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LDRs

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.

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LDRs

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  1. LDRs

  2. 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.

  3. This graph shows the LDRs resistance against light intensity. Remember, the resistance of an LDR decreases as light intensity increases.

  4. 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.

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