Fritzing

Components: --------------------   * npn Transistor 2N3904 * Resistance 100 kOhm * 5V DC Supply * LDR * Resistance 220 Ohm

Idea:

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Control the LED ON/OFF using the npn Transistor.

Case (1): when there's light, the transistor will be in cutoff region,

and the LED will be OFF.

Case (2): when there's dark, the transistor will be in active region,

and the LED will be ON.

And the LDR will control the operation region of the transistor.