I am building a 8-bit full-adder circuit using the 7432 for the OR gate, the 7408 for the AND gate, and the 7486 for the XOR gate. I am only on the first bit and I am already having problems.

The carry light immediately goes to high as soon as I apply the power. It will not go low, it stays on. I have my two inputs, A and B, attached to 10 LEDs by two DIP switches. Each input has its own DIP and LED. I did this so I will be able to see the numbers I am adding.

The carry light remains on no matter if either input is high or low. When either input is high, there is no sum. The carry continues to stay high.

The layout (see picture) is:
From left to right: 7486 7408 7432

In the pic:

  • the two yellow wires are inputs A and B. A is on pin 1 of the 7486 and B is on pin 2 of the same chip
  • The white wire is the sum, from pin 6 of the 7486.
  • The orange wire is the carry from pin 3 of the 7432.

The schematic:

  • The schematic shows the first full adder, along with the pin layout of each chip. It's the first bit, so I tied the carry to ground. The carry is pin 5 on the 7408.

What I have done:

Replaced all IC chips;
Replaced the breadboard;
Replaced the 10 LEDs;
Since it's the first bit, there is no carry. Therefore, I tied the carry to ground;
Using 320 Ω resistors for the LED segment;
Checked my connections with the schematic quite a few times: The connections are correct.

What I have not done: Given up.....yet.

enter image description here      
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  • \$\begingroup\$ (Much better. Effective line breaks in markdown: append two blanks to preceding line.) \$\endgroup\$
    – greybeard
    Apr 15, 2023 at 5:13
  • \$\begingroup\$ Thank you @greybeard. A commentor mentioned pull up/down resistors for the DIP switch. I never thought about that. I was thinking about putting a 1k pull up resistor. \$\endgroup\$ Apr 15, 2023 at 5:31
  • \$\begingroup\$ A 1k pull up should work nicely with any 74X series IC I remember or dare to imagine. \$\endgroup\$
    – greybeard
    Apr 15, 2023 at 5:49

2 Answers 2


Your single full adder has two outputs, one of which behaves in an unexpected way.

Do more measurements/observations!
And log the results.

Connect indicators (current limited LEDs) to the inputs of the/each gate showing an unexpected output level.
(Some are bound to be driven by gate outputs: you'd work your way from output to input here.)

If you had a single input leading to unexpected behaviour, you'd work in the other direction, from input to output.

If you are using CMOS gates, MF3's comment puts the finger on it: standard CMOS gate inputs need a "defined" level.

There are habits to follow to keep out of trouble: Put a buffer capacitor in the middle of each pair of power rails - anything in low μF should do.
If you plan to do "fast" switching, including logic, get used to placing "decoupling" capacitors immediately at "every power supply pin" - with TTL, 0.1μF used to be common.

With CMOS, give every input a defined level, GND being common.
Should you get bitten by undefined input level a second time, place a 100k-ish resistor from every 5-hole row to GND and just keep them there: life is short.

Measure often.
Keep a "lab log", enter at least every unexpected observation.

  • \$\begingroup\$ (Read the last few entries in the lab log when resuming "work".) \$\endgroup\$
    – greybeard
    Apr 15, 2023 at 6:01
  • \$\begingroup\$ (To not use up a prohibitive amount of "power rail holes" for pull-down/up resistors, use (single-in-line) SIL resistor arrays.) \$\endgroup\$
    – greybeard
    Apr 15, 2023 at 6:23
  • \$\begingroup\$ Thank you for your help. You and MF3 solved it. \$\endgroup\$ Apr 16, 2023 at 4:30
  • 1
    \$\begingroup\$ @MichaelPaxinos - Hi, (a) Your comment 2 above this one starting @MF3 will not have notified them, since they have not written a comment on this answer. I recommend you delete it from here (hover over the comment, a red "delete" link will appear) & repost it below their answer. (b) Your comment directly above suggests the issue has been solved. If so, to effectively mark the topic as solved, please consider "áccepting" your choice of the best answer (i.e. click the "tick mark" next to that answer to turn its tick mark green). Thanks. \$\endgroup\$
    – SamGibson
    Apr 16, 2023 at 4:39
  • \$\begingroup\$ Done. Thank you @SamGibson. Do I need to close the question out? If so, how do I do that? I don't see a command. \$\endgroup\$ Apr 16, 2023 at 5:34

In the photo, the pin 7 of the 7432 is not connected to GND. That may explain why it can not go low.

  • \$\begingroup\$ Actually, I had the chip grounded before and it still did the same thing. I think I may have accidentally pulled it when I was taking the pic. It still doesn't work. \$\endgroup\$ Apr 15, 2023 at 0:45
  • \$\begingroup\$ It is difficult without a photo of the original circuit and a full schematic including the DIP switches. Do you have pull up or down resistors with the DIP switches? \$\endgroup\$
    – MF3
    Apr 15, 2023 at 2:14
  • \$\begingroup\$ The DIP switches were an add on. I wanted to make it easier to do the inputs. I am not using any pull up/down resistors. That's a good idea. That never occurred to me. Would a 1k be workable? \$\endgroup\$ Apr 15, 2023 at 3:25
  • \$\begingroup\$ The puzzle has been solved. I didn't have pull up resistors. 1k didn't do it, but 10ks did. Everything is working as it should. I finished up to 4 bits on the adder, I will do the other four tomorrow. Thanks to both of you. There is no way I would have thought of pull up/down resistors \$\endgroup\$ Apr 16, 2023 at 5:31

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