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 similar as the active element used.
With special circuit concepts this way will even the smallest amounts of light can be detected.
Here are some links to schematics

Here you will find Schematic Circuit Diagram for DIY and further information on Light detector
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40KHz Light Detector with photodiode	www.discovercircuits.com/PDF-FILES/NewPDF/40KHZLSRDET1.pdf
Infrared light detectors, eg for robot control	www.ki.privat.t-online.de/Schaltungen/IR-Detektor/ir-detektor.html
Light Detector with LDR and LM741	www.uoguelph.ca/~antoon/circ/sensor2.htm