the principle of underground metal detector
Underground metal detector is the application of advanced technology, it has a wide range of detection, positioning accuracy, resolution and strong, easy to operate and so on. Metal detectors are mainly used to detect and identify hidden underground metal objects. In addition to its military applications, it is also widely used: security inspection, archaeological, prospecting, looking for scrap metal, also known as “iron detector” is a good helper waste recycling.
Underground metal detector using the principle of electromagnetic induction, the use of alternating current through the coil, resulting in a rapidly changing magnetic field. The magnetic field in the metal object can be induced eddy current. Eddy current will produce a magnetic field, in turn, affect the original magnetic field, causing the detector beep.
Built-in high-frequency oscillator by the transistor VT1 and high-frequency transformer T1 and other components, is a transformer feedback LC oscillator. T1’s primary coil L1 and capacitor C1 constitute LC parallel oscillation circuit, the oscillation frequency of about 220kHz, the inductance of L1 and C1 of the capacitance of the decision. T1 secondary coil L2 as the oscillator feedback coil, the “C” end of the oscillation tube VT1 base, “D” termination VD2. Since VD2 is in the forward conduction state, for high frequency signals, the “D” terminal can be regarded as ground. In the high-frequency transformer T1, if the “A” and “D” terminals are respectively the leading ends of the primary and secondary coil winding directions, the feedback signal input from the “C” terminal to the base of the oscillating tube VT1, The formation of positive feedback and self-excited high-frequency oscillation. Oscillator feedback voltage and the size of the coil L1, L2 turns ratio, turns ratio is too small, the feedback is too weak, not easy to start, too much oscillation waveform distortion caused by metal detectors will greatly reduce the sensitivity . Oscillation tube VT1 of the bias circuit by the R2 and diode VD2 composition, R2 is VD2 of the current limiting resistor. Since the forward threshold voltage of the diode is constant (about 0.7V), it is applied to the base of VT1 through the secondary winding L2 to obtain a stable bias voltage. Obviously, this kind of steady voltage bias circuit can greatly enhance the stability of VT1 high-frequency oscillator. In order to further improve the reliability and sensitivity of metal detectors, high-frequency oscillator through the regulator circuit power supply, the circuit by the regulator diode VD1, the current limiting resistor R6 and decoupling capacitor C5 composition. Oscillation tube VT1 emitter and ground indirect two potentiometers in series, with the role of the emitter current negative feedback, the greater the value of its resistance, the stronger the negative feedback, VT1 amplification ability is lower, and even make the circuit Stop vibration. RP1 is the coarse adjustment potentiometer of the oscillator gain, RP2 is the fine adjustment potentiometer.