Dilemma in my Sleeper Casio Calculator Build - Page 1
Forum section: electronics / projects

author: louishep
author_rank: Newbie
timestamp: June 03, 2025, 05:00:41 am
content: So I don't really have any project progress to speak of, I was just hoping for a discussion how to decide what direction to go in. This is my first post on this forum, so if there's a more appropriate place for this a discussion like this please let me know.I really like "sleeper" builds of all kinds, and for a while now, I've really wanted to modify a calculator to "enhance" it, but without giving anything away in its appearance. Kind of likeproject where someone put an ESP32 in a TI84. However, the TI-84 is already pretty advanced, so I thought it would be more fun to do it with a simpler "scientific" (non-graphing) calculator.It seems like most of these modern scientific calculators use Chip-On-Board for their main processor, so given the lack of information on them, I think the strategy of simply adding additional hardware to the existing processor is a no-go. So the approach I'm going for is to replace the entire internal mainboard with my own.I chose the CASIO FX-300, because it has one of the better displays I've found and I like the overall aesthetic. Unfortunately, the display is exactly what I'm stuck on. I had dreams of somehow reverse engineering and keeping the original LCD, but the connector looks like a lot to deal with... (see the attachment for close-up) I found another post on this forum about someone would dealt with a similar soldered-on ribbon cable and it didn't look fun. (https://www.eevblog.com/forum/repair/desoldering-ribbon-cable-of-ipod-touch-5th-gen-how/) I do own a hot-air rework station so I am keeping that option in mind, but I've never tried anything like this.On the other hand, I've been searching on aliexpress, mouser, digikey, and I haven't been able to find a suitable replacement. (2.4cm x 6cm of viewable screen space with a similar look.)So I'm basically wondering if someone might have a lead on a better source for LCDs that would fit for my use case, or if someone has any information or experience that could help me in the very daunting task that would be reusing the original display.Thanks,Louis

author: jwet
author_rank: Frequent Contributor
timestamp: June 04, 2025, 05:02:03 am
content: You won't find any equivalent LCD's. LCD's of the calculator variety are all custom, its relatively cheap to do this- a few thousand bucks. Dave Jones here on his blog ran through a design for a custom LCD. It might be doable for a replacement LCD but would be ton of work and would likely take a few iterations.I don't quite understand what you're goals are but I would leave the calculator mostly alone and add a peripheral ESP32 or similar board for connectivity.Though the TI Calcs use a COB type hardware design, internally, they base processor was a Z-80 or were not long ago. It would seem straightforward to write a bit of Z-80 code to talk to an ESP32 and parse its output to something useful and leave the display to be serviced by the standard CPU.I don't know what's in the CASIO but its likely some standard architecture- Z-80's and 6502 derivatives were common.The easiest way to get there is probably to take an old android phone and make it look externally like a calculator.Dubious character and legitamacy but a good hack if you're smart enough to pull it off.

author: louishep
author_rank: Newbie
timestamp: June 04, 2025, 11:51:59 am
content: Hi jwet,Thanks for confirming that finding equivalent LCDs would be a no-go, I suspected that would be the case.I definitely agree that it would be easier to use existing hardware so I don't have to reinvent the wheel in terms of interfacing with all the externals. The reason I was put off from that is because the chip is covered in that black goo, there isn't any information about it online, and it doesn't seem to be using any external memory that I can easily access/dump. So even though I might have a clue about the architecture of the processor, I'm not sure that programming it would be so straightforward...Even on some of the older calculator models from the 90s, where the chip is more easily accessible, I'm not sure how I would go about reprogramming it without any info. (It seems like most of these used custom chips and public information about them is scarce.)I understand it might be hard to understand my goalsit's not solving any practical problem, since I'm finished with school, so it's more of a reverse engineering exercise/token project. I just like builds where old technology is modernized and I think turning an old calculator into a kind of PDA would be cool!With that in mind, I don't think modifying an android phone would suit my purposes.

author: jwet
author_rank: Frequent Contributor
timestamp: June 04, 2025, 02:00:12 pm
content: A few pointsThere are calculator hobbyist groups around that hack the living hell out of calculator internals. The biggest group is for HP Calculators but there are active communities for TI and other calcs, google around a bit.Most modern or semi modern calcs use flash memory and are programmable in circuit. The tell tale sign is a little cluster of 4-6 pins on the board; that is an ISP connector. (in system programming). There was a hack not long ago, where hobbyists took an HP-20 business calculator, made new keyboard legend labels and a new code base and turned it into a advanced scientific called the WP-35 IIRC.There is a company called Swiss Micros that make HP like improved calcs. They use a little ARM and are pretty approachable- the fit an finish is every bit as nice as the HP's. There are guys who make calc kits and sell on Tindie. They are mostly HP-41 like clones but very legit. Calculator came out when I was in high school in the early 70's and were the first glimpse of what was to come later in electronics.I understand- have fun.

author: ArgyllGargoyle
author_rank: Contributor
timestamp: June 04, 2025, 02:34:56 pm
content: Instead of replacing the LCD, I wonder if you could put an OLED display -under- the existing LCD?Perhaps that would yield a display that looked 'stock' until you turned on the secret OLED mode which would (hopefully) shine through the LCD?Might be hard to work around the zebras, though...

author: louishep
author_rank: Newbie
timestamp: June 04, 2025, 05:01:28 pm
content: That would be super cool... but yeah those LCDs probably do use a zebra connector under that opaque aluminum backing, no idea how I would get around that...

author: ajb
author_rank: Super Contributor
timestamp: June 04, 2025, 06:48:10 pm
content: Perhaps you could sacrifice the PV cell and button battery above the display and fit a new display that was the correct width but taller? They probably won't be much use for an ES32 or similar anyway, unless you put a fair bit of work into minimizing the power consumption. The upper portion would be obscured by the case, which is a bit of a waste, but if it's tall enough the PV cell window could be used as an auxiliary display area.There are some transparent OLED displays that could be interesting for this if you can find the right size. With the right backing film they should look fairly close to the original display while it's off, but with better resolution. Contrast might be better or worse depending on viewing conditions and the backing you use.

author: louishep
author_rank: Newbie
timestamp: June 04, 2025, 08:18:09 pm
content: EDIT: I looked into the transparent OLED you mentioned but I wasn't able to find one that is as big or bigger than my desired viewing area. (2.4cm x 6cm). Is there one that you found?I think the option you described of using a more modern LCD that basically fits would be the easier route. (Provided I can find one, which isn't a given...) But I guess that isn't as appealing to me!Take this iPod classic mod that basically uses the same approach I want to use (gutting all the internals):Really impressive, I like how they made the UI look like the original, and even implemented clickwheel functionality. Arguably, most would prefer the modern LCD over the old one. But I think it would be even cooler if someone could pick up that iPod, start using it, and not realize it isn't a meticulously well-kept original until they see the "spotify streaming" option in the menus.

author: ajb
author_rank: Super Contributor
timestamp: June 04, 2025, 10:16:59 pm
content: I haven't actually looked at them myself, sorry. I just saw the one my coworker got from Ali Express, but it was smaller than what you're looking for.

author: dnparadice
author_rank: Contributor
timestamp: June 05, 2025, 02:38:36 pm
content: This looks like a fun project. I love calculators.Instead of looking for a screen, try to find another calculator with a similar size screen and see if you can hijack the screen out of the other calculator. Sometimes the off brand calculators do not use such sophisticated manufacturing technology as Casio so its easier to hack their parts.like this:https://www.aliexpress.us/item/3256807290284926.html?spm=a2g0o.productlist.main.1.5b536f4avaOmqI&algo_pvid=4ab61db7-43e8-465d-86f6-01b58bdef8b9&algo_exp_id=4ab61db7-43e8-465d-86f6-01b58bdef8b9-0&pdp_ext_f=%7B%22order%22%3A%223622%22%2C%22spu_best_type%22%3A%22price%22%2C%22eval%22%3A%221%22%2C%22orig_sl_item_id%22%3A%221005007476599678%22%2C%22orig_item_id%22%3A%221005008490431239%22%7D&pdp_npi=4%40dis%21USD%2119.29%218.49%21%21%21137.80%2160.63%21%402103209b17491344467684683e9052%2112000040910601129%21sea%21US%210%21ABX&curPageLogUid=pg1TfZl7lO3F&utparam-url=scene%3Asearch%7Cquery_from%3A

author: ledtester
author_rank: Super Contributor
timestamp: June 06, 2025, 04:07:16 am
content: You might get some ideas by looking at the PicoCalc project.Search for "clockworkpi.com" and "PicoCalc". They sell a kit but also make all the design files available on github.

author: louishep
author_rank: Newbie
timestamp: June 07, 2025, 01:24:21 am
content: Here's an update/question. I decided to go down the route of reverse-engineering and potentially using the original display. To do this, I figured I would need to probe the display while it is in use. I chose the TI-30XA, as it has a simpler display of ten 7-segment digits, along with a few dedicated light-up words. Even this had 32 connections to probe, so I figured I would need to solder them. There was a little strip of pad that I thought I could put some flux and a dab of solder on to attach a wire. But when I tried the pads oxidized fast. (Or I ripped them up? Can't really tell what happened. It seemed like they just instantly turned black.) I tried on the 8 leftmost pads in the attached picture.This seems like a problem that would have been encountered before. Are there any better methods out there?

author: ledtester
author_rank: Super Contributor
timestamp: June 07, 2025, 02:28:46 am
content: With a hobby/Xacto-knife gently scrape away the solder mask to reveal the traces. Then solder small gauge wire to the exposed traces.See below for what I did -- I used wire from a parallel printer cable. 30 AWG wire will also work well.All of the wires are soldered to the copper board mainly to hold them in place.I test a segment by desoldering two wires from the plate and applying an alternating signal across them from two GPIO pins of an MCU.

author: louishep
author_rank: Newbie
timestamp: June 07, 2025, 02:37:16 am
content: Wow, awesome!!!! I'm glad I now have a model for exactly what I'm trying to do, thank you. Did you end up desoldering the ribbon cable of that display to use it elsewhere?

author: louishep
author_rank: Newbie
timestamp: June 07, 2025, 02:42:43 am
content: By the way, dnparadice, that sounds like a great idea, but I personally don't want to do it that way because this would stall my progress for the entire time I'm waiting for it to get here, and I'm trying to get most of this project done within the month of June.

author: ledtester
author_rank: Super Contributor
timestamp: June 07, 2025, 03:01:30 am
content: Quote from: louishep on June 07, 2025, 02:37:16 amFor this display I actually didn't go beyond mapping out the pins.Here's another thread I started about connecting to calculator displays...https://www.eevblog.com/forum/projects/making-connections-to-a-calculator-lcd-zebra-ribbon-cable/There's some talk about conductive adhesive and t-bar soldering tips and also see Reply #12.

author: louishep
author_rank: Newbie
timestamp: June 07, 2025, 03:20:11 am
content: Thanks, great read, those videos you found are helpful as well. Did you ever succeed in connecting to that display? Also, did you ever find/use that T-bar soldering iron tip? I was trying to find one that would fit in my Weller WLC100, (I think the form factor for the tips is called "ST series"), but nothing came up.