Woodstock Display Upgrade (a sight for sore eyes)

[markhen]

I got my (first) HP-25 in engineering school in the late seventies, and I still have it, and use it. Some years ago, I thought, “wouldn’t it be nice to have a more readable display?”. I gathered some components to attempt this, but it took several years for me to complete the project. I’ll post several pictures of the completed prototype for your viewing pleasure!

There are several really cool features of this project, and a couple of not-so-cool ones – the highlights are:

Cool:
- Much bigger, more readable display
- A couple of display content enhancements (more on this in posts below)
- Upgrade does not modify the base calculator (it can be removed without trace)

Not-so-cool
- The displays ALONE cost over US$100 per calculator (slightly less if 10, 20 or more are bought)
- The processor chip used is impacted by the global chip shortage (aka very hard to get at the moment)
- The power consumption of the display is a little higher than the original display – but since we’re all using modern batteries with higher capacity, this is really not an issue
   

[markhen]

The upgraded display module has 2 proximity switches integrated into its top-left and top-right corners (capacitive switches). These allow changing display modes without any modifications to the calculator housing. While a touch/proximity switch is activated, a red LED in the top-middle of the display lights (this LED also lights under low battery condition).
   

[markhen]

The display module has 2 “content enhancements”, when activated by the touch/proximity switches:

“RUN” Mode Display Enhancement
If the calculator is displaying an exponent (in the right-most 3 digits (sign, digit, digit), and the exponent matches an SI exponent prefix (10^24 .. 10^-24), the module will not display the numeric exponent, but will instead display an up arrow (for exponent) and the alphanumeric SI prefix. Since I work most of the time in ENG notation, this is SUPER handy!!
       

[markhen]

The display module has 2 “content enhancements”, when activated by the touch/proximity switches:


“PRGM” Mode Display Enhancement
If the calculator is in PRGM mode, instead of displaying the key-coordinates of the recorded key strokes, the display modules will show the program step in alphanumeric text that matches the keyboard (no more remembering those strings of digits!!).
           

[markhen]

The display module is a plug-in module that replaces the original module. The only connections required by the upgraded module that are not provided by the original connector are display power and ground. On my prototype I am using “pogo pins” (springy pins) to contact traces on the main board, but I am thinking to go to 2 soldered wires for more reliable contact (which will still allow almost completely “invisible” removal of the module it required). I think the processor used might be the ONLY one in the world that can pack all these features into a single chip that will fit under the display!
           

[markhen]

What’s next?

I picked the HP-25 as the first target for this upgrade since it’s my all-time favorite calculator! I have an EXTENSIVE HP collection, and of course there are other models that I’d like to support:

- Panamatik HP-2xE – Bernhard’s amazing ACT was part of my inspiration for this project. I have upgraded calcs and HP-01 watches with Bernhard’s kits, and we collaborated on a couple of things previously. My upgraded HP-25 display does ALMOST work with the HP-25E ACT, but there are a few “wrinkles”… I intend to talk w/Bernhard to see if we can iron these out…

- HP-29C – Since the display module decodes the PRGM codes, some (easy) tweaks to the code will be required. From an initial investigation, I think there are also some slight differences in the display interface timing vs. the HP-25.

- HP-21, HP-22 - I think this module will work with both models, something I can test.

- HP-27 – I’m not sure if the HP-27 PMOS levels affect the display interface? Maybe someone here knows?

Although all the basic functionality is working, there is still some PCB cleanup, some code cleanup and a couple of residual bugs to chase, as always <sigh>

[Steve Simpkin]

That is so Awesome! I use ENG display mode a lot too.
I bought my HP-25 in 1977 while in 9th grade. I used it for almost 10 years through High School, 4 years of college and the beginning 2 years of my first engineering job as an EE. It still works great and is calculator I am most attached to.
Very cool project and I look forward to seeing more pictures (especially of the insides).

Edit: I see you posted more pics while I was writing
That is so profesional it looks like it was done by HP!

[markhen]

Insides! Hard to see much, side views
       

[rprosperi]

Very, VERY nice! Thanks for sharing this here.

These days, for me, the biggest challenge to using Woodstock machines is indeed seeing the display. The LEDs have not degraded in the ensuing 40 years, but my eyes sure have!

I hope upgrades for this new display will become available, a very nice upgrade making Woodstocks easier to use is something I never expected.

[tangdfx]

Very cool .

But the led dot matrix display is too expansive.

I live in China and can find the reasonable price for these LED module.

HCMS-2915 is about 10 dollars.

[pauln]

That's really really cool!

How do you deal with "-1.2345678-25" with the 12 LED display? It seems that the "." takes a full LED.

[markhen]

Bob - thank you,, I'm glad you enjoy :-) I will update all developments here on the forum.

Tangdfx - this module uses HCMS-3906 and HCMS-3916 which are more expensive (3.3V - 5V versions). What are prices on these parts in China? I can try to see if other HCMS could work, Vdisplay is nominally < 5V, but in practice I'm seeing close to 5V (in sample of 2).

Pauln - nice catch :-) The DP does indeed take a full digit. The code looks at the calculator display and decides if it has enough digits to display all digits, or not (for example, your "-1.2345678-25" case). If there is no minus sign on the mantissa, it will use the left-most digit to display a digit (this is something the base calculator never does (in "RUN" mode). In the event the display mode and display value exceed 12 digits, the least significant digit is dropped (truncated). This could easily be changed to round (vs. truncate).

[tangdfx]

Hello，All
Here is the example of hcms-2915 and Hdsp-2531(the yellow one)
[attachment=10979]
[attachment=10980]

[markhen]

Steve Simpkin - thank you for your gracious comments! I was indeed editing while you were viewing!

Cheers

[markhen]

tangdfx - the HCMS-2915 has luminous intensity 1/10 of the HCMA-3916. In this application that would mean MUCH higher battery consumption (for equivalent brightness). From the approved Broadcom resellers in the USA the 2915 is less cost than the 3916, but it is still $50 (qty 1). Although I know components sometimes are lower cost in China, it seems unlikely this could be available for $10?

   

[BobVA]

SO cool! Thanks for posting!!!

[Dave Britten]

This is great! If there's ever a drop-in-replacement board made available, I'd definitely be interested. Engineering prefix mode is one of my favorite things about Casio scientifics.

[tangdfx]

Hello，

Would you consider HPDL1414 which is more similar with the origianal LED and can show chars.

But the packaging will be a problem.

[vaklaff]

(08-07-2022 10:44 PM)markhen Wrote:  I’ll post several pictures of the completed prototype for your viewing pleasure!

Beautiful!

When I was going through the pictures my first thought didn't go to my LED calculators, my eyes still work fine (knock knock!), but to my DM15L. Just for pure looks. I bought it to replace my 15c but I can't come to terms with its pixelated LCD font. But red LEDs? That could be a game changer :-)

[pauln]

(08-09-2022 08:11 AM)tangdfx Wrote:  Hello，

Would you consider HPDL1414 which is more similar with the origianal LED and can show chars.

But the packaging will be a problem.

A 14 or 16 segment display would be interesting, reminiscent of the HP-41C (but LED). I'm not sure it will be able to display all the ENG units though.