I'm trying to build a transistor level half-adder. I am using each transistor as a NOR gate (see below).


simulate this circuit – Schematic created using CircuitLab

The half-adder logic circuit I am using is this one:


simulate this circuit This is the full circuit:


simulate this circuit

I have built this circuit in my spice program and it works, but it didn't work when I built it on a breadboard.Breadboard circuit

I'm pretty sure I wired it correctly, is there something I'm missing?

  • 1
    \$\begingroup\$ Show your full schematic please. \$\endgroup\$
    – Andy aka
    Commented Mar 29, 2022 at 16:24
  • \$\begingroup\$ Have you noticed how a switch as an input to your transistor circuit does not match the case where a transistor circuit drives another transistor circuit? \$\endgroup\$
    – Justme
    Commented Mar 29, 2022 at 16:31
  • \$\begingroup\$ @Justme I'm sorry I didn't understand could you clarify \$\endgroup\$
    – joupy
    Commented Mar 29, 2022 at 16:36
  • \$\begingroup\$ Isn't the output also just connected to ground. Also, since you say the spice simulation works, show it, and aince you say breadboard doesn not work, in what way it works then? What does as expected, what doesn't? \$\endgroup\$
    – Justme
    Commented Mar 29, 2022 at 16:41
  • 2
    \$\begingroup\$ "but it didn't work" - In what way did it not work? What inputs did you give it and what outputs did it give back? What measurements have you made to debug the issues? \$\endgroup\$
    – brhans
    Commented Mar 29, 2022 at 17:36

1 Answer 1


On the breadboard, rotate all 3 transistors by 180 degrees.

  • \$\begingroup\$ Yup. In the photo you supplied, your current orientation has the collectors at the top and these are all grounded (through A-E13), so clearly should have been the emitters. \$\endgroup\$
    – Dan
    Commented Mar 29, 2022 at 18:41
  • \$\begingroup\$ @joupy Are those transistors on the breadboard 2N3904s as you have shown in the circuit diagram? \$\endgroup\$
    – user173271
    Commented Mar 29, 2022 at 21:06

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