Incandescent Displays

Prior to the semiconductor explosion, much ingenuity was devoted to the task of displaying numbers electronically. This section is a catch-all for the many obscure numeric readouts that have been designed around the humble light bulb, usually either by bending light through light pipes and masks, or shaping it with slides and lenses. It is not known for certain when the 7 segment display layout first appeared in human culture, but it is probable that it predates the release of the first LED displays, which makes segmented number displays lit by incandescent lamps a forgone conclusion.

Though not as well known as Nixie tubes, vintage incandescent displays have enjoyed a renaissance amongst electronics enthusiasts in recent years, as their optical nature typically allows them to be converted to multi-color LED displays with little difficulty.

 


Dialco Incandescent Display
 

Devices included in this entry:

Dialco 7 segment with decimal point display, unknown P/N (PCB back with driver socket; pictured in thumbnail)
Dialco 710-0300-009 7 segment display (plastic back with PCB mount lugs)


This could be well considered an anachronistic device; a 7 segment display lit by incandescent lamps, one bulb per segment. Dialco made a myriad different variants of this basic design, some with spade or PCB mount lugs, some with attached drivers, all with an assortment of different bulb voltages. Incandescent displays can be difficult to date, but it is likely that these devices were produced well after the invention of the LED, and only exist because early LEDs were very dim, and could not have economically illuminated a digit of this size. The standard diameter midget flange lamps installed in these devices are held in with screws and spring terminals, and can be easily replaced.

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Dialco Incandescent Display
Alco MSM-5A
 

Alco manufactured a vast line of "mosaic indicator" incandescent segmented displays. The MSM-5A is a fairly typical example, a functional duplication of a modern LED seven segment display, done with lamps instead of LEDs. The bulbs in the MSM-5A can not be replaced - the entire display is a plug-in unit, designed to be discarded when one of its bulbs burns out. The display is an exercise in Cold War-era manufacturing excess; machine screws, aircraft grade aluminum, and gold traces are all present and accounted for. Of particular note to the home experimenter is that the lamps can be non-destructively replaced with 3mm LEDs, allowing the full color range of modern LED technology to be used in the display.

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Alco MSM-5A Incandescent Display
MSC 351 Series
 

Devices included in this entry:

MSC 351-25977-009 light pipe module (5 digit numeric display, shown in thumbnail)
MSC 351-25977-010 light pipe module (8 digit numeric display)
MSC 351-35577-007 lamp and decoder module


Light pipe displays like this one are a close relative of the Dialco display pictured above, in that they use incandescent bulbs to light a seven segment pattern on a single plane surface. Unlike the comparatively economical Dialco display however, a light pipe display uses bundles of fiber optic cables to present a fixed bitmap image of a number. The display seen here is actually equipped with sixteen segment light pipe arrays, but the unused fiber optic cables have been covered with epoxy. Each plastic-encased driver module contains an Intersil 5600 read only ROM chip which functions as a character decoder for the individual lamps. Unlike the Dialco display above, the lamps in this device are non-replaceable.

The lamps in this display can be non-destructively replaced with 3mm LEDs by the modern electronics hobbyist, but the combination of non modular digit construction and awkward digit spacing make this device a poor choice for a project.

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MSC 351 Series Fiber Optic Incandescent Display
IEE 10000-1819-B Projection Display
 

Before the arrival of reliable, easy-to-use LED displays like the Monsanto MAN-1, companies came up with a vast array of impractical methods to display numbers and characters. Projection displays, such as the one shown here, are one of these fringe display types, and were designed to compete directly with the neon filled Nixie tubes that were the leading display of the time. Also known as a "One Plane Readout" or "In Line Display", these devices were invented by IEE in 1956 for use in their proprietary industrial control systems; the devices proved so popular with customers that IEE began producing and selling the displays as a stand-alone product.

A projection display functions like a miniature slide projector, only the "slides" are numbers, each with its own separate lamp for electrical control. In this IEE display, light from one of 12 different lamps is projected through one of 12 focusing lenses, which directs the light beam onto the appropriate number mask. Light exiting the digit mask passes through a second set of lenses, which bends the light to project the image of the digit mask onto the center of a fogged plastic screen at the front of the display.

Projection displays are complex devices with numerous parts, and were expensive to manufacture compared to a Nixie tube. For the modern hobbyist though, projection displays have several significant advantages over shaped cathode displays like Nixie tubes. Projection displays have replaceable bulbs, allowing the color of the displayed number to be changed simply by replacing the bulb with an appropriate LED light source. The digit masks in most projection displays are also replaceable, which allows for many custom display types to be quickly and easily produced. During the Cold War projection displays were quite popular for industrial control systems, since custom symbols and warning messages could be easily added to the digit mask.

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IEE 10000-1819-B Projection Display
IEE 220H-6409-B-328
 

IEE was almost surely responding to the hilarious bulk of their earlier projection displays when they designed this more advanced model. Taking up less than half the volume of larger displays like the 10000 series unit shown above, this model also has an easy access clamshell case, which significantly reduces the difficulty of changing the digit mask.

This model provides a perfect example of how projection displays were tailored to industrial control applications: the standard numeric mask has been replaced with a more custom unit that displays entire preset phrases.

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IEE 220H-6409-B-328 Projection Display
IEE 340 Series Projection Display
 

Devices included in this entry:

IEE 340-0000-B-A11 numeric display (numeric characters 0 through 9)
IEE 340-06-A729-1 symbol display (plus and minus symbols)


Not all projection displays are bulky; the 340 series display, made by IEE, is so small that it could be pocketed and lost amongst loose change. These devices are truly minuscule; the digit height is only 13mm and the projection face is 12mm wide and 20mm tall... not much larger than a standard plastic DIP-packaged LED display. The 340 is much deeper than a LED display however, the complex projection lens system of IEE's larger displays has been replicated within in miniature. IEE made several different variants of the 340 series displays with different depths, the 04-A985-1 shown here is one of the shallowest, making it likely the smallest projection display commercially released. This example displays some positively useless characters; Miles, FM, DEG, F/S, F/S/F and two underscore characters are among the choices available for the prospective builder. Fortunately the display's clamshell case is only secured by two threaded screws, which can be removed for easy replacement of the slide containing the projected characters. The 340-0000-B-A11 is a more typical numeric version of this display, with a slide containing the numbers 0 through 9.

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IEE 340 Series Projection Display
NLS Edge-Lit Display
 

Edge lit displays represent another class of electronic numeral display that came into wide use during the narrow time in human history when electronics had advanced enough to cause electrical number displays to be desperately needed despite the fact that solid state LEDs had not yet been invented. Edge lit displays consist of a stack of glass plates with numbers etched or dimpled into their faces. These plates are enclosed in a frame and lit from above or below by an array of incandescent lamps. Edge lit display are bright and easily visible and have a rather distinctive appearance, but as with other stacked-digit displays, the numbers located in the rear of the digit stack are obscured by other digit outlines and can be rather hard to read.

The edge lit display example shown here, manufactured by NLS, is a fairly typical example of edge lit display appearance. NLS popularized the use of edge lit number displays in it's line of first generation digital voltmeters, which used electromagnetic uniselectors to measure and display a voltage measurement on a rack of edge lit displays. In modern times, edge lit display have become popular for their use in vintage clock and electronics projects where incandescent bulbs can be easily replaced with LEDs

Special thanks to John Taylor, of Taylor Electronics Services, for donating this unusual part.

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NLS Edge-Lit Display
KGM Electronics IND-1803 Numerik Indicator
 

Not all edge lit number displays use flat plates for their display faces. This unusual device, a IND-1803 'Numerik Indicator' made by KGM, uses curved plastic plates to bend incoming light around a 90 degree turn before reaching the digit display area. This allows the display's incandescent lamps to be conveniently mounted on the rear of the device instead of the top and bottom faces as in many other edge lit displays. In the photo the display's cover has been removed and colored LED's have been used to show the light-bending plates in action. Instead of decimal points, this display makes use of a pair of left and right-centered dots which can act as either a decimal point or a colon separator depending on need. This display is designed to be mated to a special matching socket; the spring loaded clips mounted on the top and bottom of the display hold it in place when it is inserted into it's socket.

One of the most famous uses of this model of display was as the 3 digit bomb countdown timer shown in the James Bond movie Goldfinger. Even Bond villains lose patience trying to find enough of these to make a 6 digit clock.

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KGM Electronics IND-1803 Numerik Indicator
Stanley Readout Edge Lit Display
 

Not all edge lit displays are designed to display numbers. This Stanley Readout display contains five dimpled plates that can be combined to generate a number of different 'waveform' type patterns. Five small lamps soldered to a PCB at one end of the display provide the light needed to illuminate the plates. This PCB is attached to a terminal strip along the bottom of the display with wires; the example shown here has no solder on its terminal strip, suggesting this display was never installed in a piece of equipment. Strangely, no provision is provided to replace the lamps, leading us to believe this display was intended for a piece of consumer equipment that would have been disposed of instead of repaired.

Note that in the photo, two of the five plates are illuminated to make the displayed waveform.

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Stanley Readout Edge Lit Display
Signaltron ST-15 Split Flap Display
 

Another fringe display technology, split flap displays make projection displays seem almost practical. Split flap displays use a synchronous motor and gearbox to rotate a spindle of spring loaded flaps to display numbers. The numeral flaps are split down the middle and held open by small metal fingers, hence the name "split flap display". A rotary switch attached to the spindle of the motor controls power. When power is applied to the pin for the desired number, the motor will rotate the spindle until the desired number is displayed, at which point the rotary switch will open and shut down the motor. The result is a slow, noisy display that fills a room with the sound of flaps slapping together every time it changes, and has the added feature of needing to be oiled regularly.

Although this is obviously not an incandescent display, this is the best location for such a device until we get around to adding a 'Clacking Split Flap Display' section.

Signaltron ST-15 Split Flap Display
Staver 402L Vane Display
 

The 402L, made by Staver, is a large electromechanical vane display designed for use in scoreboards and other large format applicatons. The seven mechanical vanes of the display are controlled by electromagnets, each electromagnet contains a U-shaped core which wraps around the shafts of each vane. Reversing the magnetic field rotates the shaft of the vane, causing it to slide into the body of the display. The magnetic field is retained in the core even when power is removed, allowing a number to be displayed continuously even with no power applied. An attached PCB provides a diode tree which converts the control voltages for the 7 bidirectional coils into 14 'on-off' signals, 7 signals to set the vanes to a specific number, and an additional 7 signals to clear the vanes.

Special thanks to Brian Stuckey, of TubeclockDB, for donating this unusual part.

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Staver 402L Vane Display
Ferranti-Packard Flip Dot Display
 

Flip dot displays are another 'flap' based electromechanical display in the same family as the vane and flap displays shown above. A flip dot display uses an array of magnetic disks suspended above a matrix of coils to display alphanumeric data. Each coil has a U-shaped bar passing through it, which stores the magnetic state imparted when an electric current passes through the coil. The U-bars are connected to the pivot points of each magnetic disk, causing the disks to flip to match the magnetic state of the bars. Much like core memory, the U-bars store a magnetic state even when power is removed, allowing images to be loaded and stored in the display without any external latches or secondary memory hardware.

This Ferranti-Packard model is made up of an array of five separate flip dot 'bars' each containing seven dots. The coils operate at 5V, and are attached to a diode matrix that prevents a pulse on a given row from inadvertently flipping adjacent dots.

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Ferranti-Packard Flip Dot Display
Burroughs SD-11 Sphericular Display
 

At first glance, the Burroughs SD-11 may appear to be a standard projection display, but it is in fact a sphericular display. The SD-11 operates in a similar manner to a lenticular device, but instead of being composed of bar-shaped elements, it is composed of a grid of circular elements. The front panel of the SD-11 is a complex two-dimensional array of faceted refractors which generate the displayed digit; there are no lenses or digit masks as in a standard projection display. The angle of the light striking the refractor controls which digit is displayed, by applying power to one of eleven bulbs inside the device. The digits zero through nine can be displayed with or without decimal point. Eliminating the bulky lenses used in projection displays allows for the manufacture of a display that is a fraction of the size of competing devices. The displayed characters on the SD-11 are not as clear as those displayed by most projection displays, but the visual appearance of the sphericular characters is quite distinctive.

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Burroughs SD-11 Sphericular Display

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