Muxcard, a DIY credit card size computer | Hacker News Reader

Muxcard, a DIY credit card size computer

github.com

290 pointssargstuff102 comments

Recent and related:

A self-powered computer in actual credit-card size (~1mm thick) - https://news.ycombinator.com/item?id=48251528 - May 2026 (11 comments)

Open articleView on HN

I went to the page expecting to rant about how it's not actually credit card size because of the thickness and was for once pleasantly surprised! Kudos to the author! It looks great!

I was thinking all that too and considering commenting about being sick of those credit card size claims, but after seeing the footage I am genuinely impressed. Great work there.

Love how the point of this entire thing comes across!

I'll be that guy I guess then... they stated on their page that credit cards are 0.8mm while the muxcard is 1mm and yet they still claim it is "literally the size of a credit card"... not to mention that they carved out an NFC card, not a credit card.

Yes it's still impressive either way, I'm not debating that.

I did notice that difference too. But previous "credit card size" projects have all been several mm (as in couldn't fit a wallet designed for credit cards). So 1 mm is... pretty sweet!

It said that the specification states 0.8mm, but that many real world cards are thicker. Are credit cards actually 0.8mm?

Did we read the same text? He wrote 0.76mm.

This post - the title made me remember ... ( as a credit card is about the same size as a business card )

A Linux Business Card CD is a miniature, credit-card-sized optical disc containing a stripped-down, bootable Linux operating system. They hold around 50MB to 100MB of data and were highly popular in the early-to-mid 2000s

More info at https://en.wikipedia.org/wiki/Bootable_business_card

yeah, also reminded me of wifi sd-cards: https://hackaday.com/2016/06/30/transcend-wifi-sd-card-is-a-...

this is really cool. I didn't know we had these

Or the Rex 6000 or other PCMCIA cards:

https://en.wikipedia.org/wiki/REX_6000

I had one of those - I think it is still in my office desk.

These things were cool! I believe I had some drivers installed via some of them, and a Kubuntu livecd.

Seth Schoen (<https://news.ycombinator.com/user?id=schoen> at HN) was lead dev in building one of the best-known instances of these, the Linuxcare Bootable Business Card (LNX-BBC), and has occasionally commented on that here:

<https://hn.algolia.com/?dateRange=all&page=0&prefix=false&qu...>

Business card CD ROM's were a nice idea in the day.....

.... trouble was they would often 'misfeed' when using a tray style CD Rom drive and jam in the mechanism, meaning you had to dismantle the drive to get the card out.

Understandably, this would quickly piss off people you gave the card to. This helped make the cards rather unpopular.

https://en.wikipedia.org/wiki/Shaped_compact_disc

Reminds me more of the smart card with small chip in it - SLE4442 -> https://firmware.buspirate.com/devices/sle4442

What fun!

I’d love to also go the opposite direction, a full-sized laptop with an ESP32 running tiny386 and Windows 95 ^_^

https://www.hackster.io/news/he-chunhui-s-tiny386-turns-the-...

This could be great if the rest of the chassis was filled with batteries.

Now you’re talking!

The laptop is 4kg but the battery life is 6 months

diy 486 schematics (around year 2000)[0]. no longer available. Discription/overview[3]. :( But, current linux doesn't support 486 cpus.

Was turned into something commerical (?2002[2])[1]. Perhaps modern version would be the old AMD cyrix chips?

-------------------------

[0] : http://boole.stanford.edu/cebit/

[1] : https://megagames.com/news/matchbox-pc-smallest-world

[2] : https://ptacts.uspto.gov/ptacts/public-informations/petition...

[3] : https://www.paloaltoonline.com/morgue/monthly/1999_May_26.MI...

Developer here :)

Just saw this and love how I got the 100th or so "Does it run DOOM?". Even now officially an issue on GitHub. Does that mean I now have to deliver?

> Does that mean I now have to deliver? Well, if you'd like to, you're free to do so! If not, somebody else could do it. You're not your audience's slave

I know it was intended as a joke but still..

So... it's not DOOM-complete?

Teenage Alan T. would be so disappointed... :D

Well, using something related/similar to turing-complete correspondence:

----

If it can fetch/display files from the internet, yes, it can run doom[5]. Or make use of curl, curl-doom[18]

-----

stripped down doom can run in 4MB (doom on a smart toothbrush[7]). ESP32 has support for up to 4MB external PSRAM[1]. ESP32 port of doom[2]. lcd screen might provide for faster refresh / less blury images under magnifying glass.

Alternatively, adding an expansion port to be able to use 'doomchip' might make it easier hide game play while at work[8] / stay awake between screen frame refreshes. (and/or simulateously play tetris & 1d pacman; add Badgey functionality[19] )

But should one go with micropython[3], reflectionsOS[4] or not waste space and use a 2 stage 512k bootloader for dns.doom[5]?

contikiOS[6] for dns.doom[5] might be the correct fit without needing to add 4mb psram / doomchip expansion port(s).

-----

Misc doomchip information:

   doomchip non-tech article(2020)[11].

   doomchip developer comments[12]. Should be able to get at least 2fps!

   doomchip better expanded upon in a 'heads up, 7up' thread -> slice[9]/doomchip[10].

----

Surprised there's no dns doom.wad / I.D.Software Application (IDSA) rfc "Spit.wad: A DNS string encoding using Unicode for doom.wad graphics" version of rfc3492: "Punycode: A bootstring encoding of Unicode for Internationalizaed Domain Names in Appliations" (IDNA) [13]

(IDSA) rfc might permit formally unifying things like unicode-line-stacker[14], stacked diacretics generator[15], unicodeart[16] and "awesome symbols and characters in a bash prompt"[17].

-------------------------------------------------------------

[0] : "Can it run doom?" : https://canitrundoom.org/

[1] : https://docs.espressif.com/projects/esp-idf/en/latest/esp32/...

[2] : https://www.techeblog.com/doom-electronic-badge-emfcamp/

[3] : https://github.com/NoosaHydro/2.4inch_ESP32-2432S024/tree/ma...

      https://github.com/wojciech-graj/cydoomgeneric

[4] : https://github.com/frankcohen/ReflectionsOS

[5] : https://undercodetesting.com/dns-can-run-doom-how-2000-dns-r...

[6] : https://contiki-os.org/

      esp32 https://github.com/contiki-ng/contiki-ng/discussions/3022

[7] : doom on a smarth toothbrush : https://www.youtube.com/watch?v=cO-Are8053g

[8] : https://hackaday.com/2020/05/13/the-doom-chip/

[9] : https://github.com/sylefeb/Silice

[10] : https://github.com/sylefeb/Silice/blob/master/projects/READM...

[11] : doomchip article : https://www.techeblog.com/doom-chip-game/

[12] : https://threadreaderapp.com/thread/1258808333265514497.html

[13] : https://www.rfc-editor.org/info/rfc3492/

[14] : https://github.com/axesilo/unicode-line-stacker

[15] : https://codepen.io/lingyloll/pen/ooMaYO

[16] : https://www.unicodeart.com/gallery/

[17] : https://unix.stackexchange.com/questions/25903/awesome-symbo...

[18] : https://codeberg.org/xsawyerx/curl-doom

[19] : https://memory-alpha.fandom.com/wiki/Badgey

How optimal is the PCB density? Do you think there is significant room for improvement to have a smaller PCB and larger screen and/or battery?

The PCB can be way smaller, but finding a flexible screen is the issue, there is a 2.9 inch one almost twice as big but the battery would suffer :/

?? add relevant wifi/port to connect to a projector/projection glasses / laser projector[0]

3 old micro ibm hard drives combined into rgb color space / diy laser image projector[1] might be able provide a bigger picture without exceeding DIY credit card size. kinda depends on laser size.

Ummm... a smartphone internal projector module more compact than diy stack of ibm microdrives / laser.[2]

Not worrying about size: esp32 laser dac : https://www.youtube.com/watch?v=9YASnlB_t_U

----------------

[0] : diy laser video projector : https://asktheman.xyz/

[1] : https://www.youtube.com/watch?v=fEPicBSYeNQ

    : https://www.youtube.com/watch?v=bl1e54QGJk4

[2] : https://www.youtube.com/watch?v=ET7jP2OsxzA

Yes ;)

Last week (87 points, 7 comments) https://news.ycombinator.com/item?id=48251528

Thanks! This project is cool enough that I think we can tolerate another thread about it, especially since that post didn't get too much frontpage time. I'll put a link to it at the top though.

Thanks! Arbitrary application of rules is nothing short of what I would expect from CCP LOVER DANG.

... but both threads are DOOMed ...

It's so interesting how transparent my emotions are to total strangers on the internet!

Is this an ops we shouldn't have put this dupe in the SCP, or an ops the original submission should have been a different URL?

yer dang right

> A fully working computer that is literally the size of a credit card.

Nit: A (chip) credit card is already a fully working computer :)

Only if it is inside a specially designed radio field and with no independent IO. Feels like a battery and IO justify the 'fully working' differentiation.

I've never heard a definition of a computer to include its power source.

IO is of course required.

Think that was a early computation thing -> CRT memory[0][1]

Perhaps the 'city block' heaters requiring similar sized AC condensors to provide control over Maxwell's daemon was the inspiration for the unix daemon process.

Optical computing makes fuzzy computations so much easier -> outgassing[2]

------------------------------------------------------

[0] : https://spectrum.ieee.org/the-birth-of-ram

[1] : https://bunkerofdoom.com/lit/CRT-scopes/CRT_memory/index.htm...

[2] : https://spectrum.ieee.org/adhesive-outgassing-nasa-standards

Interesting philosophical question: Is a tower PC that's not plugged into anything (neither power nor a keyboard or monitor) a computer? Does computation happen if nobody can perceive it? And is a computer a computer even between two CPU cycles?

> no independent IO

I would challenge that! How is a smartcard different from a server in a qualitative sense? Both get all their I/O over the network.

Some cards even have a display, fingerprint reader, or can blink an LED (the latter unfortunately only indiscriminately when powered up, not in response to any computation, I'm afraid).

Is a bare SoC a computer? You can poke the pins to provide power and I/O.

The interesting thing about this project is that this computer can function independently within a credit card sized space.

asic computer doesn't have cpu cycles. So, a computer is still a computer between two cpu cycles.

Very cool! Love it...

But...

The battery is likely to be squeezed quite a bit after this is put in my wallet, and in my pocket.

Lithium batteries do not like to be squeezed. They tend to signal their distress with some type of heat, usually accompanied with a small fire and probable smoke as well.

A distressed battery is very insistent upon everyone to see it's state of mind...

This is discussed in the post.

Nice light computational module for huds device. Random search engine project pick:[0]

Perhaps also basis for a table coaster 3d hologram[1]

Adding to card thickness / modularity:

   pop-up hologram display.[2][3]

   projected keyboard/display[4] & gain ability to respond to text messages!

--------------------------------------------------

[0] : https://www.instructables.com/Smart-Glasses-V2/

[1] : https://www.youtube.com/watch?v=xHbQRcIWxC8

[2] : https://www.youtube.com/watch?v=vRo8m_a6F54

[3] : https://www.youtube.com/watch?v=wM_Byrv9iBI

[4] : https://www.youtube.com/watch?v=gTnHEzshdIQ

Hidden in here is the coolest part, that the author made flex PCBs at home

Just in time for DEFCON. We built many of these types of badges

https://www.techeblog.com/doom-electronic-badge-emfcamp/

I love these kind of projects. M5Stack Cardputer Zero launched on Kickstarter last week and already hit their goal

https://www.kickstarter.com/projects/m5stack/cardputerzero

Kiisu[0], credit card sized footprint computer with modular add-ons.

-------------------------------------

[0] : https://github.com/kiisu-io/

__This__ is where all those trusted app parts should go - a smart card with e-ink display that can provide high security assurance level and where I won't mind that it's locked down because it has only one purpose.

__Not__ to my smartphone, effectively preventing me from modifying the system in the name of security. A banking app can use a card like this and on the display I could for example see where a transaction would go and then I could accept it, possibly even with a biometric identification.

This would enable me to keep my smartphone customizable and banking apps secure at the same time.

[apologies for the rant]

And it would communicate using?

NFC, with the smartphone, but the smartphone would not need to be trusted, it would just relay encrypted + signed data.

This is great, and I love it, and I hate to be saying this, but it's not literally the size of a credit card, it's 0.2mm thicker.

Fair enough, but I acknowledged that and it's 0.24mm thicker if we want to be exact. Here's a quote from my Git Repo:

"Official ISO7816 smartcards are specified at 0.76mm thickness, but many real-world cards slightly exceed this in practice. The target for this project was simple: Stay around ~1mm total thickness and preserve the illusion of a normal card."

Hey, works for me, I just got OCD from the title's usage of the word "literally".

Prologium is depositing thin film solid state batteries onto flexible ceramic insulators. They have some demos of single cells that appear to be thinner than 1mm continuing to operate after bending in half.

https://prologium.com/tech/core-technology/

I want this, but only for one thing: email.

I already use an pwnagotchi, and it works great for this - but its a bit bulky.

If I can get this set up and working, it'll be my main interface to email.

Run Unix v6 on it :) 16 bit and works with like 80kb of ram

Fuzix might be another option: <https://fuzix.org/>.

Works on the RISC-V 32-bit processor, sometimes.

EPS32-OS : https://github.com/ESP32DE/Boot-Linux-ESP32S3-Playground

> For the first prototype, I soldered each wire individually...

I salute you sir. Did you have a better solution for later versions?

cad/stl/cnc related files for 'automated assembly' via 3d printing pcb[1] / pick-n-place machine[2][3] would be nice.

Catchy, higher order point/fundamental theory of calculus application of "Move over cassette tapes, adhesive tape has memory, too"[4]

-------------------------------------------------

[1] : "Making a 3d printed circuit board that works" : https://www.instructables.com/Make-a-3D-Printed-Circuit-Boar...

[2] : pick-n-place machine : https://en.wikipedia.org/wiki/Pick-and-place_machine

[3] : No solder! : https://www.electromaker.io/blog/article/affordable-pcb-prot...

[4] : "Move over cassette tapes, adhesive tape has memory, too" " https://news.ycombinator.com/item?id=48161242

Wow. This is actually credit card sized.

Can it run DOOM?

I've got the question like 100 times easily, and I love it.

And yes, if you accept ~0.7FPS

You mean, 1.5 SPF :-D :-D

Wouldn't that be doom scrolling?

"Can it run DOOM?" : https://canitrundoom.org/

I would love if the screen could take up more space, even at the expense of a little extra thickness.

I think that there could be a wider screen if such formats are available. Once we have betavoltaic batteries, the entire card can be screen.

I'm not sure beta voltaics will ever reach LiPo densities. All materials I know would be unwise to place in your wallet, or anywhere near your body.

If we are OK with a battery and a beta voltaic source, a tritium one is reasonably safe and can trickle charge the battery when the device is in deep low power mode. The battery can still be charged by the induction coil.

That actually exists, with even the same CPU but no NFC: https://www.xteink.com/products/xteink-x4

That looks like it's the size of a smartphone. It's 7 times thicker... I'm not sure that qualifies as a little thicker, plus all the other dimensions.

Coincidentally, the xteink x4 has the same CPU, an e-paper screen and is close to credit card sized.

If you think 1.33x1.28x7.76 times bigger is close to the same size. I don’t.

love this. would be cool if we can see and perform all kinds of banking txns on this. Think ledger but all in one card. Super cool. Even cooler would be card to card money transfer without use of swipe machines

If "ledger on card" interests you, then you might enjoy Japan's FeliCa cards. They store balance locally on the card so you can pay very quickly, no network required.

Do these cards solve the electronic cash problem (in a completely different way than cryptocurrencies)? What I mean is that

- Are the card readers special/trusted issued by bank/govt in some way? Or you can transfer money from one card to another yourself fully offline?

- Is there any requirements that the transfers have to be eventually communicated to the bank by one of the parties to be fully resolved?

- Has someone managed to create fake cards with fake money in it, or this is impossible by design?

As I understand it, these cards work basically the same way as transit card systems in other countries, like the SF Bay Area's "Clipper" cards.

The overall model is similar to tap-to-pay debit cards. They're only used for consumer-to-business payments. When you tap the card, the card sends over an account number / signature / etc, which the merchant sends to a central clearinghouse to finalize the transactions.

The main difference is that the card itself keeps a running balance of how much money the customer has available to spend. In many cases, this gives the merchant enough confidence to e.g. let you through the train turnstile without actually waiting for the central clearinghouse to confirm the transaction. (I think in practice they usually send all the transactions in batches, daily or weekly or something.)

The readers do some trusted-computing/secure-enclave type stuff but are not especially hard to obtain; I think there have even been cases where companies like Nintendo have built them into consumer products, so that you could e.g. tap your card to your Nintendo 3DS to buy a video game.

I imagine there's a bit more security on the machines that let you load money on the cards, but it's probably not completely impossible to make a fake card. But the low value limit (usually a couple hundred dollars, depending on the card provider), the inability to get cash out of the system (often you can't even buy things like postage stamps), and the fact that you'll get caught relatively quickly (once the central clearinghouse notices the transactions don't match up) make it unattractive to do it in practice.

Thanks for the detailed answer.

Ah, quick read / then dissapointment on re-read -- not banking on flexing to get an idea to fly -> https://techxplore.com/news/2026-03-birdlike-robots-greater-...

So, no AI seagul training to do machine banking/swipe money transfers.

It seems like it might be a little expensive for a business card...

Let's see Paul Allen's card.

Or, it would be interesting to see how this would fare in American Psycho.

A credit card with chip is a computer without power.

How do you recharge it? Do you have to swap the battery?

Hey, developer here :)

I used an ultra thin LiPo, so you can actually charge it. USB is obviously not an option but it uses magnetic pogo pins on the back side ^^

Try NGK EnerCera for battery.

Do yourself it!

dyi = Do Yourself It?

selfie thing.

legendary

It's not a computer.

It’s got more horsepower than my first desktop computer

Right, it doesn't compute. Apologies for the lie.

In what way is it not a computer?

Your definition of "computer" is incorrect.

First thought: cool! Second thought: e-waste

(same reaction as single-serve coffee pods, circa 2023)

Good point. Ideally it would be the opposite of waste if it can save you from several cards. But banks would never certify such a multi-card system unless a big company pushes it forward.

Otherwise I'm sure people might use this to hack some terminals :P

?? e-waste recycling ??

Reply on news.ycombinator.com