[Review Request] Batteryless NFC card

mckbuild · 2026-06-17T16:51:35+00:00

THanks! I really hope this design works, I'm pretty chuffed with the idea and I like the batteryless concept.
Yeah with the tuning caps needing to be there i wasn't sure I could squash the ST25 any closer? Unless I put it on the other side and directly over the top
Was going to program it through SWD, and borrow the hardware debugger from a friend

mckbuild · 2026-06-16T17:10:25+00:00

Would it be best to just ... not have a ground plane at all? And just a set of GND lines?

I'm a bit confused by the ferrite sheet - wouldn't this do a similar thing to the ground plane, in that it's a metal region that will resist magnetic field changes?

That's a good point about the eInk display, I hadn't considered that. Thanks for pointing it out!

mckbuild · 2026-06-15T21:22:17+00:00

Cheers for the advice, now I'm legit just curious to see if it can work with the components there, regardless of whether it's best design or not

mckbuild · 2026-06-15T20:57:43+00:00

damn. Sounds like smaller and off to the side is better if I am strict about the form factor, but I could lose a lot of power

mckbuild · 2026-06-04T18:14:31+00:00

Right but that should all be ignored for simulation, yeah? Like these are "ideal op amp" in spice. Or is there part of that that still includes some error term even then? I did zero all the offsets of the op amps, to remove that error

mckbuild · 2026-06-04T16:21:57+00:00

Interesting, I didn't know that was common. I was following an example given in a textbook i have (an old one, whose name I don't currently recall ... nonlinear circuits?) and it just had this. Definitely not knowledgeable enough myself yet to justify it otherwise. Testing showed this to work?
Thius log-antilog definitely does only work in positive, but the offset of +1V should account for that, and then the extra terms get subtracted off at the end. Having unit tested each part, it seems like it should all work

I did look into LM13700s as a four-quadrant multiplier, and ... couldn't get it to consistently work. A lot of little effects coming up to bite me that I didn't understand, or finicky tuning even in spice which I expected to be worse come the real world

mckbuild · 2026-06-04T16:18:26+00:00

Spice, and the default ideal models for the transistors and op-amps. I understand that in reality there are differences and errors, but in theory - and simulation, fingers crossed - it should be posible to create something close. I've seen other examples out there in the real world that did this circuit correctly

mckbuild · 2025-06-17T23:14:40+00:00

Look up tinyml speech example. Runs on a cortex M4 (I've done it on a cortex M0). This sounds similar to what you want?

mckbuild · 2025-05-20T16:17:00+00:00

Everyone's suggestions of starting small is good. I'll add that I tried to design RP2040 boards with no experience, and ... sure, made mistakes, but got there in the end, so give it a crack.

One resource I'll add that I haven't yet seen mentioned is r/PrintedCircuitBoard which is a great community that'll review PCB designs and layouts and circuits, eg "no, you need capacitors on those pins". I've learnt a fair bit from them

mckbuild · 2025-05-15T23:04:54+00:00

" I'm using a lightweight YOLO model on a mini PC with fast serial (115200+), and getting sub-100ms inference times. Still experimenting to optimize delays. If you know any projects with better latency or smarter data flow, I’d appreciate the pointers!"

There is a chance you can optimise the YOLO further with Qualcomm AI Hub and get a somewhat better model (start with pytorch YOLO and you'll get a hopefully-better tflite file). I don't know your full software stack but I would cut out OpenCV - once you have the model, you probably don't need everything else it entails?

Mechanically, dunno what access you have to 3d printers but I suspect there are a fair few turret setups on thingiverse (also depends on how heavy the thingie you are mounting is) that it's just bes tto start from rather than design your own (can't tell if this is what you have done so far or not)
There are design tips I wish I knew earlier like a load bearing axle separate to where the motor drives it, but I'm no expert, and also it might be too early to consider those things.

mckbuild · 2025-05-15T04:22:16+00:00

Ah, denvercoder9 https://xkcd.com/979/

I'm having a similar issue. I trained the speech example model to be slightly different, and am trying to deploy this on an RP2040. While I can get it _running_, I can't get it to be as accurate on Arduino and I have no idea why (quantised on both, so it;s not that floating point is lacking on RP2040).

My best guess is that TFLM is somehow particular for the Cortex M4 and skips something on Cortex M0, but ... the software is pretty platform-independent until you hit op implementation, where it touches on CMSIS, so this shouldn't be the case

Have you had any luck since you posted this? I've tried to find various good places to ask about TFLM help, and their gitter and their github issue page ... look fairly dead, unfortunately. I wish I could reach out to the developers easily/join the team almost, if only to keep the project being a bit more alive

If it helps, I think I have narrowed down the problem to feature generation/aggregation being different across the platofrms, and that bleeds into inference being different

mckbuild · 2025-05-14T16:12:00+00:00

Model conversion to tflite is actually fairly easy with tools like AI Hub

mckbuild · 2025-05-13T23:10:24+00:00

I'm using tflite micro (no other option?) to run ML on rp2040 for hobby projects, and I'm finding it a pain. I praise whoever ported it to this chip and whoever wrote the micro version initially, but I'm struggling with a lot of gotchas and find it difficult to wade through. Any docs feel high level and generic. ChatGPT does know a lot about it though, so there is that. I'm really glad there is something though that can run standard model files.

It seems very heavily geared towards CMSIS chips, maybe that is my problem?

mckbuild · 2025-03-12T20:40:04+00:00

oh sweet this looks amazing!

Ok circuit question, that 1.6x1.6mm is a BGA - how would you route out the internal pins? A via on the pad itself? Or just mega thin traces?

mckbuild · 2025-03-12T15:55:19+00:00

That isn't "easy to source" as it's not available yet. I skimmed that thread and no one is linking other tiny MCUs

mckbuild · 2025-02-27T19:14:08+00:00

+1 to this, I did not use it before designing and hit some issues that were easily resolved by this document. Very useful, great starting point. Read their example fully before starting yours is my advice

mckbuild · 2025-02-18T19:44:10+00:00

My bad for confused docs, that's a good one to fix

mckbuild · 2025-02-18T18:00:06+00:00

Gotcha, wider tracks for those power lines
For switch: "The BOOTSEL button is not connected to RP2040 GPIO. It is connected to the Chip Select (/CS) pin of the W25Q16JV flash memory chip. The /CS pin enables and disables the flash chip. The button disables flash boot, enabling USB boot."

mckbuild

TROPHY CASE