Arduino-Compatible Photosensitive Sensor Module with 3-Pin Cable
Now you can measure light levels with this sensor module. This module uses a Light Dependant Resistor (LDR), or Cadmium Sulfide (CdS) photocell, to produce an output voltage that varies with light level. Connect it directly to your Arduino to build a night or day sensor, a sun tracker or combine it with a laser module to make a laser trip wire.