If the front end of this circuit is not Re200b, it is also a sensor with the same function. The circuit is realized by an inexpensive operational amplifier LM324. Two of them are amplified and filtered, and the passband voltage is amplified by about 3800 times. The other two operational amplifiers do comparative shaping output, the output level is close to 0V, and the high level is greater than 3 V, which can be connected to 5 1 single chip microcomputer.