looking for low current switch LEDs, any proposals?

[Eszett]

Hi! I'm considerung using Switch-LEDs for my keyboard, all in series, driven by USB. So without multiplexing or LED driver chip, I have only about 300mA to spare.. So my question is, are there reasonably cheap switch-LEDs with only a few milliamps forward current, e.g. 5mA? I couldn't find any!

criteria:
- small enough to fit in standard Cherry MX switches, e.g. 2x3x4 mm
- should be white
- should have betw. 1 and 6 mA forward current
- reasonable price

Do you have any recommendations?

[vvp]

You will be able to connect at most 2 LEDs in series since they typically have forward voltage about 1.8 V or more.
And 3*1.8 > 5V you get from USB.

[Eszett]

Sorry, I mean in parallel, of course..

[vvp]

I want to try these: KINGBRIGHT L-7104ZGCK
But I want to use a matrix with a led driver, e.g. STMICROELECTRONICS STP08CP05MTR
So that I do not need to bother with proper currents for possibly different LEDs.

But I did not get to it yet (and probably will not get to it within a year) so I do not know how well it would work.

Do not search based on LED forward current rating. Almost all of them have it at 20 mA. But they will work even with 1 mA. Concentrate to find LEDs with the biggest brightness per 1 mA. You want highly efficient LEDs so that they are bright enough even at low currents.

Good luck.

[Ratfink]

vvp, I can tell you first-hand that those ST LED driver chips are great. I used the 24-output version in a rather silly project of mine, and it performed admirably as an LED matrix driver. If I were designing a keyboard with backlighting, I'd definitely reach for one or more of them.

[vvp]

Good to know!

I would not bother that much with led driver. But these simple shift register like led drivers are dirt cheap, suitable for a quick software control in a matrix connection, and I can combine different LEDs in the same matrix without fiddling with resistors. The higher level (and more expensive) drivers with a bunch of embedded functions (like e.g. pwm brightness settings) seemed like a pain to use in keyboard matrix where I want to control each led brightness individually. Well, the worst think about the higher level LED drivers was that their settings were slow to change. I was looking for something quick as simple. So that I can e.g. share LED matrix columns with key matrix columns. Therefore it must be quick if I want a low latency switch scanning.

Maybe I'll get to LED backlight sometimes but it is low priority for me.

[Eszett]

@vvp Ah, so there aren't any 5mA LEDs with the dimensions 2x3x4mm at all? Alright. Your proposal to drive high-efficiency LEDs (which are specified for 20mA) with 5mA sounds interesting... I'm considereung that right now.

[vvp]

Maybe there are LEDs rated at 5 mA. But there will be few of them. Consider the forward current rating more like a maximum you should not get over. Not like the current you actually must drive the LED at. You will find a chart of Luminous Intesity based on Forward Current in the LED data sheet. This will be almost always linear. Therefore, you can just assume that when a LED is rated with e.g. 1.9 cd at 20 mA then it will give you 1.9/20*5 = 0.475 cd at your 5 mA. Search for LEDs in T1 (3mm) package. These will fit (if you do not use some non-standard extremely flat keycaps) and there will be a lot of them.
Maybe read also this post: post286360.html#p286360
Just so that you know how to select the right resistor for your LEDs.

[Eszett]

Consider the forward current rating more like a maximum you should not get over.

Yes, I had this error in reasoning, thanks for pointing me on the right track.