Photoresistors - definition
Photoresistors - definition
A Photoresistor (also known as a Light Dependent Resistor, LDR - Light Dependent Resistor) is an electronic component whose resistance varies with the intensity of the light falling on it. In the dark, its resistance is very high (even on the order of megohms), whereas as the light increases, the resistance decreases, often to several hundred ohms (Ω). This phenomenon is due to the photoconductivity effect - in a semiconducting active material (usually cadmium sulphide, CdS), light generates additional charge carriers, increasing the conductivity.
Photoresistors are passive elements with no internal voltage or current source. They do not exhibit polarisation, which means that they can be incorporated into a circuit at will, without regard to the direction of current flow. Their operation depends mainly on the wavelength of the light and the ambient temperature, which affects their sensitivity and response time. Photoresistors' response to changes in illumination is not instantaneous - it is characterised by a certain delay (rise time and fall time), which is why they are not suitable for detecting fast pulses of light.
Photoresistors are used in light sensors, automatic lighting control systems, electronic devices that respond to changes in brightness, and simple alarm or measurement systems due to their simplicity of operation, low price and ease of integration into analogue circuits. However, due to the use of cadmium-containing compounds, their use may be restricted by hazardous substances legislation (e.g. the RoHS directive).
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