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The examination room uses a hand-held metal detector to effectively prevent cheating

2019-03-06

With various entrance exams, judicial exams and more exam exams becoming more and more important, various examination rooms are also equipped with various security inspection devices, such as hand-held metal detectors, signal maskers, and ID card recognizers, effectively solving the cheating in the examination room. The problem.

Handheld metal detector working principle

Its basic working principle is that the 15kHz sinusoidal voltage generated by the oscillator feeds the detection coil. The detection coil consists of a main transmission ring, two sets of auxiliary coils, an input transformer and an output transformer. The two sets of secondary rings are located on both sides of the main launch circle, and they are symmetrical with respect to the distance from the main launch circle and they are interlinked to form a differential coil. When the 15kHz sinusoidal alternating voltage generated by the sine wave oscillator is fed to the main transmitter ring through the input transformer, the alternating current is generated on the main transmitter ring by a relatively large alternating current and the two auxiliary coils are cut. Because the two secondary rings are symmetric with each other and are interlinked with each other, they induce at the same time a 15 kHz induced potential equal in magnitude and opposite in direction (with a phase difference of 180°) to cancel each other out. (We cannot have two pairs due to the process relationship.) The ring is completely symmetrical to the main ring, and the external shielding material affects the output transformer. There is always an mV-level unbalanced signal output, and we expect it to be as small as possible. When the detection ring has no metal entry, there is only a weak 15kHz unbalanced signal output. After this signal is amplified, it becomes a direct current voltage and is blocked by the blocking capacitor and cannot enter the post-amplifier. In a relatively stable state and still.

Once the metal enters the detection ring, the metal is in an alternating magnetic field of 15 kHz, generating a phenomenon of induced potential, eddy current, etc., which will destroy the relative balance of the detection circle and generate a low-frequency pulsating potential difference. The pulsating potential difference is contained in the original. Equal amplitude unbalanced signals are sent to amplifier inputs for amplification. After the metal signal passes through the first-stage amplifier, the amplitude has been amplified by 3,000 to 4,000 times, and then sent to the detector to take out the useful signal from the unbalanced signal and then sent to the ultra-low frequency amplifier, and gain 1000 times more amplification. At this point, the metal signal has changed from the original microvolt level signal to a volts level after several amplifications, and the trigger trigger voltage is reached. This triggers the relay to activate the relay, sends an alarm signal, and automatically controls the desired control object. . For example, when the load power is turned off, the signal light is turned on, and the signal sound is emitted. Based on these, we can judge the presence or absence of metal.

Gold exploration network examination security program

Hand-held metal detectors: In recent years, college entrance examinations, postgraduate exams, civil service exams, etc., prevent candidates from carrying other cheating tools such as wireless headphones, and each examination room will be equipped with hand-held detectors. At present, many provincial education systems in the country are equipped with such products. The GARRETT brand handheld metal detectors are compact and easy to operate and are used in security inspections, factory damage prevention, and education.


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