R1 and R2 form a voltage divider. The potential at A will be about 0 Volts. R1 and R2 could be replaced with a potentiometer. This would make the switch over level adjustable.
R3 and the LDR form another potential divider. The potential at B depends on the light level. As it get darker, the potential increases.
When the potential at B is greater than the potential at A, the 741 operational amplifier output goes high and the LED comes on.
R4 limits the current through the LED to less than 20mA.
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