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
Q2__________NPN Phototransistor
RELAY_______9V Relay
MISC________Board, Wire, 9V Battery Snap (if battery used), Knob
For R1
1. R1 Adjusts sensitivity