A Daylight-Readable Bar Graph Display in the 70s Wasn’t Cheap

The driver board with display attached; the row of lamps is visible on the right hand side.

LEDs weren’t always an easy solution to displays and indicators. The fine folks at [Industrial Alchemy] shared pictures of a device that shows what kind of effort and cost went into making a high brightness bar graph display in the 70s, back when LEDs were both expensive and not particularly bright. There are no strange materials or methods involved in making the display daylight-readable, but it’s a peek at how solving problems we take for granted today sometimes took a lot of expense and effort.

The display is a row of 28 small incandescent bulbs, mounted in a PCB and housed in a machined aluminum frame. Holes through which to view the bulbs are on both the top and front of the metal housing, which allows the unit to be mounted in different orientations. It was made as a swappable module, its 56 machined gold pins mate to sockets on the driver board. The driver board itself consists of 14 LM119 dual comparators, each of which controls two bulbs on the display.

An example of a Wamco minitron bar graph display. Each window contains an incandescent filament. [Source: industrialalchemy.org]
[Industrial Alchemy] believes that the display unit itself may have been a bit of a hack in its own way. Based on the pin spacing and dimensions of the driver board, they feel that it was probably designed to host a row of modular units known as the Wamco minitron bar graph display. An example is pictured here; they resembled DIP chips and could be stacked side-by-side to make a display of any length. Each window contained an incandescent filament in a reflective well, and each light could be individually controlled.

These minitron bar graph units could only be viewed from the top, and were apparently high in cost and low in availability. Getting around these limitations may have been worth creating this compatible unit despite the work involved.

Display technology has taken many different turns over the years, and you can see examples of many of them in one place in the Circus Clock, which tells the time with a different technology for each digit: a nixie, a numitron, a 7-segment thyratron tube, a VFD, an LED dot display, and a rear projection display.

Soviet-Era 7-Segment Display, Built Like a Tank

In a way, all 7-segment displays are alike; at least from the outside looking in. On the inside it can be quite another story, and that’s certainly the case with the construction of this Soviet-era 7-segment numerical display. From the outside it may look a bit sturdier than usual, but it’s still instantly recognizable for what it is. On the inside is an unusual mixture of incandescent bulbs and plastic light guides.

The black-coated blocks of plastic on the left (shown from the rear) act as light guides. The holes are for nesting the incandescent bulbs. Note the puzzle-like arrangement of the uniquely shaped pieces.

The rear of the display is a PCB with a vaguely hexagonal pattern of low-voltage incandescent bulbs, and each bulb mates to one segment of the display. The display segments themselves are solid blocks of plastic, one for each bulb, and each a separate piece. These are painted black, with the only paint-free areas being a thin segment at the top for the display, and a hole in the back for the mating bulb.

The result is that each plastic piece acts as a light guide, ensuring that a lit bulb on the PCB results in one of the seven thin segments on the face being lit as well. An interesting thing is that the black paint is the only thing preventing unwanted light from showing out the front, or leaking from one segment to another; usually some kind of baffle is used for this purpose in displays from this era.

More curiously, each plastic segment is a unique shape apparently unrelated to its function. We think this was probably done to ensure foolproof assembly; it forms a puzzle that can only fit together one way. The result is a compact and remarkably sturdy unit that shows how older and rugged tech isn’t necessarily bulky. Another example of small display tech from the Soviet era is this tiny 7-segment display of a completely different manufacture, which was usually used with an integrated bubble lens to magnify the minuscule display.

Bask In The Warm Glow Of DIY Incandescent Bulbs

With most of the apparatus and instruments we now take for granted yet to be developed, the early pioneers of the Electric Age had to bring a lot to the lab besides electrical skills. Machining, chemistry, and metallurgy were all basic skills that the inventor either had to have or hire in. Most of these skills still have currency of course, but one that was once crucial – glassblowing – has sadly fallen into relative obscurity.

There are still practitioners of course, like [2SC1815] who is learning how to make homemade incandescent light bulbs. The Instructable is in both English and Japanese, and the process is explained in some detail. Basic supplies include soda-lime glass tubing and pre-coiled tungsten filaments. Support wires are made from Dumet, an alloy of iron, nickel, and cobalt with an oxidized copper cladding which forms a vacuum-tight seal with molten glass. The filament is crimped to the Dumet leads and pinched into a stem of glass tubing. A bulb is blown in another piece of tubing and the two are welded together, evacuated with a vacuum pump, and sealed. The bulbs are baked after sealing to drive off any remaining water vapor. The resulting bulbs have a cheery glow and a rustic look that we really like.

Of course, it’s not a huge leap from DIY light bulbs to making your own vacuum tubes. That’s how [Dalibor Farny] got started on his handmade Nixie business, after all.

Disco Bulb Keeps the Party Spinning

Even if you don’t like disco, you might like the slick moves that went into this project. [W&M] built a miniature motorized mirror ball inside of a standard incandescent light bulb, and the results are something to dance about.

Short of blowing a glass bulb, building a motor, and growing the wood, this is about as scratch-built as it gets. Much of the woodworking is done on a metal lathe, and this includes the base of the mirror ball itself. As with all good thing-in-a-bottle builds, the ball is too big to go in the bulb, so [W&M] quartered it, drilled a few holes, and ran a string through the pieces so they can be carefully glued and drawn back together into a sphere. He even cut up mirror tiles and painstakingly applied them with tweezers.

This disco bulb is meant to be hung from the ceiling and wired into mains like a regular mirror ball. [M&W] stuffed the guts from a small USB wall charger into the handmade beech base to provide clean power for both the geared motor that spins the ball and the tiny LED that illuminates it. Slip into your best leisure suit (or sweat suit, we won’t judge) and hustle past the break to watch the build video.

We don’t see a lot of disco balls around here, but we did see a disco icosahedron once.

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The Secret of Twinkling Christmas Lights

With the holidays over, many of us are braving the elements to take down all those holiday lights. LED lights have largely taken over the market, but in some places, you can still get classic incandescent bulbs. There are some effects that LEDs can’t quite mimic yet. One of those is the magic of “twinkling” light sets, which [Alec Watson] explains in a Technology Connections video. Everyone has seen bulbs that flash, and strings that dim. But the twinkle effect until recently has been hard to describe.

Typical flashing bulbs use a bimetallic strip. As the filament of the bulb heats up, the strip bends, opening the circuit. Then the strip cools and closes the circuit again. Twinkling lights do exactly the opposite. The bimetallic strip shorts the bulb out rather than open the circuit. Twinkling sets also use a lot of bimetallic strip bulbs – typically every fifth bulb has a strip. The result of the bulbs being shorted out is that all be the bulbs in set see a higher voltage. This makes the entire strip shimmer in time with the flashing. That’s where the twinkling magic comes from.

It occurs to us that the voltage on the strip would be a great source of random seeds. Sure, you’d have to replace bulbs now and again, but how many people can say they get their random numbers from a set of Christmas lights?

If you’re curious how incandescent Christmas lights can blow and not take out the whole strip, check out this article about anti-fuses.

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Tiny Light Bulb Flasher Vies for World’s Record

We’re going to go out on a limb here and declare this minuscule incandescent light flasher the smallest such circuit in the world. After all, when you need a microscope to see it work, you’ve probably succeeded in making the world’s smallest something.

Even if it’s not record breaking, [Ben Krasnow]’s diminutive entry in the 2017 Flashing Light Contest, which we recently covered, is still pretty keen. For those not familiar with the contest, it’s an informal challenge to build something that electrically switches an incandescent light on and off in the most interesting way possible for the chance to win £200. [Ben] says he’ll donate the prize money to a STEM charity if he wins, and we’d say he has a good chance with this flea-sized entry.

The incandescent lamp he chose is a specialty item for model makers and scale railroad enthusiasts; we’d heard of “grain of wheat” bulbs before, but this thing is ridiculous. The bulb makes the 4.6 mm diameter SR416 hearing aid battery that powers the flasher look enormous. The driver is a clever Schmitt trigger inverter with a tiny RC network to flash the bulb at about 1 Hz. The video below shows the flasher working and details the development and the build, which featured spot welding to the battery. [Ben] has even spec’d precisely how many Joules of energy will rupture the thing steel cases on these cells — we suspect involuntarily through trial and error.

[Ben]’s entry in the contest is now our favorite, and not just because he’s been a great friend to Hackaday with such classic hacks as watching a phonograph needle with an electron microscope and a homebrew CT scanner. This circuit is genuinely fascinating, and we hope it inspires you to try to top it. There’s a little less than a month left in the contest, so get to it.

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Flash a Light Bulb, Win a Prize

How many geeks does it take to flash a lightbulb? Judging from the list of entries in the 2017 Flashing Light Prize, so far only seven. But we suspect Hackaday readers can add to that total.

The goal is almost as simple as possible: build something that can flash an incandescent light bulb for at least five minutes. The system actually has to power the bulb’s filament, so no mechanical shutters are allowed. Other than that, the sky is the limit — any voltage, any wattage, any frequency and duty cycle, and any circuit. Some of the obvious circuits, like an RC network on a relay, have been tried. But we assume there will be points for style, in which case this sculptural cascading relay flasher might have a chance. Rube Goldberg mechanical approaches are encouraged, as in this motor, thread, stick and switch contraption. But our fave thus far is the 1000-watt bulb with solar cell feedback by Hackaday regular [mikeselectricstuff].

Get your entry in before August 1st and you’ll be on your way to glory and riches — if your definition of rich is the £200 prize. What the heck, your chances are great right now, and it’s enough for a few pints with your mates. Just don’t let it distract you from working on your 2017 Hackaday Prize entry — we’re currently in the “Wheels, Wings, and Walkers” phase, so maybe there’ll be a little crossover that you can leverage for your flasher.

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