LED Displays - Electronic Calculators and Clocks

Texas Instruments did not only invent the Integrated Circuit (IC) but demonstrated in 1967 with the Cal-Tech project that a portable electronic calculator can be designed using only 3 LSI circuits (Large Scale Integration) with a complexity of about 1,000 transistors, each and some additional shift registers for data storage. While rather large high-voltage Nixie tubes worked well with electronic desktop calculators, decided Texas Instruments to develop a compact, solid state thermal printer for the Cal-Tech feasibility study and when Canon introduced in April 1970 with the Pocketronic the first portable calculator based on Texas Instruments' LSI chips, it made consequently use of a serial thermal printer. Sanyo followed soon with the slightly larger ICC-82D using Nixie tubes while Sharp introduced in November 1970 the EL-8, an even smaller portable calculator using a VFD (Vacuum-Fluorescent-Display). Meanwhile Monsanto had developed with the MAN1 the first commercial available Seven-Segment LED Display with 0.27" (7 mm) character height using 14 individual GaAsP (Gallium Arsenide Phosphide) chips for the segments and 1 LED chip for the decimal point. Monsanto followed soon with the monolithic, miniaturized MAN3 display with 0.115" (3 mm) character height that could be operated at currents as low as 1 mA per segment and was hence a perfect choice for electronic pocket calculators like the Bowmar 901B. When Texas Instruments announced on September 17, 1971 with the TMS1802NC the first available standard calculator building block on a chip, it was supported by two pre-configured Multi-Digit Calculator Numeric Seven-Segment LED Displays (DIS40, DIS95) based on the TIL360 arrays, the corresponding segment drivers (SN75491) and digit drivers (SN75492) and even the 1KS/6KS Klixon™ keyboard.

When Texas Instruments announced in April 1972 the TI-2500 Datamath portable calculator and the desktop models TI-3000 and TI-3500, the calculators used different display technologies. While the TI-2500 was using the red DIS40/DIS95 LED Displays, relied the two desktop models on orange gas-discharge displays with either 8 or 10-digits marketed by Burroughs as Panaplex™ display. Main disadvantage of these technologies was their high power consumption, later calculators switched VFD technology before finally replaced by LCDs (Liquid-Crystal-Displays).

With LED Displays being one of the main cost drivers in early portable electronic calculators, their manufacturing technology was changed multiple times over a rather short time span between 1971 and 1975 before more or less extinguished before the end of the Seventies.

• Single-Digit Module LED Displays

Monsanto pioneered Seven-Segment LED Display based on individual GaAsP (Gallium Arsenide Phosphide) chips and introduced in 1971 with the MAN3 (clear epoxy, flat package) a very compact monolithic design with a housing of just 0.185" (4.7 mm) width, allowing very compact Multi Digit Display Modules having a character spacing of about 0.2" (5.1 mm). The more common MAN3A switched back to a hybrid design with individual chips for the segments and decimal point, improved the contrast of the LEDs with changing the color of the housing to a dark red epoxy, and introduced an inline package for through-hole mounting in addition to the flat package suitable for surface mounting. Both the dimensions and pin-out of the MAN3A evolved into a de facto standard and TI's DIS279 kept its dimensions while adding a small magnifying lens that the later DISXXX dropped.

Technology

Monolithic Construction: One GaAsP (Gallium Arsenide Phosphide) chip for both segments and decimal point bonded on a 10-pin lead frame and hermetically encapsulated in red epoxy without (MAN3) or with integrated magnifying lens (DIS279).

Full-Hybrid Construction: Seven individual GaAsP (Gallium Arsenide Phosphide) chips for the segments and one LED chip for the decimal point bonded on a 10-pin lead frame and hermetically encapsulated in red epoxy with a flat surface (MAN3A, DISXXX).

• Multi-Digit Module LED Displays

First Multi-Digit Calculator Numeric Seven-Segment LED Displays with Right-Hand Decimals introduced together with the TMS1802NC single-chip calculator circuit.

Technology

Semi-Hybrid Construction: The Digit and Decimal GaAsP (Gallium Arsenide Phosphide) LED chips are mounted on a ceramic header which is then hermetically sealed with a glass cover. Character-to-character spacing is maintained when multiple displays are mounted end-to-end.

• Calculator Displays using Multi-Digit LED Modules

Work-in-Progress. 

• Calculator Displays using Single-Digit LED Modules

Work-in-Progress. 

Technology

The individual Single-Digit LED Modules with one character, each are soldered on a double-sided printed circuit board (PCB) and magnified by a clear acryl lens with a cylindrical shaped surface (DIS115, DIS134).

• Calculator Displays using LED Chips and Protection Lens

Bowmar pioneered the development of calculator displays with individual GaAsP (Gallium Arsenide Phosphide) LED chips bonded directly on a printed circuit board (PCB) instead of a lead frame and consequently dropping manufacturing costs of a complete display module dramatically. Most Bowmar Optostic displays were available as standard products but Bowmar some TI specific part numbers with the leading DIS designation, too. 

Technology

Full-Hybrid Chip-on-Board (COB) Construction: Seven individual GaAsP (Gallium Arsenide Phosphide) LED chips for the segments and one LED chip for the decimal point per character bonded on a double-sided FR4 printed circuit board (PCB) and protected by a red acryl lens with a flat surface. Some designs are only partly populated (R7H-122-9).

Full-Hybrid Chip-on-Board (COB) Construction: Seven individual GaAsP (Gallium Arsenide Phosphide) LED chips for the segments and one LED chip for the decimal point per character bonded on a double-sided FR4 printed circuit board (PCB) and protected by a red acryl lens with a cylindrical shaped surface (R7H-172-9).

• Calculator Displays using LED Chips and Magnifying Lens

While Bowmar's approach of bonding individual GaAsP (Gallium Arsenide Phosphide) LED chips for the segments and decimal points directly on a printed circuit board (PCB) instead of a lead frame reduced the costs of a complete calculator display module significantly, resulted the manufacturing method with about 50 to 100 individual dies per display in a rather low yield and time consuming process. The advantage of a very good readability of the large characters was bought with a price, the surface area of the GaAsP LED chips added up quickly.

With the Calculator War starting in 1975, manufacturing costs of basic 4-function electronic calculators were under extreme pressure and the LED display was one of the main cost drivers. The manufacturing costs of a Display Module are calculated with:

With the die cost roughly proportional to the die area, testing and packaging costs roughly proportional to the number of dies, and the final test yield mostly inverse proportional to the die area, goals are well defined: Keep the die size as small as possible for a set of requirements agreed on and multiple competitors of Bowmar consequently developed calculator display modules using very small, monolithic GaAsP LED chips with an attached magnifying lens to improve readabilty.

Technology

Monolithic Chip-on-Board (COB) Construction: GaAsP (Gallium Arsenide Phosphide) LED chips integrating one character with seven Segment Bars and one Decimal Point bonded on a ceramic substrate and protected by a clear acryl lens with a spherical shaped surface. Some designs are only partly populated (SK-3-307).

Monolithic Chip-on-Board (COB) Construction: GaAsP (Gallium Arsenide Phosphide) LED chips integrating one character with seven Segment Bars and one Decimal Point bonded on a double-sided FR4 printed circuit board (PCB) and protected by a clear acryl lens with a spherical shaped surface. Some designs are only partly populated (LAB-B-2).

• Clock Displays using LED Chips, Reflector, and Protection Lens

Work-in-Progress.