Photoresistor (LDR)

A resistor whose value changes with light intensity is termed a photoresistor. This type of passive, opto-electronic component is frequently abbreviated to LDR (Light Dependent Resistor). A photoresistor's action is based on an internal, photoelectric effect. The energy (photons) of light impinging on a semiconductor releases valence electrons from their lattice bonds. The higher the light intensity, the greater the number of released charge carriers. Consequently, the electric resistance drops as the light intensity rises.

Variable Resistors !

Photoresistors are passive, electronic components whose light dependence stems from an internal, photoelectric effect.

Photoresistors are made of mixed crystals based on cadmium sulphide (CdS) and lead sulphide (PbS). Because a photoresistor's electrical resistance drops sharply on exposure to light, it is necessary to protect the photoresistor in bright conditions against damage by excessively high currents. One problem posed by photoresistors is their relatively delayed response, inversely proportional to brightness and usually amounting to a few milliseconds. For this reason, circuits equipped with LDRs only achieve switching frequencies of up to roughly 100 Hz.

A photoresistor's characteristic data include:

Dark resistance R0 (value one minute after complete blackout); standard values for R0: 106 ... 108 W
Bright resistance RH, measured at a light intensity E = 1000 lux; standard values for RH: 102 ... 104 W
Response time (up to a few milliseconds)
Maximum sensitivity wavelength
Temperature coefficient

To achieve a linear change in luminance, you can cover the LDR partly with a strip about 1.5 cm wide and adjust the illuminated area to vary the light quantity impinging on the LDR's surface. 

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