LED light control electronic circuit design

　　 Working principle: Transistors VT1 and VT2 form a complementary self-excited multivibrator, and the oscillation feedback network is composed of capacitor C. The base bias circuit of VT1 is composed of resistor R and photoresistor RG. During the day, the indoor light is strong, RG is exposed to strong light and has a low resistance, the base of VT1 is at a low level, VT1 and VT2 are both in the cut-off state, and the light-emitting diode LED does not emit light. At night, the indoor light is weak, the resistance of the photoresistor RG increases, and the base potential of VT1 rises. When it rises to about 0.65V, the oscillator starts to work, VT2 is turned on and off alternately, and the LED will flash and emit light according to the oscillation frequency. VT1 can use silicon NPN transistors such as 9011, and β≥100 is required; VT2 uses germanium PNP transistors such as 3AX31B, and β value is greater than 30. LED can be used as a red or green round light-emitting diode depending on personal preference.

　　Component selection: RG uses MG45 photoresistor, and the difference between light resistance and dark resistance should be as large as possible. R uses RTX-1/8W carbon film resistor. C uses CD11-10V electrolytic capacitor. Power supply G uses two No. 5 batteries. Other components are shown in the figure.