LDR (wikipedia) are light sensitive devices most often used to indicate the presence or absence of light or to measure the light intensity. An LDR can be applied in light-sensitive detector circuits and light-activated and dark-activated switching circuits.
How does LDR sensor work?
The resistance of a photoresistor decreases with increasing incident light intensity. In other words, it exhibits photoconductivity. A photoresistor is made of a high resistance semiconductor. In the dark, a photoresistor can have a resistance as high as several megohms (MΩ), while in the light, a photoresistor can have a resistance as low as a few hundred ohms.
Variation in resistance with changing light intensity
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 above). 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.
Setting up the circuit for LDR sensor
The circuit of LDR sensor is explained below
One end of LDR is connected through jumper wire(red wire ) to the 5V power supply in the evive board.
Another end of LDR is soldered with 10K resistor. Through the common junction of LDR and 10K resistor, a jumper wire is taken and hooked into the analog input(here we have used A1 pin number in the evive board) of the evive board.
The free end of the resistor is grounded with the help of jumper wire.
The following arduino code controls the led state using the ldr sensor.