Now, it may have become apparent after explaining how ADC's work, that the Arduino can measure only an analog voltage. But our LDR changes its resistance depending on the amount of light falling on it. So how can we effectively change the voltage across the Arduino depending on the change of the resistance of the LDR?
We use a voltage divider!
A voltage divider is a very fundamental and simple circuit which basically divides a larger voltage into a smaller one depending on the values of the resistors in its circuit.
A voltage divider uses two resistors in series with an input voltage and can output an output voltage that is a fraction of the input.
A voltage divider
Image source: https://cdn.sparkfun.com/r/600-600/assets/4/0/3/a/e/511948ffce395f7f47000000.png
The output voltage depends on the resistances and the input voltage by the following equation:
Now, using this circuit, along with our LDR, we can manipulate the voltage input into our Arduino. All we have to do...