Here is a circuit of dark activated LED using SCR. An SCR or (silicon controlled rectifier) can also work as a switch in electronic circuits like transistors. In this circuit SCR is working as a switch also. When LDR will receive light its resistance decreases and the SCR will remain close
but when it will not receive light its resistance increases, which will switch on the SCR and the LED become activated. The 50K variable resistor is used to adjust the amount of dark on which you want to activate the LED.