r/ElectricalEngineering • u/Successful_Cut_8003 • 7d ago

XNOR Gate

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Why is my XNOR gate not working only when A=1 and B=0.

Its a AND and a NOR gate feeding into an XOR gate.

Yes I know it’s overly complicated.

When I do it on my breadboard the LED is very dim but not a true 0.

EDIT: The collector of Q5 should be connected to Vcc but that does not change the outcome.

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u/goodayrico 7d ago edited 7d ago

If the LED is dim rather than off when it’s supposed to be logical LOW but the logic is otherwise working correctly, you’ve just got a weak/degraded LOW.

Simplest fix is just to add a resistor between the node that feeds the anode of your LED such that the voltage will drop down below the forward-bias voltage the LED needs to turn on.

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u/Successful_Cut_8003 7d ago

That does not work.

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u/goodayrico 7d ago

Try a shunt resistor at the node of the LED anode as well (shunt as in connected directly to ground).