Category: Computer Science

It’s been about a month, and here’s the third video explaining how computers work from the ground up.

In this third video I discuss flip flops. Starting with two not gates hooked together I discuss how a bi-stable circuit operates, extend this using NOR gates to Set/Reset flip flops, build data flip flops, edge-triggered data flip flops and 8 bit registers which can store up to 8 digit base-two numbers. I also talk a little bit about signal delays through transistor circuits, setup and hold times, and I also briefly introduce edge-triggered Set/Reset flip flops, J/K flip flops and T flip flops, as well as how to use them to build a counter.

All of this, of course, literally comes from the ground up, since everything can be built using 2N3904 transistors, a few LEDs, 330Ω and 10kΩ resistors, and a few switches to wire it all up.

My hope is to release subsequent videos about once a month, give or take.

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So, as promised, about a month later, the second video in my series of videos explaining how computers work from the ground up.

And I mean truly, the ground up; on a breadboard next to me I have a working 1-bit adder circuit made entirely out of 10KΩ resistors, 2N3904 transistors, a few LEDs and 330Ω resistors, and a few switches to demonstrate how it works. Of course once we get passed flip flops and tri-state gates I may not have enough transistors sitting around the house (or enough breadboard space) to actually wire these things up…

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I’ve also created a separate channel for all of these videos, and have a playlist of my introduction videos. Once a month (give or take) I hope to add future videos that continue down this path until we have something that conceptually works.

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Now as a footnote, the reason why the hands-on stuff is important to me goes to my finding an error in the first video, which caused me to rush out a new version of the first video in the series.

That’s because when I used the logic gates from the first video–well, nothing worked. And they didn’t work for reasons I explain in the re-cut video linked below.

I think it’s important to note all this, in large part because science.

Remember: the difference between science and screwing around is that in science, we document our results. And we confess our mistakes when we make them.

So here’s the first video in the series–re-edited and re-uploaded.

Here’s the thing. I started with NPN transistors because my intent is not to start with some sort of weird theoretical “see these diagrams, that’s why it works” nonsense.

I think one thing that is sorely missing from computer science is the hands-on part: the part where you actually touch something tangible and see it work. Sure, you can buy an Arduino and plug it into your USB port and download a program you got from the Internet–but is that the same thing as wiring up individual transistors and seeing basic logic gates working before your eyes?

So I’ve uploaded a new “Quick Video” which shows all five gates in the first “Introduction to Digital Computers” video, wired up on a bread board. I’ve even included show notes with a complete circuit diagram for all five circuits, as well as links on Amazon if you decide to do this yourself.

At the most basic level, computers are not this abstract thing. These circuits are tangible parts, working on basic principles: transistors wired in series or in parallel, outputs attached to the collector(s) or the emitters(s), base being supplied with power (on) or not (off).

Zeros and ones are not theoretical things; they represent something real: power off (zero) or power on (one).

Anyway, here’s the video itself.

Thanks for watching.

I did some minor tweaks and reshot the opening sequence. This first in a series of videos will attempt, over the next few months, illustrate the design of a basic 8-bit computer built from transistors on up.

The entire series can be found on YouTube, and linked to the new Videos section of this blog.

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As I mentioned with my last post, the goal here is to describe how computers work all the way down to the level of transistors. My goal with these videos is to show how, with enough resistors, transistors and other discrete components you could build your own working computer entirely from scratch.

Second, I see this as a foundational set of videos which launch into a second set on computer programming. One reason why I think this way is because while it’s easy for us to think about computer programming as writing some code–we never really stop to think why we write code as a sequence of instructions. We never really give much thought to “what’s under the hood” or why, for example, the most common paradigm for programming is the sequence of steps and not, for example, a Prolog-like set of assertions which is then evaluated by resolving the validity of the assertions in the list of statements.

(After all, in the past there have been many attempts at building analog computational devices that work more like Prolog: establishing a circuit or mechanical system which asserted a truism, then asking questions of the system in order to get answers.)

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Edited April 22, 2018:

The first video contained a fatal mistake, so I’ve corrected the error and updated the links to the corrected video.