Electronics lessons: The Light dependant Resistor

Following on from the last lesson which was about temperature dependant resistors called thermistors, I thought I'd move on to discuss light dependant resistors.

One of the very basic competent in electronics is the resistor, (that was covered a long time ago.)
Fixed value resistors are good and very useful in a lot of applications, however, you find that other times you need something that will alter in response to external stimulus.

Just as the thermistor can change it's resistivity in accordance with temperature, the Light dependant resistor changes it's resistivity in accordance with the amount of light that falls onto a sensor.

The standard LDR is a round package, about 1cm across and 7mm tall, it has a window at the top of it where the light falls on it.

Light dependant resistors have a symbol that is the same as a symbol for a resistor, but with a circle drawn around it, with two arrows pointing towards that circle, (the circle may be omitted in some schematics.

As light falls on the light dependant resistor it's resistivity goes down, when the window is covered, or when the sun goes down/lights go off etc, the resistivity goes up.