0
\$\begingroup\$

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.

schematic

simulate this circuit – Schematic created using CircuitLab

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

schematic

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.

\$\endgroup\$
  • \$\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\$ – Dave Guenther Apr 7 '15 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 '15 at 18:42
8
\$\begingroup\$

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

enter image description here

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.