Monitoring the output of a light-dependent resistor, or photoresistor, allows the Arduino to know how light or dark it is. When the light falls below a certain level, the Arduino turns on a couple of LEDs.

A light-dependent resistor, or photoresistor, is a sensor whose resistance decreases as the amount of light falling on it increases. When it is dark, the resistance of a photoresistor may be as high as a few MΩ. When it is light, however, the resistance of a photoresistor may be as low as a few hundred ohms.

In this experiment, we will connect a photoresistor to an Arduino analog input and read the value with the analogRead() function. Depending on the value the Arduino reads, the program will then set pin 3 HIGH or LOW to turn on or turn off the LED night lights. The threshold value is 150. When the analog value read is less than 150, the Arduino will turn the LEDs on. When the analog value it reads is below 150, the Arduino will turn the LEDs off.

Wiring Diagram

You connect the components as shown in the diagram below. Connect the LEDs to pin 3 of the Arduino. The two 470 ohm resistors are current limiting resistors. One lead of the photo resistor is connected to 5V, the other to one lead of the 1 kohm resistor. The other lead of the 10 kohm resistor is connected to ground. This forms a voltage divider, whose output is connected to pin A1 of the Arduino. As the light impinging on the photoresistor gets stronger, the resistance decreases, and the voltage output of the divider increase. The reverse happens, when the impinging light gets weaker.