The 5mm Light Dependent Resistor (LDR), often referred to as a photoresistor, is a low-cost, analog light sensor whose resistance changes with the intensity of light falling on its surface. In darkness, its resistance is high (hundreds of kilo-ohms to mega-ohms), while under bright light it drops to a few kilo-ohms or less, making it ideal for basic light sensing and automatic control applications.

Key Features

• 5mm radial package – Compact round sensing head suitable for breadboards and PCBs.
• Analog light sensor – Resistance decreases as light intensity increases, usable as a simple voltage divider.
• Wide resistance range – High resistance in the dark (hundreds of kilo-ohms to mega-ohms) and low resistance in bright light (hundreds of ohms to a few kilo-ohms).
• Visible-light response – Spectral peak around 540 nm, similar to human eye sensitivity.
• High voltage tolerance – Typical maximum DC voltage rating up to 150 V and power dissipation around 90–100 mW depending on model.
• Easy microcontroller interface – Works with Arduino, ESP32, STM32, Raspberry Pi and other 3.3 V / 5 V systems via an analog input.

Applications

• Automatic lighting control – Street lights, night lights, garden/pathway lights, and day/night switching.
• Ambient light sensing – Adjust LCD/LED brightness based on room light levels.
• Security and alarm systems – Light-based triggers, window/door tamper detection, and beam-break alarms.
• IoT and DIY projects – Light loggers, environment monitors, smart home automation nodes.
• Educational use – Teaching basic analog electronics, sensors, and voltage dividers.

Technical Specifications

How to Use with a Microcontroller

To interface the 5 mm LDR with Arduino, ESP32, or other microcontrollers, connect it in series with a fixed resistor (e.g., 10 kΩ) to form a voltage divider, then feed the midpoint to an analog input pin. As light increases, the LDR resistance falls and the measured voltage changes accordingly.

Why Choose a 5 mm LDR Sensor?

• Simple, passive component with no polarity and no dedicated driver IC required.
• Low-cost and robust — suitable for large-volume and educational projects.
• Compact 5 mm head fits breadboards, perfboards, and compact PCBs.
• Good sensitivity over the visible spectrum with a response similar to the human eye.
• Ideal for threshold-based light detection, rather than precise lux measurements.

Important Notes

• LDRs are not suitable where RoHS restrictions on cadmium strictly apply — always confirm material compliance from the specific manufacturer.
• Use within specified voltage and power dissipation ratings to avoid long‑term drift or damage.
• Allow a short settling time after sudden light changes for stable readings, as response is in the millisecond range.
• For consistent results, keep the sensing surface free from dust, fingerprints, and coatings that attenuate light.