LDR – Photoresistor

Nov 082019

What is an LDR?

LDR (Light Dependent Resistor) is a light-sensitive resistor whose resistance changes with the light intensity that falls upon it. Photoresistor (photo resistor) is manufactured with crystalline structure materials using its photo conductive properties. The most common crystals used are cadmium sulfide and cadmium selenite.

What are the normal values of an LDR?

Normal values for an LDR go from 50 ohms to 1K (1000 ohms) when it is fully illuminated and from 50K (50 000 ohms) to several megohms when it is in the dark.

Photoresistor advantages

They are available in different sizes and shapes.
Need a small voltage for its operation.
They are bidirectional and cheap.
Its resistance does not change instantly when it goes from darkness to light or from light to darkness, and the time LDR takes on these two processes is not always the same.

Photoresistor disadvantages

As photoresistors do not change their value instantly, they cannot be used in many applications, especially those that need a lot of accuracy in time (the time it takes to change their state from dark to light or from light to darkness).
Another disadvantage is the different values a photoresistor may have for a specific light intensity that falls upon it. Its typical response time is approximately 0.1 seconds.

LDR applications

There are many applications in which a photoresistor is very useful. There are cases where the accuracy of the changes is not important, and neither is the exact value of its resistance.

For example,

In automatic streetlights. A very simple circuit using LDR automatically switches ON lights in the evening after sunset. The same circuit automatically switches OFF lights when sunlight falls on the LDR in the morning.
Alarm systems
Automatic emergency light system
Automatic luminous neon signs
A lot of circuits and projects

Photoresistor symbols are