This works same as the other dark detector circuit. When light is falling on LDR, its low resistance drives transistor Q1-BC547 into conduction. This keeps transistor Q2 cut-off due to low base bias. The D1-LED does not get power as long as ambient light falls on LDR. When the resistance of LDR becomes high in darkness, transistor Q1 stops conducting and transistor Q2 starts conducting to turn on the D1-LED.