Let's say I have a logic stage doing an operation on 2 inputs, giving me an output.

How could I use this output to forward it into another set of logic stage, without having to put everything in the same source to ground circuit ?

For example, I'm trying to imagine something like such:

1st circuit:   A and B = Result1
2nd circuit:   C and D = Result2
3rd circuit:   Result1 and Result2 = Result3

I think I should do it like such (resistors removed for simplicity):

enter image description here

But if I have lots of things to put together, I think I would get an headache debugging and calculating all possible voltages and current in this recursive design. I would like to be able to make all parts one at a time, to make sure they work, and then put them together later.

So this is the first thing that came in my head, but of course it wouldn't make sense, as all my inputs would be ignored:

enter image description here

So I thought about optocouplers but, isn't that a weird solution ? (I couldn't find an optocoupler in Circuit Lab, so I tried to make one on my own):

enter image description here

Thanks !

  • 1
    \$\begingroup\$ You could try pull-up resistors (to get a voltage output instead of a current-to-ground-output). Or use complementary mos (noth N and P mosfets). \$\endgroup\$ Commented Mar 19, 2016 at 20:26
  • \$\begingroup\$ logic gate \$\endgroup\$
    – tcrosley
    Commented Mar 20, 2016 at 9:22

1 Answer 1


Pat yourself on the back!

This conundrum is exactly why logic chips come in standardized "formats", such as "TTL-compatible" (it literally means "transistor-to-transistor logic"), CMOS standards (HC, HCT, ALVC, etc.) and so on. It's why we care about "open drain" versus "push/pull", and factors such as propagation delays and power consumption.

I think your next step is to learn about those standards, as well as realize that instead of using lots of CMOS transistors, you can get faster CMOS circuits by using standard 78-series/4000-series logic chips. Knowing how to use CMOS transistors is great; but in a large system, it becomes quite a job to manage them all.


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