5mm LDR (Light Dependent Resistor), i.e. Photo Resistor, is a basic electronic component that changes the electrical resistance depending on the light intensity.

It is one of the most popular and most used sensors of electronic and robotic projects.

"5mm" refers to the diameter of the sensor (usually the width of the circular, plastic or epoxy protective sleeve).

This size is the most preferred type as it provides easy integration into breadboards and circuit boards. Use of 5mm LDR Since LDR is a resistor, it produces a resistance value that cannot be read directly by microcontrollers (such as Arduino) as voltage.

A Voltage Divider Circuit needs to be installed to convert this to voltage.

Voltage Divider Circuit SetupSerial Connection: The LDR is connected in series with a fixed-value resistor (e.g. 10 k $\Omega$).

Voltage Application: A constant voltage (e.g. 5V) is applied to this series circuit.

Measurement: The voltage value taken from the point where the LDR and constant resistance meet is connected to the Analog Input Pin of the microcontroller (such as A0-A5 in Arduino).

As the light intensity changes, the resistance of the LDR changes, which causes the voltage at the junction of the circuit to change.

The microcontroller detects the light level in the environment by reading this voltage.

Fields of Application5mm LDRs are used in a wide range of projects thanks to their simple and cost-effective: Smart Lighting Systems (Photocells): Automatically turn on street lights or garden lights when the environment is dark.

Robotics: Light-sensitive robots (Line-Followers or robots directed at the light source)Alarm Systems: Detecting the interruption of a light beam.

Screen Brightness Adjustment: Automatic adjustment of the device screen brightness according to the ambient light.