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5mm LDR (Light Dependent Resistor), i.e. Photo Resistor, is a basic electronic component that changes the electrical resistance depending on the light intensity.
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It is one of the most popular and most used sensors of electronic and robotic projects.
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"5mm" refers to the diameter of the sensor (usually the width of the circular, plastic or epoxy protective sleeve).
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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.
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A Voltage Divider Circuit needs to be installed to convert this to voltage.
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Voltage Divider Circuit SetupSerial Connection: The LDR is connected in series with a fixed-value resistor (e.g. 10 k $\Omega$).
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Voltage Application: A constant voltage (e.g. 5V) is applied to this series circuit.
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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).
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As the light intensity changes, the resistance of the LDR changes, which causes the voltage at the junction of the circuit to change.
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The microcontroller detects the light level in the environment by reading this voltage.
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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.
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Robotics: Light-sensitive robots (Line-Followers or robots directed at the light source)Alarm Systems: Detecting the interruption of a light beam.
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Screen Brightness Adjustment: Automatic adjustment of the device screen brightness according to the ambient light.


