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It 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, underground pipes and cables, and even underground. Treasure, found metal objects buried in the ground. Metal detectors can also be used as a tool for youth defense education and popular science activities, and of course it is an interesting entertainment toy.
working principle:
It can be seen from the circuit block diagram of the metal detector that the metal detector is composed of a high frequency oscillator, an oscillation detector, an audio oscillator and a power amplifier.
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.
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, etc., 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 The negative pulse signal is output, and a low level signal is output on the load resistor R4 through the 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 emitter of VT2 and the ground, the base of VT2 is reverse-biased, VT2 is in a reliable off state, 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.
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.
Power amplifier:
The power amplifier is composed of a triode VT5, a speaker BL, and the like. The positive pulse audio signal output from the multivibrator is input to the base of VT5 through the current limiting resistor R9 to turn it on, generating a transient strong current in the BL, and driving the speaker to sound. Since the VT5 is in the switching state and the on-time is very short, the power amplifier is very power-saving and can be powered by a 9V laminated battery.
The principle of high frequency oscillator for detecting metal:
Adjusting the gain potentiometer of the high-frequency oscillator just happens to put 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, due to the electromagnetic induction phenomenon, an eddy current is generated in the metal conductor, so that the energy loss in the oscillation circuit is increased, the positive feedback is weakened, the oscillator oscillation in the critical state is weakened, and even the oscillation cannot be maintained. The required zui low energy and stop vibration. 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.
The working principle of the 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 from 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 a temporary 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 VT3 exits the saturation conduction state, and the collector current begins to decrease, resulting in a decrease in the VT4 collector current, VT4 set. The electrode 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 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 discharges, 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 its oscillation frequency is about 330 Hz.
Debugging and using:
In addition to the sensitivity adjustment potentiometer, the metal detector circuit has no adjustment part, and the circuit can work normally as long as the welding is correct. When the whole machine is static, that is, when the speaker does not sound, the total current is about 10mA. When the sound of the metal speaker is detected, the current of the whole machine rises to 20mA. A new laminated battery can work for 20 to 30 hours.
If the newly welded metal detector is not working properly, first check whether the components on the circuit board and the wiring are soldered incorrectly, and then measure whether the battery voltage and the power supply loop are normal. The voltage of the Zener diode VD1 is between 5.5 and 6.5V. Do not weld the polarity of VD2. Do not weld the primary and secondary ends of the oscillating coil in the disc.
Before using the metal detector, you need to adjust the length of the probe. Just loosen the black rubber, push and pull the rubber sleeve to the appropriate length, and then rotate the inner tube of the glue to tighten the cable and make the tip of the handle face up. After the Zui, the rubber gel is tightened to lock the rubber sleeve. Thus, when holding the detector handle, the thumb is just right next to the sensitivity adjustment potentiometer.
When adjusting the sensitivity of the metal detector, the detection disc (oscillation coil) should be away from the metal, including the paper with aluminum foil, then turn the sensitivity fine adjustment potentiometer knob to turn on the power switch, and rotate it to half position, then adjust the coarse potentiometer knob. Make the speaker sound stop, then fine-tune the fine potentiometer after Zui, so that the speaker sound just stops, then the sensitivity of the metal detector is high. When detecting metal with a metal detector, the speaker will make a sound as long as the detection disc is close to any metal, and the sound will stop automatically when it is far away from a certain position.
This metal detector has a high sensitivity. When it is used to detect large pieces of metal, the detector disc will emit sound when it is 20cm away from the metal object. It can be detected as small as a paper clip or even a pin, but the probe coil must be close. Small metal objects. Since metal detectors use the electromagnetic induction of the oscillating coil to detect metal objects, it is possible to detect the covered metal objects through non-metallic objects such as paper, plastic, masonry, soil, and even water layers, so it is practical. For example, when decorating a house, use it to detect the wires or steel bars in the wall to avoid construction hazards and safety hazards; and metal detectors for security inspection are based on this principle.
Principle and manufacturing method of metal detector
metal detector: