|HP seven-segment display patent|
Message #1 Posted by Eric Smith on 29 Feb 2008, 7:57 p.m.
I just happened across this patent that I'd formerly overlooked:
US patent 3719849
HP patented putting the decimal point inside lower part of the seven-segment display, rather that to one side of the digit, as was more common. This was used on the Classic series (35/45/55/65/70/80), 67, 97, 19C, and Topcat series (91/92/95C/97).
I'd always wondered why they did that, and thought that perhaps they believed it improved legibility. The patent mentions that but also that it reduces manufacturing cost.
The Woodstock series went to the more common use of a decimal beside the digit, mainly to get more digits in less width by not "using up" a whole digit position for the decimal point.
The other thing I found interesting about this was the construction technique. I've never inspected HP's calculator displays closely enough to see how they were constructed, but have taken apart larger seven-segment displays, and those larger ones used multiple separate LED dice with light pipes to form the digits. It hadn't really occurred to me that the small ones might actually have a semiconductor substrate the size of the entire digit.