Because I don't think so. I've only heard of metal detectors. When people talk about metal detectors, they think of mine detectors, which engineers use to detect buried mines. Metal detector is a special instrument for detecting metals. In addition to detecting mines with metal shells or metal parts, it can also be used to detect wires hidden in walls, buried water pipes and cables, and even explore underground treasures and find buried metal objects. The metal detector can also be used as a tool for national defense education and popular science activities for teenagers, and it is certainly an interesting entertainment toy. Working principle The high-frequency oscillator is composed of transistor VT 1 and high-frequency transformer T 1, which is a transformer feedback LC oscillator. The primary coil L 1 of T 1 and the capacitor C 1 form an LC parallel oscillation loop, and its oscillation frequency is about 200kHz, which is determined by the inductance of L 1 and the capacitance of C 1. The secondary coil L2 of T 1 is used as the feedback coil of the oscillator, and its "C" is connected to the base of the oscillator VT 1 and its "D" is connected to VD2. Because VD2 is in the forward conduction state, the "D" terminal can be regarded as the ground of the high-frequency signal. In the high-frequency transformer T 1, if the "A" terminal and the "D" terminal are the first terminals in the winding direction of the primary and secondary coils respectively, the feedback signal input from the "C" terminal to the base of the oscillator VT 1 can make the circuit form positive feedback and generate self-excited high-frequency oscillation. The magnitude of oscillator feedback voltage is related to the turns ratio of coils L 1 and L2. If the turns ratio is too small, it is difficult to start 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 oscillator VT 1 consists of R2 and diode VD2, where R2 is the current limiting resistance of VD2. Because the forward threshold voltage of the diode is constant (about 0.7V), it is applied to the base of VT 1 through the secondary winding L2 to obtain a stable bias voltage. Obviously, this voltage stabilizing bias circuit can greatly enhance the stability of VT 1 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, which consists of a voltage stabilizing diode VD 1, a current limiting resistor R6 and a decoupling capacitor C5. Two potentiometers are connected in series between the emitter of oscillator VT 1 and the ground, which has the negative feedback effect of emitter current. The larger the resistance value, the stronger the negative feedback effect, the lower the amplification ability of VT 1, and even make the circuit stop vibrating. RP 1 is a coarse potentiometer for oscillator gain, and RP2 is a fine potentiometer. The principle of high-frequency oscillator detecting metal adjusts the gain potentiometer of high-frequency oscillator, which just makes the oscillator in a critical oscillation state, that is, just makes the oscillator start to vibrate. When the detection coil L 1 is close to a metal object, eddy current will be generated in the metal conductor due to electromagnetic induction, which will increase the energy loss in the oscillation circuit, weaken the positive feedback, weaken the oscillation of the oscillator in the critical state, and even stop the oscillation because the minimum energy required for oscillation cannot be maintained. If this change can be detected and converted into a sound signal, it can be judged whether there is a metal object under the detection coil according to the presence or absence of sound. The oscillation detector consists of a triode switch circuit and a filter circuit. The switching circuit consists of triode VT2 and diode VD2, and the filtering circuit consists of filter resistor R3 and filter capacitors C2, C3 and C4. In the switching circuit, the base of VT2 is connected to the "C" terminal of the secondary winding L2. When the high-frequency oscillator works, the oscillation signal coupled by the high-frequency transformer T 1 turns on VT2 in the positive half cycle, and the collector of VT2 outputs a negative pulse signal, which passes through the π-type RC filter and outputs a low-level signal on the load resistor R4. When the high-frequency oscillator stops oscillating, there is no oscillating signal at the "C" terminal, and since the diode VD2 is connected between the emitter of VT2 and the ground, the base of VT2 is reversely biased, VT2 is in a reliable off state, and the collector of VT2 is in a high level. After passing through the filter, a high-level signal is obtained at R4. It can be seen that when the high-frequency oscillator works normally, a low-level signal is obtained at R4, and when it stops oscillating, it is a high-level signal, thus completing the detection of the working state of the oscillator. Audio oscillator The audio oscillator adopts a complementary multivibrator, which consists of triodes VT3 and VT4, resistors R5, R7 and R8 and a capacitor C6. Complementary multivibrator adopts two different types of transistors, among which VT3 is NPN transistor and VT4 is PNP transistor. Connecting complementary circuit can strengthen positive feedback. When the circuits are working, they can alternately enter on and off states, thus generating audio oscillation. When VT3 is on, R7 is both a load resistor of VT3 and a current limiting resistor based on VT4. R8 is the load resistance of VT4 collector, and the oscillation pulse signal is output by VT4 collector. R5 and C6 are feedback resistors and capacitors, and their values will affect the oscillation frequency. Working principle of complementary multivibrator When the power supply is turned on, the base of VT3 is forward biased because it is connected with bias resistors R 1 and R3. Assuming that the collector current of VT3 is in the rising stage, the base current of VT4 will also rise accordingly, which will lead to a sharp increase in the collector current of VT4 and a rapid increase in the collector potential of VT4. The current output by VT4 will charge C6 through R5 connected to it and flow to ground through the base of VT3, which will further increase the base current of VT3. In this way, the strong positive feedback makes VT3 and VT4 quickly enter the saturated conduction state, and the collector of VT4 is at a high level, which makes the multivibrator enter the first transient process. With the power supply charging C6 through saturated VT4 through R5, when the base current of VT3 drops to a certain extent, VT3 exits the saturated conduction state, and the collector current begins to decrease, resulting in the decrease of collector current and collector potential of VT4. This process further aggravates the rapid drop of the charging current to C6, and the base potential of VT3 drops sharply, which makes VT3 turn off, the collector of VT4 drops to a low level rapidly, and the multivibrator turns to the second transient. When the multivibrator just entered the second transient, the right end of its capacitor was positive and the left end was negative because C6 was charged before, and now the right end of C6 is at a low level to ground. Because the voltage across the capacitor C6 can't jump, the base of VT3 is strongly reverse biased by the negative potential at the left end of C6, which makes the two transistors turn off for a long time. When C6 discharges, the current flows out from the right end of the capacitor, mainly through the emitter junctions R5, (R8), R9 and VT5, and then flows back to the left end of the capacitor C6 through the power supply, R6, R 1 and R3. Until the discharge of C6 ends, the power supply continues to charge C6 reversely through the above circuit, and the left end of C6 is positive. When the potential at both ends of C6 rises to 0.7V, VT3 begins to enter the conducting state, and after strong positive feedback, it quickly enters the saturated conducting state, causing the circuit to turn over again, repeating the previous transient process, and so on, and the circuit produces self-excited multi-harmonic oscillation. It can be seen from the working process of the circuit that when C6 is charged, the resistance of the charging resistor R5 is small, so the charging process is faster and the circuit is in a saturated conduction state for a short time. In C6 discharge, many related resistors need to flow, and the total value of discharge resistors is large, so the discharge process is slow, that is to say, the circuit is in off time for a long time. Therefore, the duty cycle of VT4 collector output waveform is very large, and the pulse width of positive pulse signal is very narrow, and its oscillation frequency is about 330Hz. Power amplifier The power amplifier consists of a triode VT5 and a loudspeaker BL. The positive pulse audio signal output by the multivibrator is input to the base of VT5 through the current-limiting resistor R9, which makes it conductive, generates instantaneous strong current in BL, and drives the speaker to sound. Because VT5 is in the on-off state, the on-time is very short, so the power amplifier is very energy-saving and can be powered by 9V laminated battery. Debugging and operation methods The metal detector circuit has no adjustment part except the sensitivity adjustment potentiometer. As long as the welding is correct, the circuit can work normally. When the whole machine is stationary, that is, when the speaker is silent, the total current is about 10mA, and when the sound is detected from the metal speaker, the current of the whole machine rises to 20mA. A new multi-layer battery can work for 20 to 30 hours. If the newly welded metal detector can't work normally, first check whether the components and wiring on the circuit board are welded incorrectly, and then measure whether the battery voltage and power supply circuit are normal. The voltage of zener diode VD 1 is between 5.5 and 6.5V, and the polarity of VD2 should not be reversed. Don't weld the primary and the head and tail ends of the oscillating coil in the detection disk by mistake. Before using the metal detector, it is necessary to adjust the length of the probe rod. Just unscrew the hose, push and pull the hose to a suitable length, then rotate the hose to wind the cable tightly and make the tip of the handle face upwards, and finally tighten the hose to lock it. In this way, when holding the probe handle, the thumb is just next to the sensitivity adjustment potentiometer. When adjusting the sensitivity of the metal detector, the detection disk (oscillating coil) should be far away from the metal, including the paper with aluminum foil. Then, turn the knob of the fine-tuning potentiometer to turn on the power switch and turn it to the half position. Then, adjust the knob of the coarse potentiometer to stop the audio sound of the speaker. Finally, fine-tune the potentiometer to stop the sound of the speaker. At this time, the sensitivity of metal detector is the highest. When a metal detector is used to detect metal, as long as the detection disc is close to any metal, the speaker will make a sound, and the sound will automatically stop when it is far away from a certain position. This metal detector is highly sensitive. When it is used to detect a large piece of metal, when the detection disk is 20 cm away from the metal object, as small as a paper clip or even a pin, the speaker will make a sound, but the coil of the detection disk must be close to the small metal object. Because the metal detector uses electromagnetic induction of oscillating coil to detect metal objects, it can detect covered metal objects through non-metallic objects, such as paper, wood, plastic, masonry, soil and even water layer, so it is very practical, for example, when decorating houses, it can be used to detect wires or steel bars in walls to avoid construction dangers and security risks; Another example is the metal detector for security inspection, which is the reference material made according to this principle:

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