LDR Night Light

Photoresistor and NPN transistor switch an LED on when it gets dark.

Difficulty: Breadboard. Estimated build time: about 20 minutes. Estimated parts cost: about US$0.75. 6-part bill of materials with live supplier pricing. A dark-sensing circuit with no active ICs: a light-dependent resistor (LDR) forms a voltage divider with a fixed resistor, and the midpoint voltage drives the base of an NPN transistor.