# Designing circuits after 101 class

• Schfra

#### Schfra

Im just finishing up my first digital electronics class right now. I know how to design the logic for basic circuits and some slightly more complex ones.

I don’t quite understand how to design actual circuits for a few reasons.

1.) When we design “circuits” in class, we just draw them without resistors so I’m not sure where they should be placed in an actual circuit.

2.) We have a lab for the class, and when we design circuits for it, sometimes they don’t work like they should on paper. The TAs often tell us to simplify the circuit because without a simpler circuit there are things like timing errors.

So how do I go from a logic diagram to a circuit diagram with resistors?

And also, how do I avoid things that are correct on paper but cause problems while creating the actual circuit?

So how do I go from a logic diagram to a circuit diagram with resistors?
If you are using physical chips that contain the logic gates, you don't need resistors (except in special cases)

And also, how do I avoid things that are correct on paper but cause problems while creating the actual circuit?
Ah ... do actual electronics for several years while learning about timing errors

Schfra
Usually, device spec sheets show basic wiring. A good resource outlining various problems that are encountered in digital circuit design is Texas Instrument's
"Designing with Logic".

jim hardy, anorlunda and Schfra
Usually, device spec sheets show basic wiring. A good resource outlining various problems that are encountered in digital circuit design is Texas Instrument's
"Designing with Logic".
If you are using physical chips that contain the logic gates, you don't need resistors (except in special cases)

Ah ... do actual electronics for several years while learning about timing errors
I was wondering why some of the circuits had no resistors in them.

Is this because there is resistance built into the gates?

I was wondering why some of the circuits had no resistors in them.

Is this because there is resistance built into the gates?
Built into the chips, along with transistors. If you are really going to learn digital design you have to learn what's inside the chips, else you're not likely to understand why timing problems occur. "Ideal logic" doesn't exist in the real world, although as you've already been told, for simple circuits it looks a lot like it does.

Asymptotic and Schfra
I don’t quite understand how to design actual circuits for a few reasons.
You will get better quickly in the next few classes you take. In the mean time, keep on asking questions -- learning to "ask the right questions" is a great skill that you will use in many of your classes.

Here is a link to an old thread that you may enjoy. In it we discuss real-world circuits, and what may be wrong with them (and how to fix them). You probably won't understand all of the circuits just yet, but I think you'll start to get a feel for the kinds of questions to ask about such circuits...