dark detector
    set up and schematic
If the LDR is to be used in a generally well lit area, it is best to shroud it, so it will react only to the light source you want it to pay attention to.

Unwanted light may keep it from turning on when the primary light source is interrupted.
Sensitivity can be adjusted by changing the fixed resistor.
What about the light source.
A flashlight drains batteries too quickly, so you may be stuck with a regular light or other light source you can plug in.
If you want to use a night light that shines across the hallway, there really isn't enough light unless you concentrate it onto the LDR.  Anything brighter than a night light may work, but is too obvious. 
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The best source for light over a long distance is the laser pen.  Using mirrors, the beam can even travel around corners.

Laser pens  can be powered with a wall adapter matched to the  same power rating as the batteries that normally power it.  Every cell inside a laser pen has is worth 1.5 volts.  For example, if it has 3 cells, you need to find a wall adapter that provide 4.5 volts. 

The beam here is shone from inside a window, and travels around the outside of the house.  A speaker was set off for 10 seconds at 1000 hz  in the next door neighbors' homes, one on each side. 
The system was able to be keyed off outside by the owner.

Please be careful.  Many laser pens claim to meet safety specification, but really can damage your eyes if you are exposed over a period of time.