A typical metal detector is composed of basic circuits such as dual oscillators, mixers, and differential frequency amplifiers. When the detection coil is close to a metal object, the inductance changes, causing a change in the oscillation frequency. Although the metal detector circuit is not too complicated, it is not an easy task for beginners with limited experience and limited experience. The metal detector introduced below only needs a medium-wave oscillator, and other functions are replaced by radios, making production and debugging very easy.
The circuit is shown in the figure, and the commonly used common base capacitor three-point oscillation circuit is used, and the detailed principle will not be repeated. First use a radio to receive a medium wave radio station as a comparison signal, and then adjust C2 to make the oscillation frequency close to the radio frequency. At this time, the speaker will emit a beat frequency howling, select the appropriate tone, when the inductor coil L is close to the metal When an object changes its tone, it can be determined whether there is a metal object.
The inductance coil L is wound with a diameter of 0.52mm enameled wire around + 300mm tires for 16 turns. After the tire is removed, it is tightly tightened with insulating tape. The diameter of the detection coil can be appropriately changed according to needs. , But the detection sensitivity for smaller metal objects is reduced. In addition, when the detection coil L is close to the ground, it is easy to drift due to the influence of distributed capacitance. The coil can be shielded with aluminum foil or a soft metal mesh. Note that the shielding layer cannot form a short-circuit loop to the coil, that is, the shielding layer in the schematic One end is grounded, otherwise the oscillation circuit will not start. Other components have no special requirements and can be determined according to the conditions at hand.
As long as the circuit is welded and connected to the power supply, the vibration can be started, and the frequency adjustment range of the circuit is also wide, which is easy to succeed. The inductor coil and the circuit board are fixed on the lower end of the insulating rod, and the radio is placed on the upper end.
Under normal circumstances, the tone produced by the beat is only determined by the difference between the radio frequency and the oscillation frequency of this circuit. It has nothing to do with the tuning of the radio. Whether the tuning of the radio is accurate or not only affects the volume of the volume. The beat frequency sound can be received at the frequency point, and the tone will also change with the tuning of the radio. This phenomenon is particularly obvious at the high end of the frequency. This is the intermodulation signal generated by the radio station, the detection oscillation circuit and the radio local oscillator. For metal detection The circuit is a good thing, because it is more convenient to adjust the tuning knob of the radio than the variable capacitor of the oscillation circuit. Secondly, if the howling is the tuning product of the higher harmonics of the oscillation signal, it can also increase the sensitivity of detection. 
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