Lots of projects use a Light Dependent Resistor (LDR) to sense light levels but don't really focus on how they work.
This project shows you how to use a Light Dependent Resistor (LDR) to sense light levels, measure those levels with the Arduino and print the measurements to the Serial port.
Arduino measures Voltages on pins A0 to A5; but the LDR is a variable resistor (varying with Light). So we need to convert the varying resistance to a voltage that the Arduino can measure.
We do that by using the LDR and a Resistor in a Potential Divider circuit.
Step 1: How it Works
The top of the Potential Divider is 5V; the bottom is at 0V and the middle (connected to A0) is some value between 5V and 0V that varies as the LDR resistance varies. Remember the LDR resistance varies with Light so the Voltage at A0 will too.
If you want to be very precise and technical then you can work out the Voltage on A0 as :
Va0 = 5 * R1/(R1+R2)
where Va0 is the voltage at A0 pin, R2 is the top resistor value, R1 is the bottom resistor value;
e.g. R1 = 10k, R2 = 5k => Va0 = 5 * 10000/(10000 + 5000) = 5 * 10/15 = 3.33V
See what it looks like on a Breadboard.