LED Display


The Story of the Little Computer That Could!


Seeing Red

The first-generation HP 9100 desktop calculator used a CRT (cathode-ray tube) that displayed three number strings in scientific notation, one per line. Each string of green numbers represented the contents of one calculator register (X, Y, and Z). The CRT was an expensive display to use simply for three numbers. The tube itself was expensive and it required a high-voltage display. In addition, the display consumed a lot of space in the calculator’s cabinet. Because of its size and cost, the second-generation calculator team selected LEDs (light-emitting diodes) for their display technology.

Hewlett-Packard had gotten into the manufacture of LEDs as part of its semiconductor-development activities in California. The division making LEDs was called HP Associates (HPA). Although HPA had offered a single-digit alphanumeric display with little market success (like most HP products, it was expensive), the HPA division got its big break when it developed some small numeric displays for the HP 35 pocket calculator. The projected sales volumes for the pocket calculator had been sufficiently high that HPA had invested in a large amount of manufacturing equipment, which was needed when the HP 35 sales exploded far past projections. The net result was to transform HPA into a large-scale semiconductor manufacturing concern.

There were three initial second-generation desktop calculators: the HP 9810, HP 9820, and HP 9830 (eventually, the HP 9821 was added as well). The HP 9810 was a newer version of the HP 9100 and it needed a 3-line numeric display. Although it used three lines of numeric LED displays, they came from Fairchild, not HPA. However, the alphanumeric LED arrays in the HP 9820, HP 9821, and HP 9830 did come from HPA. So it was quite logical that the third-generation HP 9825 desktop computer would also use alphanumeric LED displays from HPA. The other two machines in the third generation, the HP 9815 and the HP 9845, used a gas-discharge 7-segment numeric array and a large CRT respectively. The HP 9825 was the last desktop computer to use LED displays from HPA. (Except of course for the HP 9831 and the HP 9835B, which were both based on the HP 9825 chassis.)

The initial HP 9825A display arrays were encapsulated in red plastic. However, the HP 9825A used a red filter over the displays so later displays, such as the ones shown below used in the HP 9825T, were encapsulated in clear plastic resulting in improved brightness.

HP9825T Detail Small02

Four-character HP 9825T display chip detail. (Photo courtesy of Dr. Achim Bürger)

HP9825A Display small02

HP 9825A display. (Photo courtesy of Dr. Achim Bürger)

HP9825T Display Small02

HP 9825T display. (Photo courtesy of Dr. Achim Bürger)



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