A photoresistor (or light-dependent resistor, LDR, or photocell) is a light-controlled variable resistor. The resistance of a photoresistor decreases with increasing incident light intensity; in other words, it exhibits photoconductivity. This module allows for easy sensing of light levels from within your micro controller application. Just connect GND and VCC to the module and a varying voltage depending on light level is available on the third pin. The photosensitive resistances can be divided into three types by their spectral properties: UV resistance, infrared photosensitive resistance, visible light photosensitive resistor.