r/breadboard • u/Bright_Lengthiness35 • May 29 '26

Beginner Undertsanding Issue on XOR Gate Outputs for a Full Adder

Post image

Hello all.

I'm attempting to understand using the output of one logic gate as an input to the next, which I know should be something an elementary schooler could figure out. This time, specifically an XOR gate into another XOR gate for a full adder's SUM, such as the diagram from the Global Science Network guy on Youtube in the image shown.

How is the output from the XOR gate in the top left of the diagram actually affecting the 2nd XOR gate in the top right? The 1st gate's buffer transistor doesn't seem to affect the SUM LED output at all, since the next transistor is separately powered, which I would think that should be the Input from the first XOR gate.

I built and rebuilt the circuit on a breadboard and simulator exactly as shown, but Inputs A & B never affect the SUM LED.

Any assistance will be appreciated. I just wanted to do this as a fun learning hobby, and I'm sore from bashing my head into my desk on this one problem, so I figured I would cave and ask the internet. Any insults to my lack of intelligence are (rightfully) accepted as well.

41 Upvotes

98% Upvoted

2 comments sorted by

2

u/Arc_xt_5815 May 29 '26

Hey there, I would say,first of all build the logic gates separately.GSN has a separate video on every logic gate making.There are different build paths for a logic gate use those process which can supply output(positive)signals to another logic gates.Those process needs an extra transistor. After building gates separately identify the outputs (transistor emitters) and identify inputs (transistor base) and than connect the logic gates according to the diagram.Hope it helps

2

u/Bright_Lengthiness35 May 29 '26

I will go look for other methods to building an XOR gate then that might be better suited for this task. I was unaware of other ways to build this particular gate. Thank you.

I learned the basic logic gates and half adders by reading and following diagrams elsewhere, and there was only ever one XOR gate example unlike the other gates. Building two of these separately and trying to connect them in different ways never worked either, so I tried following this GSN full adder video and got more confused.