by Rick Furr
One of the more
interesting things about old calculators is how they
displayed their numbers. As easy as it seems today, in
the late 60s and early 70s it was quite hard to devise a
display system for a calculator, especially a portable
one. This article will describe the construction and
operation of the major types of electronic displays both
past and present. CATHODE RAY TUBE The Cathode Ray Tube (CRT) was developed for television in the 40s. The CRT shoots a focused electron beam from the back of the tube to the front of the tube. The front of the tube is coated with phosphors that glow when they are struck by the electron beam. An image is created by moving the electron beam back and forth across the back of the screen. The beam moves in a pattern from left to right, top to bottom and then it repeats. Each time the beam makes a pass across the screen, it lights up phosphor dots on the inside of the glass tube, thereby illuminating the active portions of the screen. The intensity of the beam is modulated thus causing the screen phosphors to glow with different intensities or to even not glow at all. The desired images to be displayed are actually retraced between 30 to 70 times each second. This keeps the images continually refreshed in the glowing screen phosphors without a flicker being perceivable to the eye. The electron beam is
generated from a filament and electrically charged
cathode in the back neck of the CRT. The electron beam is
first passed through a control grid. The control grid
modulates the intensity of the electron beam. The higher
the intensity the brighter the phosphor dot it strikes
will glow. Next the beam passes through an accelerating
electrode, this will speed up the electron beam. Then the
beam passes through a focusing anode. This will focus or
tighten the stream of electrons. All of these elements
comprise the electron gun structure housed in the neck of
the CRT. NIXIE TUBE DISPLAY In a Nixie Tube display each numeral is a
complete, lighted cathode in the shape of the numeral.
The cathodes are stacked so that different numerals
appear at different depths, unlike a planar display in
which all numerals are on the same plane relative to the
viewer. The anode is a transparent metal mesh wrapped
around the front of the display. The tube is filled with
the inert gas neon with a small amount of mercury. When
an electric potential of 180 to 200 volts DC is applied
between the anode and any cathode, the gas near the
cathode breaks down and glows. The digits glow with a
orange-red color. INCANDESCENT FILAMENT DISPLAY An Incandescent Filament display is usually housed in a vacuum tube like the either the Nixie tube or the early Vacuum Fluorescent tubes. This display is typically a seven segment style of display where each display segment is formed with a conductive anode tungsten filament. A small voltage placed across a filament will cause it to heat to incandescence. They emit a yellowish-white light that can be filtered to any desired color. The filament voltage (3-5vdc) can also be varied to change the brightness level of the display. The biggest problem with Incandescent displays is they have a slow response time and they consume a large amount of current. A popular version of this type of display was the RCA Numitron. Some early electronic kits used the Incandescent Filament display. GAS DISCHARGE or PLASMA DISPLAY A Planar Gas Discharge or Plasma
Display Panels
(PDP) display utilizes the same principle the Nixie tube
does. It's construction consists of sandwiching a hollow
center layer filled with neon and a small amount of
mercury between a glass front and a ceramic back. A thick
conductive paint forms the Cathodes on the inside of the
ceramic back. The Cathodes form the segments of each
digit. Each digit is covered by a separate Anode that is
deposited on the inside of the glass front. The Anodes
are formed from a thin transparent layer of tin oxide.
When a sufficient voltage is applied between a cathode
segment and it's anode, the gas around the cathode
segment breaks down and begins to glow. Like the Nixie
tube, the digits glow with a orange-red color. Voltage
requirements for these displays are typically 180-200
volts DC). VACUUM FLUORESCENT DISPLAY MODULE The Vacuum Fluorescent display (VFD) consists of a vacuum tube in which there are three basic types of electrodes, the filament (cathode), the anode (segment), and the grid. The VFD is essentially a small Cathode Ray Tube. The filament (or filaments) is a very fine wire that is heated to a temperature just below incandescence. At that temperature it remains virtually invisible but it emits electrons. A transparent metal mesh grid covers each digit and controls the electrons emitted from the filament toward that digit. Seven phosphor coated anodes, arranged in the seven-segment configuration (that form a square eight), glow when struck by the electrons. When a positive voltage of 12 to 25 volts is applied to the grid and the anodes, the electrons emitted by the cathode filament are accelerated and attracted to the positive anode segments which in-turn glow. If the grid has a negative potential then it will block the electrons from passing regardless of the potential of the anodes under the grid. VACUUM FLUORESCENT DISPLAY TUBE VFDs were developed in Japan in 1967. Early versions of VFDs were individual digits housed in vacuum tubes like the Nixie tube and Incandescent Filament displays. VFD Phosphors can be formulated to emit red, yellow, and green as well as the more common blue-green color. Later versions would house all of the digits (and other graphics and indicators) in one large glass assembly. Currently VCRs account for 30% of the VFD market and Audio/Video products account for another 30%. Many early series of calculators like the Commodore 412F, Brother 310, and the MITS 816 used the individual digit VFD tubes. Later manufacturers such as TI and Rockwell used the integrated multidigit VFDs in both handhelds and desktops. ELECTROLUMINESCENT DISPLAY Thin-film Electroluminescent
Displays (ELDs)
use a thin film of phosphor (zinc sulfide (ZnS); ZnSe;
ZnSMn or other fluorescent materials) sandwiched between
a dielectric layer that is sandwiched between two glass
plates. Transparent electrodes (tin-oxide) are deposited
on the insides of the glass plates. When a sufficient AC
voltage (>100 volts) is applied to any of these
electrodes the phosphors will be excited and will emit
light. ELD phosphors can be mixed with pigments to emit
many colors of light including green, blue-green,
lemon-yellow, orange, red as well as white light. LED DISPLAYS A Light Emitting Diode (LED) is an special type of
diode that emits light when electricity applied to it's
anode and cathode. A typical LED requires about 3 volts
DC at 10 milliamps to begin emitting light. LEDs usually
produce red light but yellow, green and blue versions are
also now available. The LED was first marketed by Texas
Instruments around 1962. LED displays (7 or more
individual LEDs) were introduced around 1967 but were
very expensive. Calculators used LEDs that were arranged
to form either a seven-segment display or a dot-matrix
display. LIQUID CRYSTAL DISPLAY The Liquid Crystal Display (LCD) was first developed at RCA
around 1971. LCDs are optically passive displays (they do
not produce light). As a result, LCDs require all most no
power to operate. Many LCD calculators can operate from
the power of a solar cell, others can operate for years
from small button cell batteries. LCDs work from the
ability of liquid crystals (LC) to rotate polarized light
relative to a pair of crossed polarizers laminated to the
outside of the display. There are two main types of LCD
displays used for calculators today: Twisted nematic (TN)
and supertwisted nematic (STN). TN displays twist
polarized light to 90 degrees and have a limited viewing
angle. STN displays were developed to twist polarized
light between 180 to 260 degrees resulting in better
contrast and a wider viewing angle. |
References:
Alan Sobel, "Electronic Numbers", Scientific American, pp. 64-73, June 1973. "Note on the Liquid Crystal Display Industry, http://www.duc.auburn.edu/~boultwr /lcd/lcdpg1.html. "Display Technologies in Japan", http://www.itri.loyola.edu/dsply_jp/toc.htm "Sharp -- World of LCDs", http://www.sharp.co.jp/sc/library/lcd_e/indexe.htm
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