ST188 is a reflective infrared photoelectric sensor. This circuit is a circuit with functions such as proximity switches made using ST188.

The ST188's light-emitting diodes and receiving diodes are arranged side by side. Used to detect whether there is an object approaching.

When an object approaches, the infrared ray emitted by the light-emitting tube is received by the receiving tube after being reflected by the object. The receiving tube is turned on, and the collector potential decreases. Through the differential circuit composed of C4 and R11, a negative pulse is formed. After two stages of RC filtering, it is sent to 2IN+ of the operational amplifier LM358. After two stages of amplification inside the operational amplifier, an instantaneous low voltage is formed at the 1OUT terminal, and D1 flashes.