I am trying to combine logic gates (using ICs instead of trying to piece them together using transistors or MOSFETs). Below is the simplest example of what I am trying to accomplish, though ultimately I would like to be able to combine 10 or more logic gates together in series. In short, the problem I have is anytime I have an output from one of my gates it shows the appropriate voltage, but immediately when I connect that output to an input of another successive gate the voltage on that gate is almost cut in half resulting in the input being somewhere between high and low. I looked at this post and read it over a few times, but am not able to apply the answer to my problem especially if I want to create a network of logic gates rather than just combining a NOR and NOT to get an OR. I also don't have a firm understanding of TTL, which is why I am using IC packages.


simulate this circuit – Schematic created using CircuitLab

When I breadboard this out using +5V as high, I get something like this:


simulate this circuit

Again, when I leave the output of the 7408 open and look at the voltage it is 5V, but immediately when i connect that output to the input of the 7402, the voltage drops to 2.6V.

I have a feeling that I am missing something pretty basic here but have rebuilt circuits like this several different times with different gate combinations and see the same thing occur when I try to take a logic 1 output from one gate and feed it as a logic 1 to the input of another.

Am I using the ICs properly? Can anyone tell me why the voltage is dropping between the gates?

Thanks for taking the time to read this.

  • \$\begingroup\$ Thanks, markrages. The answer is plain as day - i just didn't read the pinout closely enough. I also realized that I pulled the 74LS02 out of my bag of 7432s and didn't put two and two together when asking the question. I swapped the chip out for a real 7432 and the circuit did in fact work as pin 1 on the 7432 is an input. Is it possible to remove this question all together? I don't want it to show up as noise when others search for their issues. \$\endgroup\$ Apr 7, 2015 at 15:12
  • \$\begingroup\$ It's useful enough to leave it. You're not the first person to experience bus contention. \$\endgroup\$
    – markrages
    Apr 7, 2015 at 18:42

1 Answer 1


You are connecting to the output of the '02, not the input:

enter image description here


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