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Light Dependent Resistor

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LDR Properties

The LDR Resistance decreases with increasing light intensity.


LDR Protection

The LDR is wired up with a resistor in series.
This makes a voltage divider circuit.
The series resistor is chosen to satisfy two
design constraints.

  • When the light is bright and the LDR
    resistance drops close to zero, the other
    resistor MUST ensure that the LDR
    current remains below a safe limit.
  • It is also desirable if the LDR
    resistance and the series resistance
    are approximately equal at the light
    level you are trying to detect.

Calculate the Safety Resistor

Assume the LDR resistance is small in very
bright light. In a 12 Volt circuit, if the maximum
safe current is 10 mA ...

R = V / I      R = 12 / 0.01      R = 1.2KΩ

Voltage Divider

LDR Voltage Divider

Vout = 12 x R2 / ( R1 + R2 )

As the light gets brighter, R2
decreases and the output
voltage drops.

Voltage Divider

LDR Voltage Divider

Vout = 12 x R2 / ( R1 + R2 )

As the light gets brighter, R1
decreases and the output
voltage rises.

How the LDR Works

Light Dependent Resistors (LDR) are made from a semiconductor. In the dark, this semiconductor has very few free carriers so the LDR has a high resistance. When photons of light hit the semiconductor, they add enough energy to dislodge carriers which can then flow. This reduces the resistance. The carriers are electrons.

Logarithmic Graphs

As the light level changes, the resistance of the LDR changes over a huge range. This is difficult to plot onto normal graph paper so logarithmic graphs are used.


Calculating the Safety Resistor

If the LDR is in a 12 volt circuit, the series resistor must be at least ...

R = V / I

R = 12 / 50x10-3

R = 240Ω

240Ω is the minimum safe resistance.

If the goal is to detect a light level less than 10 Lux, a more suitable resistor would be about 800Ω.

This resistor satisfies both the design requirements mentioned at the top of this page.

Test Your Understanding

At what light level does it become impossible to meet both design requirements?




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