C 1 refers to deburring and power frequency interference, and the ratio of R3 to R2 determines the magnification at this stage. C3 refers to non-response to high-frequency signals, such as interference caused by card jitter, so it is removed.
C2 is a low frequency channel, R4 is a symmetrical resistor, and short circuit is also acceptable.
R5 is an AC coupling capacitor to avoid the interference between the front and rear stages, and C5 is also called an isolated DC capacitor to avoid the mutual influence of the DC levels of the front and rear stages.
The two anti-phase parallel diodes are bidirectional limiting circuits, R 1 1 is to ensure the incomplete open-loop stability of the operational amplifier, and C6 is a low-pass capacitor to remove high-frequency interference.
The second stage is already a small square wave.
The third stage is to amplify the square wave, turn it into TTL level and send it to the single chip microcomputer.
Although this circuit is very basic and simple, the original designer must be a westerner.
This kind of analog circuit has appropriate parameters and values, and its technical index is high. It is a very profound knowledge, which cannot be solved and replaced by your familiarity with integrated circuit design software, circuit simulation software, signal and system principles, high education and long-term working experience.