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 known from company advertisements that Dialco's incandescent display product line was solidly established by the late 1960s. The examples shown here 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. In 1970 a Dialco 2 inch display could be purchased at a price of $4.00 per digit in 1000 unit quantities, a price point early LED manufacturers could not hope to touch. 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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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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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

Fiber optic 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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This compact single digit incandescent display, manufactured by MSC, is constructed out of fiber optic light pipes which have been cast in resin to form a seven-segment digit. The segments are lit by 3mm incandescent bulbs in an aluminum block, which are soldered to a small carrier PCB in the base of the device. This PCB is designed to be replaced as a single unit when a bulb fails, a pair of screws in the back of the display can be removed to gain access to the PCB module. MSC manufactured this model of display in both flying lead and edge connectors, the latter of which are designed to mate to a special keyed edge connector on a driver PCB. The modern builder who is considering replacing the bulbs with LEDs would likely have to replace the entire bulb carrier PCB in order to make the conversion.

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Although Soviet incandescent displays are not often seen in the West, they do exist, and and function much in the same way as their American-produced counterparts. Their construction style is completely different however, at least if the device shown here is any example. Each segment of this readout is a separate transparent block, molded into a complex shape to interlock with the other blocks in the display. The outside of each block has been painted with black paint, with the coating then stripped from the portion of the segment where light exits the front of the display. This black paint is the only thing which compartmentalizes the light produced by each digit; there are no plastic baffles like the ones used in many Western incandescent displays. The blocks are locked into a metal frame and held in place with individual metal machine screws. The display is very thin, much thinner than the incandescent displays produced by Alco and Dialco, and the sturdy machined aluminum face likely allows this display to take extreme punishment. A back-mounted PCB holds the 6.3 volt lamps and a standard pin header for connection. Not much is known about the maker or intended use of this display, other than that this example was salvaged from a piece of Soviet military equipment.

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An unusual item indeed, this Eaton 925H-C fiber optic display packs two digits into a complex and compact package. Intended for the cash-gilded military market, the 925H-C is a display built to almost comical excess. Each module contains a metal block which is fitted with 14 tiny straight-pin incandescent bulbs. The center contact of each bulb pierces a plastic bracket on the back of the device and acts as one of the display's pins; the entire device can be mated to a special socket for easy replacement. A rubber gasket is fitted around the base of each straight-pin bulb to hold it in place, and all contact surfaces have been gold plated. The numbers displayed by these modules are truly tiny, each digit is 7mm high and the entire module face is 9x16mm in size. Due to the 925H-C's small size, the home experimenter looking to replace the lamps in this device with LEDs would likely have to resort to using surface mount parts mounted to a custom PCB. Fortunately this display is held together with screws, and can be easily disassembled for modification.

Unlike some other incandescent displays, the 925H-C has individually replaceable bulbs. No soldering iron is required to replace a burned out lamp, though finding replacements for the unusual bulbs used in this device is likely to be challenging.

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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 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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Sometimes, you need a projection display that has all the right angles. The IEE Series 60, produced by Industrial Electronic Engineers, is an obscure numeric projection display designed for use in narrow wall-mounted enclosures. A projection display functions like a miniature slide projector, only the "slides" are numbers, each with its own separate lamp for electrical control. The Series 60 contains the same lens array, lamp matrix, and number slides as IEE's Series 10 displays, but incorporates a glass mirror that allows the projection system to be mounted perpendicular to the display face. Applying power to one of this display's twelve bulbs will cause a number to be projected onto the mirror and displayed on a frosted screen mounted on one side of the device. Projection displays are complicated and bulky devices, and the example shown here has had it's cover removed so the intricate internal construction can be seen.

Projection displays enjoyed a modest popularity in the pre-LED era of the 1960s, and were sometimes marketed as "In Line Readouts" or "One Plane Readouts", based on their lack of Nixie tube-style stacked digits. IEE projection displays were available with a number of different character sets, and the home experimenter can make their own custom slides using transparency film.

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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. When originally released in 1965, IEE claimed the 340 series was the smallest projection display in the world.

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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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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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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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At first glance, the Burroughs SD-11 may appear to be a standard projection display, but it is in fact an unusual device known as a sphericular display. 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 slides as in a standard projection display. The angle of the light striking the refractor controls which digit is displayed, through use of an ingenious pixel mask that blocks light to each pixel based on the positions of the bulbs in the back of the device. The pixel mask installed in this example can display the digits zero through nine with or without a decimal point. Eliminating the bulky lenses used in projection displays allows for the manufacture of a display that is a fraction of the weight 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. Unfortunately the SD-11 can not be easily modified to display different characters... the pixel mask is fused to the inside of the display with epoxy and is extremely difficult to remove.

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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.

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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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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