I made a couple videos to bridge beginner gap

[jhocking]

Hi, I recorded a few videos in direct reaction to this course. This course was really helpful to me, but it bugged me that it starts with the assumption that NAND already exists, leaving a gap in how to build a computer from the ground up.

Thus, I made these videos (especially the second video in the playlist) to get students over that hump!

https://www.youtube.com/playlist?list=PL5sUefBdGhUVwK08Tgef92m4l5pvsCKvC

(I actually recorded these several years ago, but somehow I never thought to post about them here)

[WBahn]

I don't think this is a very useful explanation/demonstration of a NAND gate. Your circuit works by shorting the power supply to turn the LED off. But that means that ALL of your other logic that might be being powered by the circuit is going to fail anytime any one of your NAND gates has both inputs HI.

That's if things go well. If things don't go well, you let the magic smoke out of everything. I don't recommend anyone do this demo using a NiCad or LiPo battery -- those can deliver significant current and, in the case of a LiPo, catch fire or explode if shorted.

[jhocking]

hm point well taken. My intention was for something big and simple, just built with alligator clips, but perhaps this is an unhelpful oversimplification. What would be a better design here?

[WBahn]

Perhaps the best switch-based demo uses one DPDT switch for each input. One side of one pole simulates the switch "above" the output (i.e., in the circuitry that will actively pull the output HI) and the opposite side of the other pole simulates the switch "below" the output (i.e., in the circuitry that will actively pull the output LO).

[jhocking]

thanks! I’ll see if I can design a lesson around that suggestion

[Atsab]

I left a comment on your video that I thought I'd copy here, to stimulate discussion! I LOVE the idea of what you're trying to do, and the first video (after the intro) has really helped me. But I feel it doesn't go far enough. Here's what I wrote:

This is *so close* to the explanation I've been looking for! I love your idea -- Having no experience with even the most elementary electronics, I found the start of both books "But How do It Know" and "Elements of Computing Systems", while very readable, both elide the most basic physical explanation at the fundamental level: how power turning on and off relates to the NAND gate, an abstraction which is used by both books as the fundamental axiom to build off. But I had trouble accepting this abstraction as the starting point. So I want to thank you for this video and I hope you continue to work in this direction-- You're on to something. But I personally need even more explanation here. Where it starts to get fuzzy for me is when the first (only) switch is connected to the circuit via the "middle". I'm verging on being lost already, with what that even means. Next, I can vaguely accept on your say-so that with the switch being Off, the path of least resistance is to go to the light, but I absolutely don't understand why the power does NOT go to the light when the switch is On. Needless to say by the time a second switch is introduced I'm completely lost. I think this is why there is nothing out there akin to what you're trying to do: to those like yourself who understand these basic things, it's hard to explain every little step! Remember, your audience is Me: I know NOTHING! Thank you

**EDIT***
I got the book 'Code' by Charles Petzold from my library and found the early chapters are just what I needed in terms of thoroughness and elementariness. Its description of circuits and NAND gates (which is around 70 pages long) begins with a basic explanation of electricity, for example. I'm posting this here in case anyone who was in my shoes (a non-sciencey person interested in the fundamentals of computers) ends up here looking for such a resource, as I did.