You can use the circuit to detect light levels and for example activate a LED to switch on when it gets dark!
Step 1: Parts List
Blank PCB board
270k 0603 resistor
SMT 3904 NPN Transistor (SC-70)
Any sized LED
LDR (Light detecting resistor)
Small hand drill
Step 2: Making the Circuit
Drill holes into your board if it hasn't got any already for the LDR to fit and for the power inputs.
Your LED can fit straight onto your circuit if it's a surface mount component.
Or you can extend your LED distance by running wires back to the board.
Try different batteries and voltages depending on how many LEDs you attach to the circuit, you may have to tweak your resistor slightly.
Always check each components datasheets for maximum power ratings!
Experiment with different resistors as it depends on what LDR you have, the best way to tweak the sensitivity is to place a variable resistor into the circuit first and turn it accordingly to the light levels to make sure your LED starts to turn on as it gets darker.
Any questions feel free to ask!
I use this circuit in my other Solar Arc Reactor Instructable, check it out!