Here you can learn how to make a Darkness Sensor with just few cheap parts.
It´s really simple and easily made by yourself.
Step 1: List of Materials and Schematics
What you´ll need:
1 or more LDR
Plug everything together like shown on schematics and test it.
Step 2: ... Make It More Accurate
You can make the Sensor more accurate by adding LDR.
By adding the LDRs you make the potential resist bigger so the LED dimms
over a more wide range.