U 1, R2 and R7 form an optical pulse signal receiving circuit.
C3 and R8 are AC coupling circuits, which couple AC pulse signals to the following filter circuits.
R4, C4, R5 and C5 form a second-order resistance-capacitance filtering circuit to filter out high-frequency components and transmit useful low-frequency signals to the following amplification and shaping circuit. The filtering frequency is f =1/2 ∏ RC =1/(2x3.14x1kx1u) =16hz.
In another picture, there is an amplification and shaping circuit.
The former stage is a negative feedback amplifier circuit, and the magnification is about 100k/220=455 times.
The latter stage is Schmidt shaping circuit, which shapes the filtered and amplified input waveform into a rectangular wave for P32 port to process.