The schematic shown below is a circuit of light activated LED using SCR. SCR (Silicon Controlled Rectifiers) or thyristor are physically looks like transistors but their internal circuit and working procedure is totally different as compare to the transistors. Both SCR and transistor can work as   

a switch in the electronic circuits but SCRs can handle high AC voltages easily, whereas transistor can be used for low DC voltages.
In this circuit the SCR is working as a switching device. When the LDR will receive light its resistance will decrease which will switch on the SCR and the LED becomes activated. The 50K variable resistor is used to adjust the amount of light on which you want to switch on the SCR and activate the LED. The circuit can be operated with 6V battery or power supply.

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Light Activated LED Using SCR