“Light sensors and detectors are extremely useful for microcontrollers and embedded systems and intensity monitoring is also to be done. One of the simplest and cheapest of such sensors are LDR. LDR or Light Dependent Resistors can be easily used with an opamp as comparator and detection of light can be done.
An LDR is a component that has a (variable) resistance that changes with the light intensity that falls upon it. This allows them to be used in light sensing circuits.The most common type of LDR has a resistance that falls with an increase in the light intensity falling upon the device (as shown in the image here). The resistance of an LDR may typically have the following resistances: Daylight = 5000Ω and less
Dark = 20000000Ω
You can therefore see that there is a large variation between these figures. If you plotted this variation on a graph you would get something similar to that shown by the graph shown above. It is a hyperbolic curve.”