This circuit is used to detect the ambient light. When the light is bright, it will give an audible and visual alarm, the LED will light up, and the buzzer will beep for a long time. The sensitivity can be adjusted by the potentiometer. Take an application example: call the police to wake up at dawn.
VR is an adjustable resistor used to adjust the sensing sensitivity, GR is a photoresistor, when the light is brighter, the resistance of the photoresistor decreases, the base current flowing into Q1 is larger, the voltage of the transistor collector decreases, D1 is lit, and R2 is limited Flow resistance. At the same time, the base current flowing into Q2 through R1 is also relatively large. Q2 goes into saturation. SP is a buzzer, it starts to sound an alarm after receiving current.