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/ Tips & Tricks / Tips & Tricks how to interact with light – Photoresistors

Tips & Tricks how to interact with light – Photoresistors

To use light great helpers are Photoresistors

Definition of photoresistors:

Photoresistors are light sensitive resistors whose resistance decreases as the intensity of light they are exposed to increases.

  • Photoresistors are also known as light-dependent resistors (LDRs) and are passive components that change their resistance based on the amount of light that falls on them.
    They are used in laser-based security systems to detect the change in the light intensity when a person/object passes through the laser beam.

  • Characteristics

    Sensitivity – Light dependent resistors have a lower sensitivity than photo diodes and photo transistors. Photo diodes and photo transistors are true semiconductor devices which use light to control the flow of electrons and holes across P-N junctions, while light dependent resistors are passive components, lacking a P-N junction. If the light intensity is kept constant, the resistance may still vary significantly due to temperature changes, so they are sensitive to temperature changes as well. This property makes LDRs unsuitable for precise light intensity measurements.

    Latency – Another interesting property of photoresistors is that there is time latency between changes in illumination and changes in resistance. This phenomenon is called the resistance recovery rate. It takes usually about 10 ms for the resistance to drop completely when light is applied after total darkness, while it can take up to 1 second for the resistance to rise back to the starting value after the complete removal of light. For this reason the LDR cannot be used where rapid fluctuations of light are to be recorded or used to actuate control equipment. But this same time latency property is exploited in some other devices, such as audio compressors, where the function of the light dependent resistor is to smooth the response.

    Construction of Photoresistors

    Since the discovery of photoconductivity in selenium, many other materials have been found that are light dependent. In the 1930s and 1940s, PbS, PbSe and PbTe were studied following the development of photoconductors made of silicon and germanium. Modern light dependent resistors are made of lead sulfide, lead selenide, indium antimonide, and most commonly cadmium sulfide and cadmium selenide. The popular cadmium sulfide types are often indicated as CdS photoresistors. To manufacture a cadmium sulfide LDR, highly purified cadmium sulfide powder and inert binding materials are mixed. This mixture is then pressed and sintered. Electrodes are vacuum evaporated onto the surface of one side to form interleaving combs and connection leads are connected. The disc is then mounted in a glass envelope or encapsulated in transparent plastic to prevent surface contamination. The spectral response curve of cadmium sulfide matches that of the human eye. The peak sensitivity wavelength is about 560-600 nm which is in the visible part of the spectrum. It should be noted that devices containing lead or cadmium are not RoHS compliant

    wheelson main s6
    Dusty ready to go

    Typical Applications for Photoresistors
    Photoresistors are most often used as light sensors. They are often utilized when it is required to detect the presence and absence of light or measure the light intensity. Examples are night lights and photography light meters. An interesting hobbyist application for light dependent resistors is the line following robot, which uses a light source and two or more LDRs to determine the needed change of course. Sometimes, they are used in other sensing applications, for example in audio compressors, because their reaction to light is not instantaneous, and so the function of the LDR is to introduce a delayed response.

    Types of sensors what are most used

    Lidar sensors and LDRs sensors are both types of sensors that detect light. However, they work on different principles and are used for different applications.

    Lidar (Light Detection and Ranging) sensors use laser light to detect objects and measure distances. They emit a laser beam and measure the time it takes for the beam to bounce back after hitting an object. Lidar sensors are used in self-driving cars, drones, and other applications where accurate distance measurements are required.
    We offer to try hands on Lidar sensor on our product Wheelson autonomous robot car with AI to test sensor possibilities even in dark.

    LDRs (Light Dependent Resistors) sensors, on the other hand, are passive components that change their resistance based on the amount of light that falls on them. They are used in laser-based security systems to detect the change in the light intensity when a person/object passes through the laser beam. They are also used in some consumer items like light meters in cameras, light sensors like in robotic projects, clock radios etc. They are also considered as a good infra-red detector and hence find application in infrared astronomy.

    For LDRs sensor test we have fun product called Dusty is a unique wacky robot that is equipped with photoresistors so it can move by following the light.

    robotics in future
    Dusty in action
    Willoughby Smith

    In 1873 English electrical engineer Willoughby Smith discovered that the electrical resistance of selenium varies dramatically with the amount of light falling on it. The photoconductivity of selenium eventually provided a method for converting images into electrical signals—the basis for photoelectric cells and a theoretical basis for television.

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