The LDR (light dependent resistor) is a resistor that has a value that varies depending on the amount of light it has on it.
The photoresistor (LDR) symbol is:
LDR´s range values goes from 50 ohms to 1K (1000 ohms) when it is fully illuminated and from 50K (50000 ohms) to several MegaOhms when it is dark.
The LDR is manufactured with crystalline structure materials using its photoconductive properties.
The most common crystal used are: cadmium sulphide and cadmium selenide.
The LDR`s value (Ohms) does not vary instantly when it moves from darkness to light or from light to darkness, and the time the LDR takes in this two processes is not always the same.
This is why the LDR can not be used in many applications, especially those that need a lot of accuracy in time (the time it takes to change its state from dark to light or from light to darkness) and the accuracy of the values of the LDR on the same previous statements. Its typical response time is approximately 0.1 seconds.
However there are many applications in which a photoresistor is very useful. There are cases where the accuracy of the changes is not important.
- Automatic activation night light which uses a photoresistor to activate one or more lights (lamps) when the night comes.
- Light controlled Relay where the lighting state of the photoresistor, enables or disables a relay. This kind of circuit can have a large number of applications.
The LDR is an extremely useful element, especially when we need to detect the absence of light.