In my previous blog — Kan Klive’s Karnaugh Maps Be Korrect? — I mentioned that starting sometime in 2021, I’m going to be writing a series of columns for Practical Electronics magazine in the UK teaching digital logic fundamentals to absolute beginners. (As an aside, I meant that title as a joke, but you wouldn’t believe how many emails I’ve subsequently received addressing me as “Klive” — maybe the joke’s on me LOL.)
I also noted that this is going to have a hands-on component with an accompanying circuit board. We’re going to commence by constructing some simple logic gates at the transistor level, then use primitive logic gates in 7400-series ICs to construct more sophisticated functions, starting with combinatorial and working our way up to a sequential edge-triggered D-type flip-flop, all implemented out of AND, NAND, OR, NOR, and NOT gates. From there we’re going to… but once again, I don’t want to give too much away.
The reason for my wafflings here is that I have a question. We’re going to group the gates together as they would be presented in the 7400-series devices; that is, groups of six inverters, four 2-input ANDs, and so forth (I’m not talking about the pin layout, just the gathering of the functions).
Each of these primitive gates will have two LEDs — one red, one green — associated with each of its outputs. Initially, we (yours truly and the publisher of PE, Matt Pulzer) were tempted to associate the green LED with 0V (ground, grass) and the red LED with 5V (danger, power). However, we’ve come to the conclusion that it will be better to associate the red LED with 0V (off, stop) and the green LED with 5V (on, go).
The image below shows a small prototype board containing a single hex inverter. We are using this as a test case to decide the best way to annotate the device names, signal names, pin numbers, and suchlike.

Observe the way in which the two LEDs associated with each gate output are centered on the signal line. This was the original layout Matt came up with. To be honest, I wouldn’t have thought anything more about this until he came up with an alternative possibility as shown below.

As you can see (especially if you click the image to make it larger), in this new version Matt has raised the LED pairs above their associated signal lines. Furthermore, he’s modified the signal lines so that they appear to form the shape of a positive-going pulse around the LEDs.
Matt’s idea is that when the output of the gate transitions (rises) from 0 (red) to 1 (green), the light will appear to move up. Similarly, when the output of the gate transitions (falls) from 1 (green) to 0 (red), the light will appear to fall down.
I must admit that I rather like this scheme. It’s a little different from anything I’ve seen before, and I think it adds a little je ne sais pas to the proceedings. But I suggested to Matt that it might be a good idea to post this blog to ask what your feelings were about all of this. As always, I (we) welcome your comments, questions, and suggestions.
Brilliant. Goes without saying that the offset arrangement is the way to go with the silkscreen providing the visual to assist the optical effect of the LEDs as well. And for the record, I’m also firmly in the RED=0, Green=1 camp. Too many years in the auto industry seeing stacklights on production machines I think.
The second option is better. You can add more space between 2 sets of LEDs for better identification if possible
Why two LEDs?
Surely that obscures the binary on/off nature of the signals?
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Why not do those boards to be used in breadboard ? It would be difficult or impossible to see the LEDs when upside down.
A perfect solution for people with perfect eyesight. Anyone sufficiently far away won’t notice the change in position. IMHO
I don’t mean to rain on your parade, but for those of us with some degree of color blindness, it would be better to just have one LED that is either on or off. Save the bi-or-tri-color scheme for things that have more than two states (it saves power too!)
Or you could have two LED’s in a straight line with 1 and 0 below in the silk screen. The LED’s could be Red or Green or white or blue or yellow depending on colour sensitivity of the eye ball visualising the board in use.
Just a thought what did the old DIL logic testers use as led indicators.
I like on/off. Even I can grasp that concept. Plus you save components, circuit complexity, and power. Now should it be green for “go” or “red” for “warning”?
Max I gave this some thought some time ago and thought of a graphic display which could be programmed with diagrammatic representations of any (small) logic gate or circuit, with switches,buttons or even touch icons for inputs and outputs shown as filled or empty circles. That way your one board could do just about anything from an inverter through bistables to counters and shift registers, etc. You could display Karnaugh maps and truth tables. Sure you could do all that on a PC but with a touch screen with colour it would be a great interactive learning tool. You could use it just to demonstrate what a gate / circuit did, or to quiz learners on their understanding (display a set of inputs and ask the learner to tap for a 1 or 0 output and then check their answer).
All I need is a raspberry PI with a touch screen and LOTS of time…..
Through-hole connectors are much sturdier than SMT connectors. 12 pins will have enough surface area so the SMT ones should be OK for a while … as long as you are careful. Besides, if you use TH headers, you have the option of mounting on either side of the board.
Don’t forget a pin 1 marker. Since it is for beginners, you may want to consider putting pin number silk on the corners of the connectors (1, 6, 7, 12 or 1, 2, 11, 12 depending on DIP convention or ribbon cable convention). The risk is visually cluttering up the board versus educating the reader.
The second LED placement is growing on me.
What you really need is a black LED for off. Of course, it will be hard to tell when it is on… 😉