# Seven Segment Displays

You probably haven’t thought much about them. However, they are everywhere! You might know seven segment displays from the digital watches that we all had in the 80s, making up each digit of the time, as well as from many other electronic clocks, thermometers, radios, CD players, VCRs/DVDs, calculators and even in lifts.

In fact, seven segment displays premiered in the 1968 film 2001: A Space Odyssey and were later to appear in the first digital watch – the Pulsar – which came out in 1972 and cost \$2,100. It’s ironic that by the time 2001 came, the digital watch had already gone out of fashion.

Anyway, in the early hours of the morning recently, I was staring at the clock and started wondering about the fact that although seven segments are used, only 10 different patterns are displayed. There are 128 (i.e. 2 to the power of 7) different patterns that can be displayed on a seven segment display, which is a few more than are really needed. In fact, you should only need four segments to display 10 patterns, so three of the segments are, in a sense, redundant.

I wrote a quick and dirty computer program to see which segments could be removed, or burn-out, and you would still be able to tell the difference between the different digits. It turns out that the bottom and bottom-right segments could be removed, and the standard patterns that make up the digits one through zero are still unique. This is shown in following the diagram:

However, we can do better. There are a number of variations on the patterns used for some of the digits. There are alternatives for one, six, seven, nine and zero, as shown here:

I remember my Dad’s precious digital calculator from the late 70s used the variant for zero shown here. Although, I admit I don’t recall having seen it anywhere since.

Using the software I developed, I looked for whether by using any of these alternative versions of the digital numbers, if there was a way of removing three segments from the display but still being able to tell the difference between the numbers. In fact, there was a way to remove another segment, if we use the variants for zero and six (and also nine, since we might as well have them match, although it’s not necessary). This special set of digits is shown here:

From this set of digits, we can remove both right segments and the middle segment, and the remaining patterns for the digits can still be distinguished. Although, to be honest, it is a bit strange.

It does show that we don’t need the full seven segments, and only need four. But, clearly, it’s not possible to go any fewer.

I can’t think of any applications for this four segment display, but it makes me happy to know that it’s possible to make one that is also compatible with existing patterns for the ten digits. And if the special set of digits is used, then up to three segments can fail on a display while it remains (somewhat) useful.

## 9 thoughts on “Seven Segment Displays”

1. You know, Deepti and I were just talking about the seven segment thing the other day…. hang on that’s got nothing to do with Harriet….

Lol. Just kidding.

2. I just needed to nerd-out for a bit. :) The next post will be parenting related, I promise.

3. Bob says:

I was born to be fussy, so why fight it? The problem is you’d need a special symbol to differentiate between 1 and _ (space). Otherwise you can’t tell between 12:00 and 2:00
You could use _ for space, of course, or you could use military notation (02:00).

4. I was going to say that you’ve solved your own problem here. However, there’s one more option I can see – use the alternative version of 1 and then space can still mean space in the 4-segment patterns.

5. And as a postscript to all this, I found a couple of patents that relate to a six segment display.

Application US 2007/0279320 has the top and bottom segments of a standard 7 segment display linked, so they always go on together. And the digit 7 is displayed in a sort of lower case.

Patent US 3,827,043 uses some funky curved segments to make it possible to realise ten digits with six segments. The digit 1 is not ideal though, with a bit of a kink in it.

6. Bob says:

Wow cool.

The alternative 1 is indeed the neatest solution.

I can’t believe it’s really that hard to reduce to a six segment display. Six segments is theoretically enough to get the whole alphabet in as well, with room to spare for some punctuation. So I guess asking for four is completely out of the question. But if I can get down to five I could patent it. (No one would ever use it, but I’d be able to say I have a patent.)

Which makes me ask… got any patents, Andrew? Strikes me as something Andrew-like.

7. There may be some writing systems that could do their numeric digits with less than six segments. Our “arabic” system is particularly amenable to this sort of thing, so finding a better one might be hard. Although, perhaps if we went back to Roman Numerals…

I don’t have any patents myself, but there are some Telstra ones where I’m listed as the inventor. They’re not very recent, as that sort of thing is less of a focus for Telstra these days.

8. Alex Valerio says:

hey guys…i have a problem….can you teech me hoow to use the seven segment with seven resistor……..i’l w8 for any responses…tnx a lot

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