active-low LED

Discussion in 'General Electronics Chat' started by livermor, Jul 28, 2011.

  1. livermor

    Thread Starter New Member

    Jul 27, 2011
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  2. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    If you look B-E on this part, when your input is zero you get (7V-Vbe- 0V) = 6.3V across R2.

    When your input is high you still get (7V-Vbe- 3.3V) = 3.0V across R2 for 3mA of base current, and that is still enough drive to turn on the LED.

    So something has to change, either run the LED off 3.3V, or drop a zener is series with R2. Or even 4 or 5 signal diodes.
     
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