These Funky In-Plane Displays Are Worthy of an Explanation!

We're suckers for a good, display based project.

While many of our write-ups will focus on rich, colorful, pixel-packed IPS panels, or more recently, low-power electronic ink-based displays, there's no school quite like the old school when it comes to the characteristic look and feel of yesteryear!

Before we were blessed with manufacturing techniques that have enabled our myriad modern display technologies, designers had to dwell a bit harder on how to go about displaying information to users.

From Ferranti-Packard "flip-dot" arrays to silkscreened split-flap displays, older display tech at one point was more mechanical than electronic.

As beautiful as these electromechanical masterpieces are to look at — and even more lovely to listen to, there is one thing that they are potentially lacking — and that's mechanical reliability.

Don't get me wrong, they are designed as well as could be for the constraints of the time, but they are somewhat limited in their application due to two factors — size, and the moving parts that make up their construction.

These displays will do just fine when installed in a static application — those of us of a certain age will all have seen such displays at places like rail stations (though they are now commonly replaced with like-for-like LED versions).

They will fare less reliably when installed in say, an aircraft, with dynamic loading and mechanical forces being constantly applied. So the aviation industry turned to something new for their display needs at the time — something called an in-plane display.

For its time, the in-plane (aka one-plane) display was a marvel of miniaturization. Able to display fixed frames of content — be they string value, or numerical constants, fixed at the time of manufacture, they pulled some clever tricks to cram everything needed into the miniature display module form factor.

Looking a an exploded view below, we can pick out an array of incandescent lamps, along with lenses and matrix of multiple apertures — these define what data can be displayed on the screen.

The illustration below does a pretty good job of correlating the innards of the in-plane display module, shown above, to their functional descriptions.

As with nearly every bit of retro-tech that we find, there are people who have set about finding ways of integrating them into more modern projects — either for the nostalgia, or more often for the sheer novelty of things!

After a recent flurry of activity from Twitter users @brentvoltz and @robcruickshank shone light upon the novelty of these displays, Glen Akins simply had to get involved with the fun.

With two, single digits in hand Akins has whipped up a straightforward interface board that allows his host computer to display the values "00" to "99" in all their glorious, illuminated prestige.

The devil is in the details.

Although little more than a set of PIC-controlled shift registers, the devil is in the details with this board, owing to a clever choice of speciality shift register that allows Akins to keep the parts count low on this PCB.

While many a microcontroller, or nearly any selection of shift register could deftly drive an array of LED indicators — remember, these displays are deliciously retro-tech-tastic — and instead rely upon incandescent lamps, along with their higher current consumption, in order to illuminate the display films.

While we've seen some efforts over the years to retrofit some LED light sources in place of the higher current consumption incandescent lamps that are normally found in these display units — Akins is keeping it old school, choosing to instead use his displays as-supplied, and dealing with the higher current that these lamps require.

While you could bolt a Darlington array onto many a standard shift register, wouldn't it be nice if you didn't have to double up on chips to deal with the increased current?

A Texas Instruments TPIC6C595 shift register steps up to the plate here — and while that name seems somewhat familiar, owing to the '595 suffix, it's a new part for me, and one that looks to be decidedly useful for future projects.

Looking a lot like standard shift register, up to the fourth column of symbols in the above diagram, we can see that the TPIC6C595 adds an array of open-drain FET devices — one for each output on the shift register.

This integrated device allows the array to sink far more current than the logic itself would be able to, with this part quoting up to 100mA per channel — perfect for sinking the required current necessary in order to light the signal lamps found within the lamp array of the display.

Does it work?

Of course it does, and doesn't it look great while doing so?

We'd say that taking the time to work with the existing bulbs is a worthy decision — as the look of the display is maintained, keeping that warm, soft glow of the filament lamps.

Akins is known for breathing new life into the slightly older tech of days gone by and we'd recommend following his Twitter (@bikerglen) for quick write-ups on all sorts of slightly weird and wonderful bits of retro-tech!