Underground metal detector working principle

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, pipes and cables buried underground, and even Be able to explore the underground and discover metal objects buried in the ground. Metal detectors can also be used as a tool for carrying out youth defense education and popular science activities, and of course it is an interesting entertainment toy. Metal detectors mainly use these principles: high frequency oscillator, oscillation detector, audio oscillator, complementary multivibrator

Underground metal detector

Their principles are:

1, high frequency oscillator

It is composed of a triode VT1 and a high-frequency transformer T1, and is a transformer feedback type LC oscillator. The primary coil L1 and the capacitor C1 of T1 constitute an LC parallel oscillating circuit whose oscillation frequency is about 200 kHz, which is determined by the inductance of L1 and the capacitance of C1. The secondary winding L2 of T1 acts as a feedback coil for the oscillator, with its "C" terminal connected to the base of the oscillating tube VT1 and "D" terminated to VD2. Since VD2 is in the forward conduction state, the "D" terminal can be regarded as grounded for high frequency signals. In the high-frequency transformer T1, if the "A" and "D" terminals are the head ends of the primary and secondary coil winding directions, respectively, the feedback signal input from the "C" terminal to the base of the oscillation tube VT1 can make the circuit Positive feedback is generated to generate self-excited high frequency oscillations. The magnitude of the oscillator feedback voltage is related to the turns ratio of the coils L1 and L2. The turns ratio is too small. Because the feedback is too weak, it is not easy to start. If the oscillation 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 oscillating tube VT1 is composed of R2 and a diode VD2, and R2 is a current limiting resistor of VD2. Since the diode forward threshold voltage is constant (about 0.7 V), it is applied to the base of VT1 through the secondary winding L2 to obtain a stable bias voltage. Obviously, this regulated 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 stabilizing circuit, and its circuit is composed of a Zener diode VD1, a current limiting resistor R6 and a decoupling capacitor C5. There are two series potentiometers connected between the emitter and the ground of the oscillating tube VT1, which has the negative feedback effect of the emitter current. The larger the resistance value is, the stronger the negative feedback effect is, the lower the amplification capability of VT1 is, and even the circuit is made. Stop vibration. RP1 is the coarse potentiometer for the oscillator gain and RP2 is the fine potentiometer. The principle of the high-frequency oscillator to detect the metal: adjust the gain potentiometer of the high-frequency oscillator, just to make the oscillator in a critical oscillation state, that is, just to make the oscillator start. When the detecting coil L1 is close to the metal object, an eddy current is generated in the metal conductor due to the electromagnetic induction phenomenon, so that the energy loss in the oscillation circuit is increased, the positive feedback is weakened, and the oscillator oscillation in the critical state is weakened or even impossible to maintain. Stop the vibration with the lowest energy required for oscillation. If this change can be detected and converted into a sound signal, depending on the presence or absence of the sound, it can be determined whether there is a metal object under the detection coil.

2, oscillation detector

The oscillation detector is composed of a triode switching circuit and a filter circuit. The switching circuit is composed of a triode VT2, a diode VD2, and the like, and the filter circuit is composed of a filter resistor R3 and filter capacitors C2, C3 and C4. In the switching circuit, the base of VT2 is connected to the "C" end of the secondary coil L2. When the high-frequency oscillator is operating, the oscillating signal coupled via the high-frequency transformer T1 turns VT2 on in the positive half cycle, and the VT2 collector A negative pulse signal is output, and a low level signal is output on the load resistor R4 via a π-type RC filter. When the high-frequency oscillator stops oscillating, there is no oscillation signal at the "C" end, and since the diode VD2 is connected between the VT2 emitter and the ground, the VT2 base is reverse-biased, VT2 is in a reliable off state, and the VT2 collector It is high and passes through the filter to get a high level signal on R4. It can be seen that when the high-frequency oscillator works normally, a low-level signal is obtained on R4, and when it is stopped, it is at a high level, thereby completing the detection of the working state of the oscillator.

3, audio oscillator

The audio oscillator uses a complementary multivibrator consisting of transistors VT3, VT4, resistors R5, R7, R8 and capacitor C6. The complementary multivibrator uses two different types of triodes, of which VT3 is an NPN type triode and VT4 is a PNP type triode connected to a complementary circuit capable of enhancing positive feedback. As the circuit operates, they can alternately enter the on and off states, producing audio oscillations. R7 is both a VT3 load resistor and a VT4 based limit current resistor when VT3 is turned on. R8 is a VT4 collector load resistor, and the oscillation pulse signal is output from the VT4 collector. R5 and C6 are feedback resistors and capacitors whose magnitude affects the oscillation frequency.

4, complementary multivibrator

When the power is turned on, the VT3 base is forward biased due to the bias resistors R1 and R3. Assuming that the VT3 collector current is in the rising phase, the VT4 base current rises, resulting in a sharp increase in the VT4 collector current. The potential of the VT4 collector increases rapidly. The current output by VT4 is charged to C6 through the R5 connected to it, and flows through the base of VT3 to ground, which causes the VT3 base current to rise further. With such repeated cycles, strong positive feedback causes VT3 and VT4 to quickly enter a saturated conduction state, and the VT4 collector is at a high level, causing the multivibrator to enter the first transient steady state process. As the power supply is charged to the C6 via R5 through the saturated VT4, when the VT3 base current drops to a certain level, the VT* is saturated and the collector current begins to decrease, resulting in a decrease in the VT4 collector current, VT4. The collector potential drops, this process further aggravates the rapid decrease of the charging current to C6, the VT3 base potential drops sharply and VT3 cuts off, the VT4 collector rapidly drops to the low level, and the multivibrator flips to the second Temporary steady state. When the multivibrator just enters the second transient state, the result of charging the C6 is that the right end of the capacitor is positive and the left end is negative. Now the right end of C6 is low to the ground, because the voltage across the capacitor C6 cannot jump. Therefore, the base of VT3 is strongly reverse biased by the negative potential of the left end of C6, so that the two transistors remain in the off state for a long time. When C6 is discharged, the current flows from the right end of the capacitor, mainly flows through R5, (R8), R9, VT5 to the ground, and then flows back to the left end of capacitor C6 through the power supply, R6, R1, and R3. Until the end of C6 discharge, the power supply continues to reverse charge C6 through the above circuit, and the left end of C6 is positive. When the potential across C6 rises to 0.7V, VT3 begins to enter the conduction state. After strong positive feedback, it quickly enters the saturation conduction state, causing the circuit to flip again, repeating the previous transient steady state process, so that the circuit generates self-excitation. Multiple resonances. It can be seen from the working process of the circuit that when charging C6, the resistance value of the charging resistor R5 is small, so the charging process is faster, the circuit is in a saturated conduction state for a short time; and in the C6 discharge, it needs to flow through many related The total value of the resistor and the discharge resistor is large, so the discharge process is slow, that is, the circuit is in a long cut-off time. Therefore, the duty cycle of the output waveform from the VT4 collector is large, and the pulse width of the positive pulse signal is narrow, and the oscillation frequency is about 330 Hz.