This metal detector, due to the use of a quartz crystal oscillator and a difference frequency signal multiplier, makes the circuit stable and extremely sensitive. Although the circuit uses five ICs, the cost is very low (30 yuan About), its use effect is no less than a few thousand yuan of imported products. This machine can be widely used in the detection of some small metal objects such as archaeology, military, etc. or metal objects with long distance and large buried depth.
Circuit design principle: see the diagram below the principle. By IC1, CD4011B and Ll and related components constitute an induction oscillator, RP1 is the frequency trimming potentiometer. The three NAND gates of IC1 form an inductive oscillator circuit, and the three NAND gates of IC2 and a crystal form the local oscillator. The local oscillator signal and the inductive signal are at a frequency difference in a NAND gate of IC1, from IC1's ⑧ The pin outputs a difference frequency signal. Due to the low beat frequency, especially the small changes in the frequency induced by tiny metal objects, if you directly listen to the beat frequency signal, it is difficult to reflect it in your ears. Therefore, the difference frequency signal can be multiplied to meet the requirements of the ear to distinguish frequency changes. Since the IC3 CD4046B phase-locked loop circuit and the IC4 CD4017B decimal distribution circuit constitute a multi-stage adjustable frequency multiplication circuit, the adjustment range is 1-9 times and a total of 9 gears, which can be selected according to the actual situation. The difference frequency signal is input from pin 14 of CD4046, and the frequency multiplier signal is output from pin ④, and then sent to IC5 (LM386) through the volume potentiometer RP2, which is amplified and output to the speaker or earphone to emit sound.
Production points: L1 uses 0.45mm enameled wire, wound into a coil with a diameter of 60cm, the number of turns is 30, and the inductance is about 1.2mH. JT can choose 400-500kHz crystal for color TV, VD is MV2105, C1~C6 can choose mica or black dot ceramic capacitor to meet the requirements. First, adjust Cl and RP1 so that the output frequency of IC1⑩ pin is the same as that of IC2⑩ pin. At this time, IC1(11) should have no frequency output, and there should be no audio in the earphone. The frequency multiplier gear can be selected with a 1x9 switch. Frequency multiplier selection, generally the smaller the metal object, the greater the depth, the higher the multiplier, on the contrary, the larger the metal object, the closer the multiplier should be lower.
In order to avoid external interference, you should choose headphones for better results. The circuit can be installed in a plastic box, and the earphone socket is installed on the handle. The overall appearance is shown in the figure below.