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Hi, i'm a teacher and i have a calcualtor project with my students : we want to take a HP-45 with a bad keyboard and make a big external keyboard made with push button switch. I think it's a matrix and i'll copy the wiring inside the bad keyboard. We also want to make a bigger display with .5 inch 7 segments LED; the problem is : will the display "drivers" be ok with that extra current taken from these bigger LED? Is there another solution like "boosting" the curent externally before the new LED (transistor).

Thanks
Michel

Your keyboard fix should be OK.

The display may need boosting, it depends on the current drawn by the original display and the new one which may be much more efficient at turning electrons into photons.

To be on the safe side I would make a booster circuit with either transistors or perhaps a buffer chip and dropper resistors.

If you are making all this it would probably be worth making a new power source for the display with bigger batteries or mains supply (just in case you overload the original power supplies).

I think there are HP45 circuit diagrams traced by someone, I don't know who might have these.

PS Have you been sub-contracted by HP to make a demonstrator model for the latest HP RPN calc???

I'm thinking about the possibilities ...by using the Opto-Isolators to interface the display's segments with the outside world.

The HP35/45/55/65 used a very tricky power saving technique to do the display. The segments are scanned individually within each digit then the display goes to the next digit. There are inductors on each segment line that store and boost the drive current. Each segment is only driven for a few microseconds. I would try directly driving your bigger LED's first. Boosting the drive signals may not be an easy task for a high school student.

The keyboard is just a matrix of swtiches, so you should be able to add a larger one very easily.
The display might be harder -- the HP LED drivers on the 'Classic' calculators (35, 45, 55, 65, 70, 80) are _weird_. The LEDs are driven from the back-emf of a set of 8 indeuctors (one per segment) along the top edge of the PCB. And it's both-ways multiplexed -- only one segment is driven at a time (not all the segments of one digit as for more normal calculators).
My feeling is that you should add buffering before wiring up an external display, but that the buffer circuits might not be totally trivial.
Poke about on the CUCPS web site. There's a text file there giving IC/connector pinouts for some of the classic models, which might get you started.

Not so simple, isn't it ?
My contributions:

1. A few days ago, someone posted a link to HP-45 patents. At that time, I followed the link, and I'm pretty sure there was a schematic in the patent.

2. The 45 emulator (java) and the source code for the firmware are available - would it make sense to interface a matrix keyboard and a modern led display to a PC running the emulator ? In my opinion, this looks more like a school project.