The Light Dependent Resistor(LDR) is, as presto wrote, a
resistor
where the
resistance is dependent on how strong the
light
that hits the
light sensitive area. Typically a LDR have a resistance
of several
mega ohms in the dark, and less than 100
ohms
in direct sunlight. The most common
light-
sensitive materials
used in LDRs are
Cadmium Sulphide(CdS) and
Cadmium selenide(CdSe)
CdS based LDRs are sensitive to about the same part of the
electromagnetic spectrum
as the
human eyes. CdS LDRs are most sensitive to light
with a
wavelength of about 520
nanometers(
cyan).
CdSe LDRs are
sensitive to
infrared and the lower
frequenciesof the visual spectrum. CdSe has a
maximum sensitivity to
light
with a
wavelength of 730 nm.
By mixing these two we can make LDRs with other
maximum sensitivities.
LDRs may, if left in a
constant light level,
develop a
temporary
memory effect. This mean that their
resistance may temporary be
shifted
LDRs are also very sensitive to
temperature variations. Their resiatance can
change by up to about 2% per
Kelvin.
Electrical symbol:
arrows pointing towards a normal
resistor symbol
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