555 and R9, C8, VT4, VT5, etc. constitute a light-controlled monostable circuit. The so-called light control uses the phototransistor (3DU type) to exhibit different impedances for different illuminations, and performs high and low potential control on the 4th pin of the time base circuit 555, or is in a waiting trigger state, or is in a forced reset state. During daylight, the VT5 is low-resistance, VT2 is positively biased and saturated, and its collector potential, that is, the 455 pin 4 is clamped at about 0.3V, so that the 555 is in a forced reset state. How low the trigger level is, the 555 will not be turned over, the thyristor will not trigger conduction, and the light will not be lit. RAM/RFM Middle Frequency Water Cooled Capacitors RAM/RFM Middle frequency water cooled capacitors Water Cooled Capacitors,Medium Frequency Capacitor,Intermediate Frequency Capacitors YANGZHOU POSITIONING TECH CO., LTD. , https://www.pst-thyristor.com