A Light Dependent Resistor (LDR) or Photoresistor is a resistor  made of a semi-conductor material which allows the Resistance of the LDR change with luminance variation. In darkness, the LDR has a High Resistance while under lighting the resistance drops to a low value. Consider using the Light Dependent Resistor in projects that function with light intensity levels such as a Daylight or Night time Switcher that turns on a home security light during night time and turns it off during daytime. You can also use the LDR in Solar power systems to automatically activate heavy loads during daytime when the solar panels are supplying backup power but disable such loads during night time when back up power is supplied by batteries.