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How do I calculate the current draw of an LED if I don’t run it at full power?
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mariush:
There are surface mount leds that can easily fit inside keys or besides keys, and they're not expensive (well, they're a bit more expensive compared to those aliexpress but not by much)

TME.EU - White surface mount leds
Digikey - White surface mount leds

0603 are a bit difficult to solder wires to but with a bit of patience and maybe a magnifying glass it's possible. 0805 is fairly easy. Bigger sizes are super easy. You can just tin solid core wire and the terminals of the led and then place the solid core wire on the terminals and tap with a soldering iron to make the connection and that's it, you have through hole leds.

Here's a couple examples:

0603 : 3.8 cents each in packs of 50:  https://www.tme.eu/en/details/ll-s194pw-w2-1c/smd-white-leds/lucky-light/
 LUCKY LIGHT LL-S194PW-W2-1C
LED; SMD; 0603; white cold; 200÷250mcd; 130°; 20mA; 2.8÷3.8V

2216 : 3 cents each in packs of 50: https://www.tme.eu/en/details/rf-k30ti16ds-ee-y/smd-white-leds/refond/
 REFOND RF-K30TI16DS-EE-Y
LED; SMD; PLCC2,2216; white warm; 6÷10lm; 2850-3050K; 90; 120°; 20mA, 2.7v..3.3v

As for how to get most efficiency, your best bet would be to boost the voltage to around 16-24v using a step-up (boost) switching regulator, and then use led driver ICs to control the current.
For example, let's say you go with the 2216 leds above, with a maximum forward voltage of 3.3v
You could go with 2 or 3  8 channel led drivers. This gives you 16 or 24 channels to play with.

With 16 channels, you could have 6 leds per channel for a total of 96 leds but you would need a voltage of minimum 6 x 3.3v = ~20v
With 24 channels, you could have 4 leds per channel for a total of 96 leds, and the voltage is lower at 4 x 3.3v = 13.2v
With 32 channels, you could have 3 leds per channel for a total of 96 leds, and the voltage is lower at 3 x 3.3v = 10v

Most LED drivers will need at least 0.5v..1v above the sum of forward voltages to work properly.

Led drivers with lots of channels are super cheap and super easy to use, basically shift registers. They can be chained together so you send data to just one chip and it's very easy to turn on and off leds individually if you so desire.
You can set the maximum current using a single resistor (for example 10mA) and then some shift registers allow you to send a command to adjust the brightness in either 0..100% or 256 levels ... basically, they reduce the current from maximum set through the resistor.

Here's some examples of such chips :

STP08DP05TTR : https://www.digikey.com/product-detail/en/stmicroelectronics/STP08DP05TTR/497-6029-1-ND/1632058
8 channels, up to 100mA per channel and 20v per channel , set max current through a resistor and can adjust "brightness" through a pin.

STP16CPC26PTR https://www.digikey.com/product-detail/en/stmicroelectronics/STP16CPC26PTR/497-11923-1-ND/2757659
STP08DP05TTR https://www.digikey.com/product-detail/en/stmicroelectronics/STP08DP05TTR/497-6029-1-ND/1632058
16 channels, up to 90mA per channel and 20v per channel , set max current through a resistor and can adjust "brightness" through a pin.
 
Here's loads of others with 8, 16 or 24 channels .. most of them can be chained together, or in the case of i2c drivers you can give each a unique i2c address : link to digikey led drivers[u/url]

Now you need a voltage regulator that would boost the 5v to 11v (for 3 leds per channel) or 14v for (4 leds per channel)
For example, here's a there are step-up (boost) led drivers, which you can power from 5v and can produce up to 12-16v

MIC2295 : [url=https://www.digikey.com/product-detail/en/microchip-technology/MIC2295YD5-TR/576-1080-6-ND/1858303]https://www.digikey.com/product-detail/en/microchip-technology/MIC2295YD5-TR/576-1080-6-ND/1858303
(datasheet)

MP1541 : https://www.digikey.com/product-detail/en/monolithic-power-systems-inc/MP1541DJ-LF-Z/1589-1858-6-ND/9555235
(datasheet)

MP1542 : https://www.digikey.com/product-detail/en/monolithic-power-systems-inc/MP1542DK-LF-P/1589-1043-1-ND/5298130
(datasheet)

MP3209 : https://www.digikey.com/product-detail/en/monolithic-power-systems-inc/MP3209DJ-LF-Z/1589-1906-1-ND/5292266 (datasheet)

LMR62014 :  https://www.digikey.com/product-detail/en/texas-instruments/LMR62014XMF-NOPB/LMR62014XMF-NOPBDKR-ND/3528062 (datasheet)

LM2731 (X or Y version) : https://www.digikey.com/products/en/integrated-circuits-ics/pmic-voltage-regulators-dc-dc-switching-regulators/739?k=lm2731 (datasheet)
x version is 1.6 mhz, y version is 600 kHz ... lower frequency is more efficient.

rdl:
There are indicator LEDs available that need only a few mA to work. Some are plenty bright even at less than a mA.
Blitzschnitzel:
Thank you, guys, so much for the help! So, I finally had time to dive into all the Datasheets and I will go with the IS31FL3731. There are even commands for this IC in the QMK firmware that I’m going to use. 
http://www.issi.com/WW/pdf/31FL3731.pdf

I don’t need the microphone or hardware shutdown, so would this setup work fine?



I am not getting completely wise from the documentation how the current limiting works. Does the chip run the LEDs at the input voltage (5v) and only limits the current?
The documentation also says not to go lower than 18k ohms for the current limiting resistor. But I only need half the possible LEDs on one side’s matrix and 2/3rds on the other side. Can I then use an even lower resistor accordingly? Meaning lower resistor more forward current brighter LEDs?
rstofer:

--- Quote from: Blitzschnitzel on September 11, 2019, 06:03:18 pm ---I am not getting completely wise from the documentation how the current limiting works. Does the chip run the LEDs at the input voltage (5v) and only limits the current?
The documentation also says not to go lower than 18k ohms for the current limiting resistor. But I only need half the possible LEDs on one side’s matrix and 2/3rds on the other side. Can I then use an even lower resistor accordingly? Meaning lower resistor more forward current brighter LEDs?

--- End quote ---

The chip will deliver current based on the resistor.  It is not delivering 'voltage' per se, it is measuring LED current and the voltage required to create that current is whatever it is.

So, when the folks who write datasheets tell you to not go lower than 18k Ohms, there is probably a reason.  Excessive output current for example.  Chips have at least two current limits:  One for each pin individually and one for the package as a whole.  The 18k is probably protecting the pin current because the designers have no idea how many output pins will be in use.  The value will also be safe for the entire package.

OR, try it and see.
Blitzschnitzel:

--- Quote from: rstofer on September 11, 2019, 06:44:48 pm ---So, when the folks who write datasheets tell you to not go lower than 18k Ohms, there is probably a reason. [...]

--- End quote ---
Ok, I'll leave it as recommended then. ;D

Does the setup above look OK? SDB needs to be pulled high so it won’t go into shutdown. But I don’t really need the resistor there, right? If it runs on 5v can it handle it on the SDB pin?
For C_FILT it says "Capacitor used for audio". Since I am not connecting a microphone, I can also ditch that, right?
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