When there is light, the resistance value of PV cell VD is low, and C 1 is positive when the left end is charged, and positive when the right end of C2 is charged. Transistor V 1 is saturated by the base current provided by resistor R3, transistor V2V3 is turned off, and lamp EL is not lit.

When there is no light, the photovoltaic cell VD presents high resistance, and the voltage at the right end of C 1 is negative, the transistor V 1 is turned off, V2V3 is turned on, EL is turned on, and the voltage at the left end of C2 is negative. When the base voltage of V 1 is charged, the transistor V 1 is turned on, the transistor V2V3 is turned off, and the lamp EL is turned off.

When there is light in VD again, the photovoltaic cell VD presents low resistance, and the power supply charges C 1 through VD, so that V 1 is turned on, the transistor V2V3 is turned off, and the lamp EL is not lit.