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    How Does a Metal Detector Work?

    Metal detectors are widely used in examinations, security checks, archaeology, prospecting, etc., bringing more convenience to people's lives. So how do metal detectors accomplish these tasks? Next, ATO will introduce the working principle, application, and structure function of the metal detector to you.

    What is a metal detector?

    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 principleHandheld underwater metal detector 8-9cm

    The basic principle of the metal detector is to use electromagnetic induction, using alternating current to generate a rapidly changing magnetic field through the coil, so that the magnetic field lines of this magnetic field pass through the metal object and form an eddy current on its surface. The eddy current in turn generates a secondary magnetic field, which in turn affects the original magnetic field, producing a signal that the instrument can receive and identify. Finally, the signal is processed and amplified to deflect the pointer of the indicator and simultaneously drive the acoustic indicator to emit an acoustic signal.

    When the detection coil is close to a metal object, the phenomenon of electromagnetic induction occurs. The eddy current generated in the metal conductor increases the energy loss in the oscillation circuit, weakens the positive feedback, weakens the oscillation of the oscillator in the critical state, and even stops the vibration because it cannot maintain the minimum energy required for oscillation. In this way, the detection coil will detect the change in the signal and give feedback to the operator through the display device. The operator can judge whether there is a metal object according to the prompt of the display device.


    Metal detectors have high sensitivity. When using it to detect large pieces of metal, the detector will emit a sound 20cm away from the metal object. As small as a paper clip, even a pin can be detected, but the detection coil must be in close proximity to small metal objects. Because the metal detector uses the electromagnetic induction of the 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 layers, so it is practical. For example, when decorating a house, use it to detect the wires or steel bars in the wall, so as to avoid construction hazards and safety hazards. The metal detectors used for security inspection are also made according to this 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

    Composed of triode VT1 and 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 200 kHz, 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 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 forms positive feedback to generate self-excited high-frequency oscillation. The magnitude of the feedback voltage of the oscillator is related to the turn ratio of the coils L1 and L2. If the turn 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.Hand held metal detector applications

    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.

    Oscillation detector

    The oscillation detector is composed of a triode switch circuit and a filter circuit. The switch circuit is composed of triode VT2, diode VD2, etc., and the filter circuit is composed 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 coil L2. When the high-frequency oscillator is working, the oscillation signal coupled through the high-frequency transformer T1 will turn on VT2 in the positive half cycle, and the collector of VT2 will Output a negative pulse signal, pass through a π-type RC filter, and output a low-frequency signal on the load resistor R4.

    When the high-frequency oscillator stops oscillating, there is no oscillating signal at the "C" terminal, and because 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 cut-off state, and the collector of VT2 It is a high level, and after a filter, a high-level signal is obtained on R4. It can be seen that when the high-frequency oscillator is working normally, a low-frequency signal is obtained on R4, and when the vibration is stopped, it is a high-frequency signal, thus completing the detection of the working state of the oscillator.

    Audio oscillator

    The audio oscillator adopts a complementary multi-vibrator, which is composed of triode VT3, VT4, resistors R5, R7, R8, and capacitor C6. The complementary multi-vibrator uses two different types of transistors, among which VT3 is an NPN type transistor, and VT4 is a PNP type transistor, which is connected to form a complementary circuit that can strengthen positive feedback.

    When the circuit is in operation, it can be alternately switched on and off, producing audio oscillations. R7 is both a VT3 load resistor and a VT4 base current-limiting resistor when VT3 is turned on. R8 is the VT4 collector load resistor, and the oscillation pulse signal is output from the VT4 collector. R5 and C6, etc. are feedback resistors and capacitors, and their values affect the level of the oscillation frequency.

    Power amplifier

    The power amplifier is composed of a triode VT5, speaker BL, and so on. The positive pulse audio signal output from the multi-vibrator is input to the base of VT5 through the current-limiting resistor R9 to make it conduction and a strong instantaneous current is generated in BL to drive the speaker to sound. Since VT5 is in the switching state and the conduction time is very short, the power amplifier is very power-saving and can be powered by a 9V laminated battery.

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