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.

 

Schematic

Parts:

Part	Total Qty.	Description	Substitutions
R1	1	100K Pot
Q1	1	2N3904 NPN Transistor	2N2222
Q2	1	NPN Phototransistor
RELAY	1	9V Relay
MISC	1	Board, Wire, 9V Battery Snap (if battery used), Knob For R1

Notes:

1. R1 Adjusts sensitivity