An LDR (Light Dependent Resistor) is a type of resistor that changes its resistance according to the intensity of light. It is a photoconductive device, meaning that its resistance changes when light falls on it. The resistance of an LDR decreases with an increase in the intensity of light, and vice versa. LDRs are often used as light sensors, light meters, automatic street lights, and in areas where light sensitivity is required. They are also known as photoresistors. LDRs are usually available in 5mm, 8mm, 12mm, and 25mm dimensions.

LDRs are made of semiconductor materials with high resistance, and they are specifically designed for their light sensitivity and the change in resistance this causes. They are nonlinear devices and have a sensitivity that varies with the wavelength of the light applied. LDRs are not typically used for color sensing because they are only sensitive to the intensity of light and not the color.

LDRs have a variety of applications, including:

• Lighting switch: LDRs can be used to automatically turn on a light at a certain light level, such as in street lights or garden lights.
• Camera shutter control: LDRs can be used to control the shutter speed on a camera by measuring the light intensity.
• Audio compressors: LDRs can be used in audio compressors to introduce a delayed response.
• Light-sensitive alarms: LDRs can be used in light-sensitive alarms to detect fluctuations in light intensity.

LDRs are not typically used for detecting motion because they only respond to changes in light intensity and not movement.