Light Dependant Resistor.

As the name suggests, a light dependant resistor (LDR)
is a component whose resistance changes when the
amount of light falling on it (called the light intensity) changes.

All you need to remember is that
the resistance goes down as the amount of light goes up.
Resistance decreases as light intensity increases,
light is low, dark is high (resistance).

Plot for a Light Dependant Resistor (LDR)

The LDR can be used in a circuit
to automatically switch lights on at night (see relay).

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