Photoresistor or Light Dependent Resistor (LDR) is a type of photoelectric conversion element whose resistance value changes with the intensity of light shining on its surface. Photosensitive resistors are mainly made of semiconductor materials, such as cadmium sulfide (CdS), cadmium selenide (CdSe), etc., which are very sensitive to light.

working principle

The working principle of a photoresistor is based on the photoelectric effect. When light shines on the semiconductor material of a photosensitive resistor, the energy of photons is absorbed by electrons in the material, causing electrons to transition from the valence band to the conduction band, increasing the number of free electrons and thus reducing the resistance value. The stronger the light, the more free electrons are generated, and the lower the resistance value. On the contrary, in dark environments, the resistance value of a photoresistor will be very high.

characteristic

Photosensitivity: The resistance value of a photoresistor changes significantly with the intensity of light.

Response speed: The response speed of photoresistors is relatively slow, usually in the millisecond range, which limits their application under high frequency or rapidly changing lighting conditions.

Cost effectiveness: Photoresistors are inexpensive, easy to obtain, and are an economical and practical type of optical sensor.

Linearity: The output of a photoresistor is usually not linear, which means that the relationship between the change in resistance value and the intensity of light is non-linear.

Spectral response: The spectral response range of photoresistors is usually concentrated in the visible light region, especially sensitive to yellow and green light.

application

Photoresistors are widely used in various fields, including but not limited to:

Automatic lighting control: used for automatic switching of streetlights, indoor lighting and other equipment, adjusting lighting brightness according to changes in ambient light.

Camera metering system: used in traditional film cameras to measure ambient light intensity and help determine the correct exposure settings.

Security system: As part of the intrusion detection system, it is used to monitor changes in light, such as the light curtain in anti-theft alarms.

Meteorological instrument: used to measure the intensity of light in the atmosphere, assisting in weather forecasting and research.

Package size of photoresistors: The packaging size of photoresistors (Photoristors or Light Dependent Resistors, LDRs) varies greatly depending on their purpose and manufacturer. Photosensitive resistors are usually not components that use surface mount technology (SMT), but traditional plug-in components with two pins, so their packaging size refers more to the overall dimensions of the component, including the pins.

Here are some common examples of packaging dimensions for photoresistors:

Small package: diameter approximately 3mm to 5mm, length (including pins) approximately 6mm to 10mm. This small package is suitable for space limited applications.

Standard packaging: The diameter is approximately 5mm to 10mm, and the length (including pins) is approximately 10mm to 20mm. This is the most common size of photoresistor in the market.

Large scale packaging: The diameter can reach 20mm or more, and the length (including pins) can reach 30mm or even longer. Large scale packaging is typically used for applications that require higher sensitivity or work in harsh environments.

The pins of the thermistor can be straight or curved to meet different installation requirements. Packaging materials are usually plastic or ceramic, and sometimes there is a transparent epoxy resin coating to protect sensitive semiconductor materials.

When selecting a photosensitive resistor, in addition to considering the packaging size, factors such as its spectral response, dark resistance (resistance value in the absence of light), bright resistance (resistance value in the presence of light), response time, and operating temperature range should also be considered to ensure that specific application requirements are met.

Due to the lack of a unified naming convention for the packaging size of photoresistors, it is best to refer to specific product specifications or data sheets when purchasing to obtain accurate size information. Manufacturers usually provide detailed instructions on packaging dimensions, pin configurations, and other related parameters in their product specifications. When selecting a photoresistor, the following factors need to be considered:

Lighting range: Ensure that the selected photoresistor's lighting range meets the application requirements.

Resistance value: Select the appropriate resistance value based on the required light sensitivity and circuit design.

Response speed: For applications that require fast response, choose photoresistors with fast response speed.

Photosensitive resistors are a simple and effective type of optical sensor. Despite their limited response speed and linearity, they are still very useful in many low-cost or simple optoelectronic applications due to their low cost, easy integration, and usability.