld metal detector When mentioning metal detectors, people will think of mine detectors, which are used by engineers and soldiers to detect buried mines. A metal detector is an instrument specially used to detect metal. In addition to detecting mines with metal casings or metal parts, it can also be used to detect wires hidden in walls, water pipes and cables buried in the ground, and even Able to explore underground and find metal objects buried in the ground. The metal detector can also be used as a tool for youth defense education and popular science activities, and of course it is also an interesting entertainment toy.
working principle
The detection principle of the metal detector consists of a triode VT1 and a high-frequency transformer T1. It is a transformer feedback LC oscillator. The primary coil L1 of T1 and capacitor C1 form an LC parallel oscillation loop, and its oscillation frequency is about 200kHz, which is determined by the inductance of L1 and the capacitance of C1. The secondary coil L2 of T1 is used as the feedback coil of the oscillator, its "C" terminal is connected to the base of the oscillator tube VT1, and its "D" terminal is connected to VD2. Since VD2 is in the forward conduction state, for high-frequency signals, the "D" terminal can be regarded as grounding. In the high-frequency transformer T1, if the "A" and "D" ends are the first ends of the winding direction of the primary and secondary coils respectively, then the feedback signal input from the "C" end to the base of the oscillation tube VT1 can make the circuit Form positive feedback to generate self-excited high-frequency oscillation. The magnitude of the feedback voltage of the oscillator is related to the turns ratio of the coils L1 and L2. If the turns ratio is too small, it is not easy to start the vibration because the feedback is too weak. If it is too large, the oscillation waveform will be distorted, and the sensitivity of the metal detector will be greatly reduced. . The bias circuit of the oscillator tube VT1 is composed of R2 and diode VD2, and R2 is the current limiting resistor of VD2. Since the forward threshold voltage of the diode is constant (about 0.7V), it is added to the base of VT1 through the secondary coil L2 to obtain a stable bias voltage. Obviously, this voltage-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 voltage regulator circuit, which consists of a voltage regulator diode VD1, a current-limiting resistor R6 and a decoupling capacitor C5. There are two potentiometers connected in series between the emitter and the ground of the oscillator tube VT1, which have the effect of negative feedback on the emitter current. The larger the resistance value, the stronger the negative feedback effect, and the lower the amplification capability of VT1, even making the circuit stop vibrating. RP1 is the coarse adjustment potentiometer of the oscillator gain, and RP2 is the fine adjustment potentiometer.
