Grüne LED-Anzeige?

HP-45eBay ist eine wunderbare Fundgrube. Letzte Woche tauchte ein HP-45 mit grüner LED-Anzeige auf. Dieser Rechner war offenbar eine Sonderanfertigung für Bill Hewlett, der die Farbe Rot nicht gut sehen konnte und darum Mühe hatte, bei einer roten LED-Anzeige die Ziffern 7 und 9 zu unterscheiden. Anscheinend war aber der Kontrast der grünen LED-Anzeige schlecht, denn Bill gab den Rechner nach einigen Monaten zurück mit dem Kommentar „Wenn man den Rechner benutzen will, muss man in die Toilette gehen und das Licht abschalten“. Doch das war nicht das einzige Problem. Auch die Batterielaufzeit war viel zu kurz. Schon nach 20 Minuten war wegen des höheren Strombedarfs der grünen Anzeige der Akku leer. Somit erstaunt es nicht, wenn diese Variante nicht in Serienfertigung ging. Hier noch der Originaltext der Auktion und ein Bild dieses besonderen HP-45. Übrigens, der Endpreis der Auktion war $1275, wow!


HP-45 mit grüner LED-Anzeige (Prototyp)

Up for auction is a truly one-of-a-kind HP-45 Calculator.

I worked at HP Labs from 1973 to 1979 as a Senior Tech. I was in the Solid State Labs (SSL) in building 1-Upper at 1501 Page Mill Rd. in Palo Alto.

Bill Hewlett was one of the roughly 10% of the public that could not see the color Red very well that the LED’s in the Calculators produced. It is at the edge of the human eye response curve, and he could not tell the difference between an “7” and a “9” for example, in the 7-segment display. He wanted to see if a Green LED Display could be made to work. I was given a new HP-45 and some Green displays that the lab made in a special batch as a test. I installed them in the Calculator but they needed to be driven with 4 times as much current to get the brightness up. There are Cathode drivers on one side of the multiplexed displays and Anode drivers on the other side. There are three DIP’s of 5 digits each. I made Emitter-followers to drive one side by filing down plastic transistors small enough to fit. I piggy-backed a driver chip on the other side by filing down the top and bottom of the chip so it would fit in the case. This allowed enough drive current to be effective.

Since the color Green is in the middle of the eye response curve, room light entered through the display and reflected. This caused glare and made it difficult to see the displays if there were a light source behind the user. So I obtained from one of the Engineers a circular polarized Plexiglas bezel and had the surface frosted. You can see this where I reflected light off the bezel in one of the pictures.

Bill Hewlett used it for a few months and gave it back to my Department Head, Dr. Paul Green. Bill’s comment was “You have to go into the Men’s room and turn off the light to be able to use it”. It never made it to production.

Paul had it in his desk for a while and gave it to my supervisor. He discovered the battery life was only about 20 minutes because of the extra power drain.

When I left HP in 1979, I asked if I could have it. He had no use for it so he gave it to me. It has been setting in my closet, unused, since then. Buy the way, it still works.

I recently retired and discovered it and am now offering it for auction.

The battery and power pack are long gone. The standard power pack cannot supply enough current. At the time I added an external small power transistor to the outside of the power pack to increase the regulator capacity enough to be used normally. This could be done today to a stock power pack. Otherwise it can be powered by a fully charged battery for up to 20 minutes. For the picture, I used a power supply. This comes with the original leather case (pictured).

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