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This handy little circuit can tell the difference between darkness and light, making it very useful for switching on and off signs, porch lights or other things when it gets dark or light.

Parts:
R1 100K Pot
Q1 2N3904 NPN Transistor or 2N2222
Q2 NPN Phototransistor
RELAY 9V Relay
MISC Board, Wire, 9V Battery Snap (if battery used), Knob For R1

Notes:
1. R1 Adjusts sensitivity