This circuit will detect when ambient light is dark and turn on an led. The trimmer could be labeled as sensitivity and helps with making field adjustment easier, resistor and ldr value depend on ambient light. You should check on field. The output of an opamp can directly drive an led but for my application i needed to drive a lcd display, together with led, which is a heavier load than standard led, hence the transistor switch. Base resistor value depends on transistor type.
