A Brief History of the Crazy Old 7-Segment Display

How old is the seven-segment display? Surely it is a product of the 1970s. After all, calculators started showing up, and the height of junior high humor was plugging 7734 into your calculator and showing it to someone upside down. Of course, for it to go mainstream, maybe they really originated in the 1960s, but no earlier than that, right? Actually, no. Sure, the LED seven-segment display had to wait for LEDs. But the actual idea is much older than that.

The concept of building numbers from a small set of reusable segments predates LED displays by decades. In fact, the basic idea appears in patents from the early 1900s and may have roots in even older mechanical signs and printing techniques.

The history isn’t entirely straightforward. Unlike vacuum tubes or transistors, segmented displays evolved gradually through a series of practical ideas rather than one defining invention.

Blacking out the Eight

While looking into the history of segmented displays, I was reminded of something I’d seen years ago in retail stores: reusable price tags printed with rows of eights.

Rather than printing every possible price, the clerk simply used a marker to black out portions of each figure, transforming an 8 into whatever digit was needed. Cover a few strokes, and the eight becomes a three. Remove a different set, and it becomes a zero or a five. It was, in essence, a manual segmented display.

Finding the exact origin of these price tags is akin to finding out where Romans bought sponges. They were inexpensive commercial supplies, not the sort of products that historians carefully documented. My recollection is from the middle of the twentieth century, but the underlying concept is almost certainly older.

Everything New is Old Again

George Mason’s 1898 21-segment display used 21 lamps and a complicated switch to display any digit or letter in a very stylish font. You can see a modern recreation of these ancient displays in the video below. While this is the basic idea, certainly, it is more ambitious than a simple 7-segment display.

I couldn’t determine that Mason’s displays were ever used for anything.

You could argue that an early 1903 invention by Carl Kinsley to draw characters telegraphically using six pens was an even better precursor, but using magnets to draw with pens on tapes hardly seems to qualify as a display, although figure 12 in the patent clearly shows the formation of numbers and even letters with this arrangement.

Digits of Patent

The direct parent of modern segmented digit displays appeared in the early 1900s (filed 1908; granted in 1910). Technically, this was an 8-segment display because it had a bar dedicated to forming a proper four, with the top-left part slanted. But removing that one segment is just an optimization. It may or may not have been the first, but by 1910, seven-segment displays were in use and not just curiosities on a workbench or dreams in a patent application.

Even for lit-up displays, the first implementations weren’t LEDs. Early displays used incandescent lamps or neon-filled tubes. By the 1930s and 1940s, segmented neon or incandescent indicators were appearing in industrial equipment and counters, instead of the common columns of ten neon bulbs, pointers, or rotating wheels.

Then, too, there were different approaches. Nixie tubes used individual character forms that lit up. Decatrons could count with ten different glowing points, each representing a digit.

Enter the LED

The real explosion came in the late 1960s when practical LED displays arrived. Suddenly, segmented displays could be compact, rugged, inexpensive, and operate at low voltages. Calculators, clocks, frequency counters, digital multimeters, and every imaginable piece of consumer electronics adopted them almost overnight.

Of course, it wasn’t just LEDs. Numitrons used seven tiny incandescent filaments. Vacuum fluorescent displays used segments with phosphor that glowed when excited. LCDs adopted the same pattern, blocking or passing light to produce the segments. But the key idea was something that lights up, arranged in seven segments.

Radio Shack’s 1976 catalog featured magnified LED displays.

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Early LED displays often had all the diodes on a single die to reduce cost. That made them very tiny. It wasn’t unusual to see displays with magnifying bubbles to make it easier to read.

The important point isn’t whether they used exactly seven segments. Many didn’t. Some divided the numerals differently or used additional pieces to produce more attractive typography. The insight was the same: begin with the most complex digit and selectively remove strokes to create all the others.

Why Seven?

One of the reasons seven segments became the standard is that they strike a perfect balance (although not everyone agrees, as you can see in the video below). With fewer than seven elements, several decimal digits become difficult to distinguish. Adding more segments certainly improves appearance, but every additional segment increases wiring, decoding logic, manufacturing cost, and the number of possible failure points. Seven is the right number for, um, numbers. But what about letters?

You can make some compromises to show some letters. For example, old computers would display hex digits using seven segments, but the A would be uppercase and the B would be lowercase. You also had to light the top segment for the 6 to make it look different from a B. But you only need A-F for hex. If you need, say, the letter S, there’s no real way to make it not look like a 5 with 7 segments. But sometimes, the letters you can make are good enough.

There were also 8- and 9-segment displays that could do better with letters and special characters. You can increase the segments further to get more glyphs.

The Union Jack

Different segment counts for displays (public domain by [errorage])

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The solution for full letter displays was to add more segments. Despite Mason, you didn’t really need 21. By the 1970s, fourteen-segment and sixteen-segment displays had become common in instruments, telephones, calculators, and video cassette recorders. Their pattern of diagonals and crossbars earned them the nickname “Union Jack” display because the arrangement resembles the British flag. These are also sometimes called starburst displays. You occasionally see 16-segment displays, too.

Even Today

Today, you have a plethora of options for adding alphanumeric screens to just about anything. Yet, you still see 7-segment displays hanging around.

The seven-segment display isn’t successful because it’s beautiful. It isn’t even especially flexible. It’s successful because it’s close to the minimum solution that works. It delivers readable numbers with very little hardware, whether the technology behind it is incandescent bulbs, neon, vacuum fluorescence, LEDs, LCDs, or even ink on a printed price tag.

Perhaps that’s why it has survived every technological transition for more than a hundred years. Good engineering ideas often outlive the technologies that first bring them to life.

And once you start looking for segmented displays, you’ll notice they’ve been hiding in plain sight for much longer than the digital age.

Featured image: [Arduino Enigma]’s marvelous Seven Segment Art Installation