Transcript LDRs

IP2.23.5 LDRs
LDRs
© Oxford University Press 2011
IP2.23.5 LDRs
 A light-dependent resistor (LDR) is a special type of resistor. Its
resistance changes as the intensity of the light falling on it changes.
 When the light levels are low, the resistance of an LDR is high. This
is because an LDR is made of a semiconductor material where the
outer electrons are bound weakly to the atoms.
 When bright light shines on an LDR, the resistance is much lower.
The light energy is transferred to the outer electrons which can then
break free from the atoms. They are then free to flow through the
LDR.
© Oxford University Press 2011
IP2.23.5 LDRs
 This graph shows the LDRs
resistance against light intensity.
Remember, the resistance of an
LDR decreases as light intensity
increases.
© Oxford University Press 2011
IP2.23.5 LDRs
 This change in resistance according to the intensity of light means that
LDRs can be used as switches.
 For example, they can be used to switch on security lights when it gets dark.
 The light shown here has an infra-red detector to switch the light on when
there is motion in front of it but there is also an LDR so the light only comes
on if there is motion at night.
© Oxford University Press 2011