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  • LED question

    I have this expression pedal with 2 LED's, a green and red that each indicate a different function. W/O going into a longwinded explanation of why i want to do this, i needed to have another LED in parallel with one of them so i can have two LED's lighting up when the green one is on. One will be in a different location on my pedal board. So i did this and it works but barely. The function works, but the LED i added is about 10% as bright as it should be. Barely visible at all. Yet the one it's paralleled with is as bright as it's always been. I figured they must have different voltage requirements. But if thats the case then that would mean the voltage is not enough to light the added one. However, that doesn't make much sense because i tried a bunch of LEDs and all had the same result, and i'm sure most were the typical 2.1v. So if thats not i maybe the current draw? And whichever it is it, what would i look for in a LED that will be as bright as the stock one when paralleled with it?

  • #2
    If I were doing that I'd use the same part number LED for each and put them in series rather than parallel. Then I'd adjust the current limiting resistor value to give the brightness that I wanted.

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    • #3
      Originally posted by daz View Post
      I have this expression pedal with 2 LED's, a green and red that each indicate a different function. W/O going into a longwinded explanation of why i want to do this, i needed to have another LED in parallel with one of them so i can have two LED's lighting up when the green one is on. One will be in a different location on my pedal board. So i did this and it works but barely. The function works, but the LED i added is about 10% as bright as it should be. Barely visible at all. Yet the one it's paralleled with is as bright as it's always been. I figured they must have different voltage requirements. But if thats the case then that would mean the voltage is not enough to light the added one. However, that doesn't make much sense because i tried a bunch of LEDs and all had the same result, and i'm sure most were the typical 2.1v. So if thats not i maybe the current draw? And whichever it is it, what would i look for in a LED that will be as bright as the stock one when paralleled with it?
      1) at the beginning you say "one" of them which means "any" of them , then you mention lighting when the green one does.
      So I'll assume you put a Led in parallel with the green one.
      2) different colours have different turn on voltage .
      Red is 1.9V ; Green is higher so if you measured 2.1V I believe you.
      Blue is much higher, don't remember now but 2.5V or higher wouldn't surprise me.
      So in parallel with Green, use *only* green.
      3) Leds have hugely different light output, as much as 100:1 .
      Pedal makers go with the best ones, to save on battery power, I do the same to make my amps recognizable on stage (no kidding, one Evangelical customer calls them "the Light of God"), while "regular" Leds, specially grab bag ones, are appalling.

      What you need is measured in "millicandles per mA" , as said there can be 100:1 differences.
      You'll find 10/20 mcd cheap ones and 5500mcd ones , here's even 20000 and 40000 mcd monsters.

      I really don't get why would anybody buy cheap Leds when impressive ones cost less than a buck each and very good ones 27 cents.

      At Mouser search for led water clear , then
      Category Optoelectronics > LED Indication > Standard LEDs - Through Hole
      then
      Applied Filters
      Illumination Color = Green


      and choose based on diameter and maximum mcd output; around 5000 is a good optimization between efficiency and pride.
      You'll end up choosing basically what the pedalmaker did.



      Most 10 cent or "assorted bags" Leds are the weakest ones, best go from 1$ to 5$ or so.

      In my book, worth it.

      You also need transparent plastic ones, best are "water clear" , which are transparent as glass, and not coloured, because colour comes straight from the Led die itself.
      You don't want "diffused" ones but transparent which have a built in lens, so they project a coin shaped dot on the palm of your hand, 4 inches away.

      There is a new "green" colour which is used in traffic lights, it's meant to help color blind people see it better, it "cheats" by having a little blue mixed in, and once you see it's emerald green colour you'll prefer it to any others.
      I do, at least.

      Not to be used in parallel with your green one but as power indicator on its own.
      Juan Manuel Fahey

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      • #4
        With different color LED's in parallel I think I would have each 'leg' of the parallel circuit include its own dropping resistor (sized appropriately) along with the LED rather than feeding both LED's from one resistor.

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        • #5
          I'm with Tom, just put them in series, then the same current lights both. In parallel you have to worry that each LED has the same junction voltage.
          Education is what you're left with after you have forgotten what you have learned.

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          • #6
            LEDs in parallel can cause brightness problems even if they're the same colour from the same batch. As Mark points out, you need each LED to have its own resistor to get even brightness. Especially so with different colours and you'll need to trim the resistor value for each LED to get the brightness the same.

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            • #7
              Originally posted by Mick Bailey View Post
              LEDs in parallel can cause brightness problems even if they're the same colour from the same batch. As Mark points out, you need each LED to have its own resistor to get even brightness. Especially so with different colours and you'll need to trim the resistor value for each LED to get the brightness the same.
              First of all, i tired red, green, and yellow with the same results with all. As to the trim resistor, in parallel that shouldn't matter if i use LEDs that draw the same right? The problem is we're talking about impossible to work with micromini surface mount stuff and no schematic. I guess the series option would be ok but i really don't want to disturb the board or cut the legs. Guess i'll just try some other and if no go i'll live with the yellow once which just barely visible. In respect to what was said about clear ones, I do have some of those so i'll try that. They are the dual color type with 3 legs so if they work i'll even have a color choice. The stock ones in the pedal by the way are the clear type, tho somewhat frosted looking like the ones i have.

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              • #8
                Interesting....i tried connecting a 1.5v cell to them and both lit fully ! Maybe the stock one requires a tiny amount of voltage and thats why the paralleled ones i've tried don't light much with the stock voltage? I can't test it now because i don't have the amp here but i wonder if thats it.

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                • #9
                  Originally posted by daz View Post
                  As to the trim resistor, in parallel that shouldn't matter if i use LEDs that draw the same right?
                  Even if they draw the same current, they can have different levels of efficiency, which affects the brightness.

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                  • #10
                    Originally posted by Mick Bailey View Post
                    Even if they draw the same current, they can have different levels of efficiency, which affects the brightness.
                    Ok, but look at my last post. I can't imagine the circuit would have less current than the button cell 1.5v battery i tried that lit them both equally.

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                    • #11
                      So you know they light up fine with 1.5V. What voltage did you measure across it when you had it lit up in circuit?
                      Originally posted by Enzo
                      I have a sign in my shop that says, "Never think up reasons not to check something."


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                      • #12
                        I didn't measure it. Are you suggesting maybe the LED in the pedal requires much less than 1.5v and thats why the added one was low? How low a voltage requirement do some LEDs go?

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                        • #13
                          Originally posted by daz View Post
                          Ok, but look at my last post. I can't imagine the circuit would have less current than the button cell 1.5v battery i tried that lit them both equally.
                          There will be a resistor in series with the power supply from the amp to the leds that will limit the current and cause the two to light up differently.

                          When each led is allowed to light up to full potential, each could draw different amounts of current from the same power source as long as the power source has enough strength to power both leds.

                          When you add the resistor each led will light up only as bright as the resistor will allow it to. If they both are exactly the same they will both light up the same. If there is a slight difference in the two leds, then they will not light up the same.

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                          • #14
                            Thats exactly what i figured. After all, in parallel there would be nothing different to each one and if they are the same why would they not light the same. So assuming thats the case, i tried about 10 LEDs figuring i've got to hit on one thats right. But nope. I guess i will just have to try a bunch more till i hit the right one.

                            Originally posted by 52 Bill View Post
                            There will be a resistor in series with the power supply from the amp to the leds that will limit the current and cause the two to light up differently.

                            When each led is allowed to light up to full potential, each could draw different amounts of current from the same power source as long as the power source has enough strength to power both leds.

                            When you add the resistor each led will light up only as bright as the resistor will allow it to. If they both are exactly the same they will both light up the same. If there is a slight difference in the two leds, then they will not light up the same.

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                            • #15
                              Originally posted by daz View Post
                              I didn't measure it. Are you suggesting maybe the LED in the pedal requires much less than 1.5v and thats why the added one was low?
                              Yes. It's probably a very low power consuption LED for pedal use to save battery life.
                              As was mentioned, it's all about the efficiency of the LED (the "millicandles per mA" that Juan mentioned).
                              It must be less than 1.5V where you are connecting it, otherwise it would light up the same as with the battery.
                              Originally posted by Enzo
                              I have a sign in my shop that says, "Never think up reasons not to check something."


                              Comment

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