Photoresistors

• Light sensitive variable resistors.
• Its resistance depends on the intensity of light incident upon it.
– Under dark condition, resistance is quite high (M: called dark resistance).
– Under bright condition, resistance is lowered (few hundred ).
• Response time:
– When a photoresistor is exposed to light, it takes a few milliseconds, before it
lowers its resistance.
– When a photoresistor experiences removal of light, it may take a few seconds
to return to its dark resistance.
• Some photoresistors respond better to light that contains photons within a particular wavelength of spectrum.
– Example: Cadmium-sulfide photoresistos respond to light within 400-800nm range.
– Example: Lead-sulfide photoresistos respond to infrared light.