The general LED light-emitting diodes need more than 1.5V to emit light. The red LED should be 1.8V green to be 2.2V or more to emit light. This circuit uses a positive feedback oscillator to achieve flashing. 
Triode signal transient polarity analysis method:
1) Marking the "+" on the base of the transistor indicates an increase in voltage.
2) When the base signal voltage increases, the base current increases or decreases.
NPN type: base voltage increases, base current increases
PNP type: base voltage increases, base current decreases
3) Analyze the phase of the signal voltage at each point along the amplifier signal transmission line to increase or decrease, and mark "+", "-"
4) After analyzing the feedback signal at the output end, it will affect the input signal.
Circuit principle: In the figure, VT1 and VT2 form a complementary ultra-low frequency oscillator. The oscillation is completed by the positive feedback of capacitor C1. When the light is bright, the resistance of the photoresistor becomes smaller, the base potential of VT1 decreases, VT1 is cut off, and the oscillator stops working. The light-emitting tube does not work. When there is no light at night, the resistance of the photoresistor becomes larger, the base voltage of VT1 increases, and the oscillator starts to oscillate. VT2 is in the alternating state of on and off, when VT2 is cut off. When the power supply is charged to C1 through R4 and R3, it obtains: left positive and negative negative voltage, its voltage is close to the power supply voltage 1.5V. When VT2 is turned on, the capacitor C1 passes through the emitter of VT2, and the collector and the power supply are superimposed on the LED. Discharge, at this time, the LED gets close to 3V and emits light. The alternating of VT2 on and off is the LED sparkle..
Tabbing Ribbon Machine,Photovoltaic Ribbon Welding Machine,Pv Ribbon Interconnect Machine,Ribbon High Speed Tinning Making Machine
Jiangsu Lanhui Intelligent Equipment Technology Co., Ltd , https://www.lanhuisolar.com