LDR (Light Dependent Resistor) is simple, cheap electronic device. Simply speaking this is a resistor which resistance varies depending on light intensity. You’ve probably seen typical LDR in some projects where light intensity has to be taken in to 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 wavelength range from 400nm – 850nm. For near infrared range (1μm – 3μm) there are PbS or PbSe materials used. For deeper infrared range (3μm – 1000μm) there are InSb and InAs.