Here we introduce a metal proximity switch circuit that is widely used in the machinery industry, as shown in the figure. The circuit consists of two parts, namely a high-frequency oscillator and a switching circuit, and its working principle is as follows. 
When the metal is not close to the probe, the high-frequency oscillator works, the oscillating signal is rectified by the voltage doubler of DV1 and DV2, and the DC voltage is obtained to make BG2 turn on, BG3 cut off, and the subsequent circuit does not work. When there is metal close to the probe, due to eddy current loss, the high-frequency oscillator stops vibration, BG2 is cut off, BG3 is electrically turned on, the photocoupler 4N25 has a built-in luminous tube to emit light, and the phototransistor is turned on to turn on the circuit and play a switching role.
The component selection probe needs to be self-made. It is wound on the φ5mm & TImes; 4mm magnetic core with φO.12mm enameled wire. The number of turns is as shown in the figure, and other components are marked as shown in the figure. Generally, as long as the components are good and the welding is correct, it can work normally.
Debug and switch on the power supply, adjust W1, and monitor BG2 with a multimeter to make the two poles c and e fully conductive. At this time, the high-frequency oscillator is in a weak state. Then close the probe with a metal object, BG2 should be cut off immediately. Then fine-tune W2 to make BG3 fully conductive. At this time, the sensitivity is high and the range is large (the sensing distance is a few millimeters to tens of millimeters). Then, according to your own use, carefully adjust W1 and W2 to make the sensing distance suitable for your use can.
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