Feasibility questions surrounding wearable display

[evilC]

I currently own a wearable off-the-shelf display - a Loy T3-HD panel, which is a P2.0 96x128px panel. It came as a backpack, but I took the display out and designed and 3D printed a mount to attach it to a t-shirt.

It has an Android app which you can use to upload images / GIFs etc and control it.

It is totally self contained, it only needs power - I run it off 2x 5000mAh 21700 batteries, which lasts about 8 hours.

It is, in effect, a similar thing to what @marcmerlin has.

However, I do not like the fact that it is a rigid panel. I was looking at maybe trying to self-build something using a flexible panel. It does not need to continually flex, I just wish it to conform to the contours of my body rather than sticking out at the edges.

I am wondering how feasible this might be - the main concern is the power requirement - I don’t want to have to wear a huge amount of batteries. 4-6 21700s for ~8hrs use is probably about the limit of what I think would be acceptable.

I don’t need very high framerates - the Loy panel can only manage about 10-15FPS, and I am OK with this, but 25-30FPS or so would be nice I guess.

Resolution wise, 96x128 is acceptable, 128x128 or above would be optimal.

It does not need to be bright - it is always used indoors and typically in the dark.

Looking at @marcmerlin ‘s setup, I am not really getting why he needs an ESP32 and a Pi. Can this not be done with just a Pi?

In terms of control, I would want something similar to what I have at the moment (Some way to change what is currently displayed via an Android phone). I do not need upload capability while in the field - so I guess some kind of BT / WiFi connection would be needed.

I found this panel (I would use 2 of them) - I wonder if it is suitable, or maybe overkill?

https://www.aliexpress.com/item/1005003671026006.html?pdp_ext_f={“sku_id”%3A"12000026734908155"}&sourceType=1&spm=a2g0o.wish-manage-home.0.0

Thoughts appreciated.

[marcmerlin]

For why do I use 2 CPUs, it’s because I wrote all the code on ESP32 until it ran out of memory to drive my current 128x192 setup (it was at its limits with 64x96), so I kept my wifi code and web server on ESP32 and moved the rest to rPi with 3 parallel outputs.
96x128 might run on ESP32 with GitHub - mrcodetastic/ESP32-HUB75-MatrixPanel-DMA: An Adafruit GFX Compatible Library for the ESP32, ESP32-S2, ESP32-S3 to drive HUB75 LED matrix panels using DMA for high refresh rates. Supports panel chaining. but it will be close to a refresh limit you’ll be happy with on single output channel. Teensy v4 with smartmatrix will go higher res, but the FPS will drop to something you may not like.

I switched to Pi with 3 output to get 3x the refresh rate (important for pictures). Note that for FPS I don’t mean data push to framebuffer I mean the how many times per second the FB is pushed to the display.
You can totally do what you want with a Pi, and the flexible panel you gave, but that will be a toal res of 128x192, same as mine (except I use P2 and this is P1.875).
I do have to warn you however that for a wearable, the pixels on the edge will fall off, and that the flexing will randomly unsolder pins on the ICs on the back of the board, don’t ask me how I know
Long story short, you’re going to have repeated issues with wearable + flexible panels and I have not found a way around it outside of replacing panels as they fail (and they do)

For power, there is no miracle, I use 200Wh for about 12h of runtime.

[evilC]

@marcmerlin Thanks for your respones.

You can totally do what you want with a Pi, and the flexible panel you gave, but that will be a toal res of 128x192

I am thinking of just using 2 of them, so 128x128

so I kept my wifi code and web server on ESP32 and moved the rest to rPi with 3 parallel outputs

So you think I should be able to do 2 outputs and keep WiFi / BT on the Pi?

I do have to warn you however that for a wearable, the pixels on the edge will fall off, and that the flexing will randomly unsolder pins on the ICs on the back of the board, don’t ask me how I know

Yeah, I used to have a 64x64 flexible one too, and needed to resolder ICs, but mainly I think cos it continually got flexed - so I was thinking with the new one to make a 3D printed frame so that it only ever needed to be flexed once, when I mounted it to the frame. Did you try this approach? I was also thinking of applying a conformal coating to the LEDs to ensure that they were protected.

For power, there is no miracle, I use 200Wh for about 12h of runtime

So 8h would be about 133Wh. Am wondering how much of that is due to the two boards (I am guessing the ESP32 is a bunch less than the Pi). Also wondering how much extra having 192px vertical is affecting this. Is the power draw directly proportional to the # pixels? So 128x128 would be 20% less power than 128x196?

Also wondering what effect framerate has on power consumption? Probably minimal? In terms of LED power, surely no real difference, because that would be surely be a factor of the duty cycle (ie brightness) of the LEDs? It would only be the extra CPU power that would affect it?

I am guessing that the Pi is a huge factor in the power consumption, seeing as I currently drive 96x128 for 8+ hours on 36Wh (2x 21700 at ~18Wh each)

[marcmerlin]

Yes, a Pi can easily output 2 or 3 channels and do Wifi or BT. I didn’t test it myself, but I’m convinced it will be fine.

3D printed frame is indeed the solution to the not repeated flexing issue, I am not equipped to print this myself and I have a friend who offered to help, but as of right now, I don’t have this and it’s not reliable. And yes, adding coating to the LEDs will absolutely help. If you find the right coating that works for you, please share with me so I can try it.

For power, my ESP32 uses very little, most of the power goes into the Pi and displays. Note that I have 6 panels of 128x64 (front and back). Fewer pixels will take less power, but if you have fewer but bigger pixels, that will take more power again. In your case 1.875 is smaller even so it should be less power than me. Yes, the Pi uses a non trivial amount of power from what I remember measuring (well, depending on the brightness of your panels, I run mine at 50%)

Honestly, build what you need first, and then figure out how much power it will need by simply running it on an amp meter or timing how long the battery lasts before it dies. You can then indeed adjust a few parameters.

Also, please share the details on what you built, I’d love to see it (and let me know if you go to EDM festivals, if so we should meet at one of them, Dreamstate Socal is in 10 days )

[evilC]

I have 6 panels of 128x64 (front and back)

I take it you mean 128x196 (front), and 128x196 (back) ?

Ah, so with my proposed 1x 128x128 setup (1/3 the resolution), my power draw should be significantly less I am guessing.

1.875 is smaller even so it should be less power than me

Oh, I did not realize that, another reason that my power should be a bunch less.

200wh for 12h = 133wh for 8h, divided by 3 = ~45 wh for 8h? In that case, 6x or even maybe 4x 21700 might be feasible.

Also, please share the details on what you built, I’d love to see it

Presuming you mean my existing off-the-shelf setup? As mentioned, it’s from a Loy T3-HD backpack

Balls, did a big long post, only to find out I can only do one image per post as I am new.

Splitting up…

Back side - this is everything (Controller board is the PCB afficed with red glue - this is as it came). Batteries are 21700 so you can judge scale. Power bank is an Xtar PB2S

[evilC]

Front side

Oh god, need to wait 20 seconds between posts too

[evilC]

Panel holder front:

[evilC]

Back (Only showing one of the curved brackets - all 4 are the same so only needs to be modelled once)

[evilC]

Showing an image (Logo of the last rave I went to - a halloween do). The panel is held on with velcro, and there is a hole in the t-shirt (Just visible at the top right of the panel) that the power wire (USB-A) goes through. That image draws 0.6A (For both the driver board and the panel).

let me know if you go to EDM festivals

I do club nights in London, UK. 1988-1995 era stuff. Well, I say nights, they’re mostly daytime these days as the clientelle is typically old gits like me

If you’re ever in the area, drop me a line!

[marcmerlin]

thank you for all the details, sorry about the new user stuff, I just upp’ed your level hopefully it will fix future posts.
I think you also only have front and not front + back like me, so that’s 2x less power

Last tip is to use USB battery packs that can output 12 or 15V (20V is fine too) and get a USB trigger cable that tells the pack to give you that 12V and send it to a barrel connector. that’s what I use now and it’s easier to travel with USB packs and recharge them
if you haven’t seen my last post: Marc's Blog: arduino - Party LED Outfit Version 7: USB Powerbanks Powered Flexible LED Panels, LED Hat, and LED Shoelaces

I live in CA, sometimes go to a UK trance festival, but that’s not every day

[evilC]

I think you also only have front and not front + back like me, so that’s 2x less power

You’re 128x396 total (2x 128x196)?, I am 128x128 total, so 3x less surely?

Yeah, I have seen way too many LiPos go up in smoke (I do FPV drones too) to even consider using those

Thanks for the heads up about the thread inserts on those panels tho - I will be wary about that.

I just pulled the trigger on parts.

2x P1.875 panels: https://www.aliexpress.com/item/1005009530836946.html
(Different manufacturer than the other one listed, as a little cheaper):

1x RPi Zero W: Lowest power version of the RPi. From what I could tell from the readme, this should be plenty fast enough? (Edit: I see you use one too, so yeah, seems good) I seem to remember reading that you need the ribbon cables as short as possible, so was also thinking as this is the smallest board, then if I have to mount it under the panel (Would rather have it on my belt), then this would be a lot more feasible

1x driver board: RGB Matrix Panel Drive Board For Raspberry Pi [Version] - ElectroDragon
Seemed to be the recommended setup?

[marcmerlin]

ribbon cable length is fine for you, the issue is people with 6 or more panels chained.
do not use the Pi ZeroW, it’s a single core slow CPU, too slow for what you want. Get the Pi Zero 2W, that one is equivalent to a Pi3.
Make sure the panels you buy do use direct addressing chips and not the newer PWM chips that do not work. Also, you may receive ABCDE panels or ABC panels. The first ones are faster, but both will work. It’s very hard to know what they will send you before you get them. I still received the wrong ABC panels after having asked for ABCDE

[marcmerlin]

I worked with electrodragon to make a smaller board: https://marc.merlins.org/perso/arduino/post_2025-03-07_Party-LED-Outfit-Version-7\_-USB-Powerbanks-Powered-Flexible-LED-Panels\_-LED-Hat\_-and-LED-Shoelaces.html
It is passive, so I do not know if it will run as fast, but I recommend you buy both and use the smallest one that works at full speed