This circuit mainly consists of a Schmitt-trigger and a LDR ("80 kOhm" left). If light decreases, the LDR will rise to something like 250 kOhm and the capacitor at the base of the first transistor (2,2 uF) ist loaded. When the shadow disappears, resistor will fall back to 80 kOhm, and the disloading current will flip the trigger for some seconds. The resistance of LDR can be variated during simulation (touch the LDR, then the tool and turn the wheel) to let flip the trigger and light the LED.
