LDRs are used to measure the light level. They are slow to react so they are not used with fibre-optic or other optical communications systems. They are used in light meters for photography and for systems that turn on lights when it gets dark. The LDR is normally used with a fixed reisitor. This prevents too much current flowing when the light is very bright. It also converts the varying light level into a varying voltage.

Desiging to Protect the LDR. The LDR is wired up with a resistor in series. This makes a voltage divider circuit. The series resistor is chosen to satisy 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.

LDR resistance decreases with increasing light intensity.

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.

More about carriers...

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.

• If the resistance is 20kΩ, the light level is about 0.42 Lux.
• If the resistance is 3kΩ, the light level is 20 lux.
• If the light level is 3 lux, the resistance is 2.7kΩ.
• If the light level is 600 lux, the resistance is about 10Ω which is close to zero.

Calculating the series resistor value.

For this LDR, the absolute maximum current is 50mA.

In bright light, assume the LDR resistance drops to zero.

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?