Electronics Principles V11

Clive W. Humphris

ACTIVE TRANSISTOR CIRCUITS: LDR Pull Up, Pull Down Circuit.  


Both the circuits here exploit the effect of pulling up or down voltages at the junction of a voltage divider resistive network.

The LDR has high dark resistance that falls to a low value when brightly lit. Therefore, the LDR can be considered as a switch under these two conditions.

However, there will also be varying levels of brightness between darkness and bright light and this will cause the voltage output to range from almost the maximum supply to nearly zero as the LDR resistance changes.

The voltage divider current can be calculated from the formula shown. Should there be a load connected at the junction it would be effectively placed in parallel with the lower resistance. See loading a voltage divider topic.


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