LDR (Light Dependent Resistor) is a simple, cheap electronic device. Simply speaking, this is a resistor in which resistance varies depending on light intensity. You’ve probably seen typical LDR in some projects where light intensity has to be taken into account. They can be used to activate light switches, alarms, adjust display brightness, and more. Light-dependent resistors can be different types. They vary in light-sensitive material used. Visible spectrum LDR is made using Cadmium Sulphide (CdS) or Cadmium Selenide (CdSe). This material is sensitive to the wavelength range from 400nm – 850nm. For the near-infrared spectrum (1μm – 3μm) there are PbS or PbSe materials used. For deeper infrared range (3μm – 1000μm) there are InSb and InAs.