LDRs or Light Dependent Resistors are very useful especially in light/dark sensor circuits. Normally the resistance of an LDR is very high, sometimes as high as 1000 000 ohms, but when they are illuminated with light resistance drops dramatically.

The animation opposite shows that when the torch is turned on, the resistance of the LDR falls, allowing current to pass through it.

This is an example of a light sensor circuit :

When the light level is low the resistance of the LDR is high. This prevents current from flowing to the base of the transistors. Consequently the LED does not light.

However, when light shines onto the LDR its resistance falls and current flows into the base of the first transistor and then the second transistor. The LED lights.

The preset resistor can be turned up or down to increase or decrease resistance, in this way it can make the circuit more or less sensitive.

1. The circuit above is a light sensor. That means light must shine into the LDR for the circuit to be activated. Draw a circuit composed of the same components that activates when it is DARK (when the LDR is covered). This is a typical examination question.

HINT: Simply swap the preset resistor and the LDR.

2. What is the role of the preset resistor ?

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