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Need help, uneven backlight on a QMK converted chinese keyboard
andresteare:
--- Quote from: nevin on Tue, 26 January 2021, 16:35:18 ---that would be overkill, but on the right track.
...you could probably use that one little board to drive the leds for the whole board.
i wish we knew the specs on the leds that are used in your board... voltage, current draw....
- sometimes different single color leds are driven at different voltages/current.
- i don't think you said.... but what's the brand/model of keyboard you're modifying?
i'll do some digging and see what i can come up with for you.
some info for starters....
https://geekhack.org/index.php?topic=55338.0
from link above:
--- Quote ---Even if you do use PWM, you still need a high current output which a typical microcontroller cannot provide. So again you'd need a buffer or to use some dedicated chip.
--- End quote ---
and you can read some of komar's explanation of how he was thinking of implementing it in the GH60
http://blog.komar.be/gh60-evolution/
--- End quote ---
Ok, I'm going to throw all the info that I think would have a chance of being useful
Brand: Gamdias
Model: Hermes 7 Colors
MCU: Evision VS11K08L, didn't found the manufacturer's datasheet, all I know it's a QFP-48 format MCU (till yesterday my dumbass thought QFP-48 was an IC design and had a datasheet from a Silicon Labs one, I recently realized QFP-48 it's a form factor so I still don't know anything about the MCU lol), I guess I can try contacting the seller to ask for the datasheet.
After hours with the multimeter I identified most pins on the MCU
This makes me question the LED circuit as a matrix because there are not much pins left on the MCU to asign LED columns, also, if I can (not greatly) make them work by just wiring a resistor that goes to a transistor conected to ground.
my guess is that it's not a matrix but a paralel circuit for each row? but after taking a look at the links you provided and checking the pictures of the front side of the PCB I don't see any resistors in that side, though there is a resistor-transistor cluster near the MCU
This and seing videos of this keyboard which show that keys can be individually illuminated makes me think "A-Ha! there's the matrix" but there are only 7 pins available in the MCU and 21 columns in the matrix (at least in the keys matrix). Also, the article mentions that a paralel circuit works by putting a resistor for each LED which doesn't look to be the case here. So the matrix circuit seems more possibly to be the case.
I'm dumb, I just realized, while writing this reply that the resistor cluster I mentioned before contains exactly 21 resistor-transistor-resistor groups, which I guess it totally confirms the LED matrix hypothesis. But where do they wire the columns to? There are 7 pins left in the MCU and there is no other IC present in the board. And if it's a matrix, why is it lighting up just by connecting the end of the LED rows?
I didn't take much pictures of the front side of the PCB and I wished I mapped it, but here are some pictures of the front side:
Hope I provided some relevant info, thanks! :)
nevin:
with this info, my guess (without seeing a schematic) would be they are driving the columns + with the resistors & fets (21) and using the rows as the - connected to the MCU to turn the rows on and off.
- it's better to drive shorter strands, as you've seen.
- i saw another suggestion to power them at multiple points on the string (row) instead of just the one end. (i'm not sure how you are driving them... + from mcu or -/ground to mcu)
- also the power circuitry can be before or after the led strings, see this for some general info: http://jumptuck.com/2011/11/07/using-leds-in-your-circuits/
some package sizes to try & determine what size leds are used in the boards: 5050 (5x5mm), 3535 (3.5x3.5mm), or 2020 (2x2mm)
some data sheets:
3528 https://www.iled.com/class/INNOVAEditor/assets/YeniDatasheets/4001-4005-4024-4025.pdf
- you can see on page 4 the different current & voltages for these particular leds change depending on color (use only as a reference)
5050 (RGB) https://cdn-shop.adafruit.com/product-files/2757/p2757_SK6812RGBW_REV01.pdf
- this is for SK6812 which is a popular alternative for the WS2812B
- this is an RGB led but useful for size specs & current draw info. an RGB draws more power than single color leds as there are 3 elements in an RGB led
what about cutting traces to the pcbs mcu and using the circuitry already on the pcb (resistors & fets) to drive the leds?
and from your measurements and the spec sheets and how it looks like they are driving the leds in columns, i'll bet 2.6v with enough current to drive the leds at 20-25ma per led will be the answer. (6 leds in a column, so 120-150ma per column @ 2.6v) 6*20-25ma
hope this info helps and gets you pointed in a forward direction. you've done a lot of work already. :thumb:
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