For many applications, you need a light detector, eg to control a robot, or as an alarm system.
It can be depending on the requirements photodiodes, LDR's, photodiodes or the like as the active element used.
With specific circuit concepts Thus, even small amounts of light can be detected.
Here are some links to schematics

Here you may find Circuit, Basics, Schematic Diagram and DIY Kit components for Light detector