LDR (Light Sensor) with Arduino

The LDR (Light Dependent Resistor) is the most common of the light sensors on the market, for years it has been used on top of streetlights to turn on the lamps at night and turn off during the day, and thus save energy and hence taxpayer money. So, let's see how to connect this sensor to the also famous Arduino. 

Figure 1 - LDR and Arduino Uno

  The sensor

The LDR, as its name shows, is a variable resistor by light, which means that depending on the luminosity the value of the internal resistance in Ohm (R|Ω) will vary, from virtually 0R, when under strong light, up to about 200kR when in complete darkness.

The circuit

Understanding that the LDR is a resistor, and can virtually reach 0R, all we need is an Arduino analog port and a 100kR resistor to create a Voltage Divider and thus we can read the voltage, in this case it varies from 0V (0 in reading), when in total darkness up to 5V (1023 in reading, considering a 10-bit board of analog input resolution), when in great light. Necessary material:

• 1x - Arduino
• 1x - Protoboard
• 1x - LDR
• 1x - 100kR resistor

Figure 2 - LDR circuit with Arduino

Follow Figure 2 to assemble the circuit.

The code

Basically the code prints if "it is dark", if the voltage received is below half of the threshold and "is illuminated" if equal or above the threshold.