Street light automatic switch control circuit

The street light automatic switch control circuit can control the street light to automatically turn off during the day and automatically light up at night. The circuit is shown in the figure.

In the figure, transistor VT1 is connected as an emitter follower, and its upper bias circuit is composed of photodiode VD1 and R3. When there is light during the day, the internal resistance of photodiode VD1 becomes smaller (about 45kΩ). Transistor vT1 has a base current to turn it on, and the current output by the emitter of transistor VT1 is added to the base of transistor VT2, so VT2 is turned on. By properly selecting the size of the base current Ib1 of transistor VT1, the working state of transistor VT2 can be controlled, so that it will enter saturation quickly after turning on. The collector current Ic2 of transistor VT2 starts relay KA1 and disconnects its normally closed contact K1-1. K1-1 is disconnected, and the AC contactor KA2 path is also disconnected, so its contact KM is disconnected, the lighting street lamp circuit is disconnected from the mains, and the street lamp goes out.

When it is dark, the internal resistance of the photodiode VD1 increases due to the lack of light, and the base bias circuit of the transistor VT1 is equivalent to an open circuit. The transistor VT1 is cut off, and the transistor VT2 also changes from saturation to cut off. The relay KA1 is released, the normally closed contact K1-1 is connected, and the AC contactor KA2 is connected. The contactor's contact KM connects the AC power of the street lamp circuit, and the street lamp lights up automatically.