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gregstm
gregstm
Senior II
May 3, 2026
Question

Nucleo Leds are too bright

  • May 3, 2026
  • 6 replies
  • 597 views

Is there a prize for the most trivial post?

Opened up a new Nucleo-L476RG board (finally worked my way through the 10+ boards I bought a number of years ago) - it comes in a cardboard box now, nice.

The Leds (LD1, LD3) were so bright, I couldn't probe anywhere around the micro or connectors without squinting. So I replaced the resistors for these Leds (1K, 100?) with 22K, things are much more mellow now.

Most people are using these boards indoors, the Leds only have to be bright enough for people to notice - not beaming out like the Eye of Sauron. 

Anyway, something for you to think about. Meanwhile, there's a few annoying clouds outside I need to shout at....

    6 replies

    David Littell
    David Littell
    Senior II
    May 3, 2026

    What, you don't do all your debugging while wearing a welder's helmet?  Safety first!

      Mike_ST
      Mike_ST
      Technical Moderator
      May 4, 2026

      Hello,

      Thank you for your feedback.

      What board revision do you have ?

      It should be something like "MB1136-C02" written on a sticker on the back of the board.

      David Littell
      David Littell
      Senior II
      May 4, 2026

      Every single Nucleo I've ever used has had insanely bright LED's, so I'm not sure a specific revision of a specific board really matters.  Just sayin'.

        Andrew Neil
        Andrew Neil
        Super User
        May 5, 2026

        I thoroughly agree!

        That's why I have a sticker on the LED of this Nucleo ST-Link:

        AndrewNeil_0-1777977278328.png

        AndrewNeil_1-1777977326187.png

         

        @Mike_ST  -  As @David Littell said, it's always been thus - not specific to any particular model or revision.

         

        To be fair, this is not just an ST thing - most electronics manufacturers seem to be on some sort of bizarre competition to have their LEDs (especially blue ones) out-shine the sun!

        A complex system that works is invariably found to have evolved from a simple system that worked.A complex system designed from scratch never works and cannot be patched up to make it work.
          Mike_ST
          Mike_ST
          Technical Moderator
          May 5, 2026

          @Mike_ST  -  As @David Littell said, it's always been thus - not specific to any particular model or revision.

          Noted. As I use the boards myself, I understand.

          I will raise the point internally.

          It will not change the boards already produced, but hopefully for the future designs.

           

            LCE
            LCE
            Principal II
            May 5, 2026

            The ST developers are working in sunny lofts and penthouses, always close to the italian or french beaches, so they wearing sunglasses anyway!

            ;)

            We recently had the same problem with a board with some internal debugging-only green LEDs in 0603 + 1k to 3.3V.
            I never specified these in the BOM, so the assembler should just use what they got (low numbers, highly priced product :D ).
            Worked well for years, but recently they mounted some new crazy bright LED which was even with 10k brighter than anything we've seen before.

             

            I haven't used a Nucleo for some time now, but yeah, the red link-LED was always on the bright side (whistle!), maybe they even changed the LED type?

              Mike_ST
              Mike_ST
              Technical Moderator
              May 5, 2026

              >> I never specified these in the BOM, so the assembler should just use what they got (low numbers, highly priced product :D ).
              >> Worked well for years, but recently they mounted some new crazy bright LED which was even with 10k brighter than anything we've seen before.

              So, you know well what is happening.

              LEDs have evolved in efficiency. Setting 10mA for a LED might be not so relevant nowadays.

                Ozone
                Ozone
                Principal
                May 5, 2026

                I tend to agree as well. Really annoying if you want to measure something on the board.

                One suggestion is to use a permanent marker  (of you favourite color) to paint onto the LED to dim it down.
                Or use something like frog tape (adhesive tape used for painting), which is somewhat semi-transparent, so you can still see the light.

                  Andrew Neil
                  Andrew Neil
                  Super User
                  May 5, 2026
                  @Ozone wrote:

                  Or use something like frog tape (adhesive tape used for painting), which is somewhat semi-transparent, so you can still see the light.

                  Indeed - see my earlier comment!

                  A complex system that works is invariably found to have evolved from a simple system that worked.A complex system designed from scratch never works and cannot be patched up to make it work.
                  Ozone
                  Ozone
                  Principal
                  May 5, 2026

                  On a related note ...

                  I recently revived a FRDM-K64F board, which I had purchased back then when Freescale was still a thing.

                  Trying an example for the onboard RGB-LED nothing worked - or so it seems.
                  The code was fine, the GPIOs are set correctly and the voltages on the LED looked ok, but no light.
                  Not sure if it ever worked, though. Or if it bothers me much.

                  The funny thing is, the LED datasheet specifies a VF of 3.3 ... 4.0V for green and blue (2.4...3.0V for red). Which seems highly optimistic for a 3.3V MCU ...

                   

                    U
                    unsigned_char_array
                    Lead III
                    May 7, 2026

                    Technically it's not the brightness it's the illuminance.
                    These LEDs are so small that even with low brightness/luminous flux in Lumen they have high illuminance in Lux.

                    Adding a diffuser/frosted cover on top would help. The diffuser spreads the light over a larger area. I often add some paper on top of bright LEDs. They act as a diffuser and spread the light over a larger area and also block some of the light. And with RGB LEDs it also helps mix the colors. A PCB mounted light pipe can also help.

                    It's certainly not trivial. It's not good for your eyes.

                    unsigned_char_array_2-1778164628040.png

                     

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