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LDR Light Dependant Resistor

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LDR’S are Light Dependant Resistors in other words the resistance changes depending on the amount of light shining on the sensor. This also means that the same LDR can be used as a Dark Sensor, so depending on how you configure the circuit below you can have the circuit switch ON when there is Light, or only switch ON when there is no Light.
This circuit is configured as a Dark Sensor. When light falls on the LDR it’s low resistance forces TR1 to conduct, this keeps TR2 switched OFF, and therefore the LED is not illuminated, and the relay is switched OFF.
Conversely when light is removed from the LDR (it becomes Dark) the high resistance forces TR1 to switch OFF which allows TR2 to switch ON. This then illuminates the LED and switches ON the relay.

The Copper Layout and Component Layout are shown below

Parts List

In this circuit we will remove a few components to make the circuit work the opposite to above, so the circuit switches ON the relay when the sensor sees light. The components to remove are TR2, R1, and the solder pads on the back that are title LINK are joined with a blob of Solder. The Printed Circuit Board is the same as above. The circuit looks like this now.