Sodding Blue LEDs...

Just needed to publicly express my frustration with these things.

It’s my first time trying to use one; and it doesn’t seem to matter how much resistance you put in, an eyeball-melting blue light comes out.

It’s now “dimmed” - I know this because all of my spare resistors are plugged into the breadboard and it is now possible to look at the sodding thing without an accute pain tearing out from the retina. But you couldn’t say it was “dim”, oh no.

The woes of high efficiency ;D

It may not be the “brilliance” of the LED that’s the problem but the sensitivity of your eyes. We have a cooker with a blue LED display and the colour appears to cause excessive flourescence which I feel is possibly a spectral sensitivity peak of the human eye. Yes I know it’s supposed by somewhere in the orange range. But have you noticed how a flashing blue light on top of certain motor vehicles also draws your eye.

Apart from that, dimming of LEDs is best achieved by PWM rather than simple current limiting resistors.


You could be right about the blue thing.

But unfortunately I can’t use PWM, this LED is destined for a circuit that the Arduino won’t have control over. It won’t need to be dimmed in the traditional variable sense - just preset to an intensity that doesn’t cut through things. :wink:

Blue LEDs come mostly in transparent housings; of course other colors as well, but we are used to green, red or yellow LEDs in scattering colored housings. This makes an enormous difference in luminous density.

have you tried coating it with a blue magic marker

Hahahah, that’s a proper DIY hack right there mate. Excellent.

I’m sure I asked for a diffused casing but they gave me clear. So it pumps out a huge amount of light in an upward direction (imagining the LED is plugged into a breadboard) and not that much in the other directions.

Unfortunately I was stupid enough to be leaning over the breadboard when I plugged the thing in with only 330r. I was quite shocked (read: blinded) at the power.

Definitely cool tech though.

Hey mate, try roughly sanding the top to give you a nice diffused effect, those bloody LEDs set out to blind us :wink:

Blue is down the spectum so apear brighter, you could diffuse the led with some fine sandpaper

I love bright blue LEDs but I do agree that you don’t want to look directly at them. The SMD wide angle LEDs are even worse.


Got myself a new pack of resistors and managed to mock-up the circuit without further damaging my eyesight.

Rather than create a new thread, I was hoping someone reading this wouldn’t mind confirming if this is safe for the Arduino.

Basically, I want

  • Arduino digi pin 12 to be “disconnected” (or whatever the term is for the not-high-not-low state); this means that 5v flows through the 100k resistor to the LED and it’s a bit dim.
  • Then, when the Arduino decides, it can send pin 12 high and give the LED a boost by applying voltage through the 10k resistor to a bright state (although not corrective eye surgery bright). I worked out that 10k and 100k resistors in parallel end up giving me about 10k resistance in total.

The bit that concerns me is switching the pinMode of digi 12 so frequently. Is this likely to cause problems?

Here’s the circuit and a bit of test code (it all works).

void loop()
  pinMode(_LEDBOOST, INPUT);
  digitalWrite(_LEDBOOST, HIGH);

Instead of that 100k ohm path to keep the led on dim you might just want to see if turning on the digital pin’s pull up resistor might work. That would save you a resistor and some wiring.

void loop()
  pinMode(_LEDBOOST, INPUT);
  digitalWrite(_LEDBOOST, HIGH);  // this will power the led with the 
                            // internal pull-up resistor at around 40k-50k ohms
  digitalWrite(_LEDBOOST, HIGH);


Hi Lefty,

Unfortunately, the supply line running through the 100k resistor is kinda part of an existing circuit that needs to stay. (Although I think it currently only has about 120R in there so I’m ramping it up). Essentially, I’m changing a red LED in a power-on/reset circuit, to a high power blue one and wacking in an extra feed to boost the LED luminosity when the Arduino (which has been inserted into the box) is influencing things.

Obviously there’s no way you could have known all that. :slight_smile:

In this circuit, I was trying to simulate the existing part of the circuit and then add in the new part.

There is no problem with changing the pinMode a lot.

Example of where it is done frequently:
To read a 4 wire touchscreen, you change the pinmode twice every time you want a reading.

That’s good to know - cheers Sci.

Blue is down the spectum so apear brighter

The human eye (I assume we’re talking about human eyes here) is least sensitive to blue light (lower frequency then green).

You see this in the weighting given to blue in the RGB to YUV conversion, which is less than half that given to red.

Peak sensitivity of the eye is in the green section of the spectrum.

We have a cooker with a blue LED display and the colour appears to cause excessive flourescence which I feel is possibly a spectral sensitivity peak of the human eye.

What you are seeing is the fact that blue light is bent less during refraction than any other colour. Consequently is is more difficult for the eye to focus blue light. This explains why blue snooker balls appear to have a glow round them.

There used to be a dolly blue factory in Backbarow in the English Lake District.

The surroundings were covered in the stuff and this gave everything a soft edge that was most unpleasant to look at. It has since been cleaned up and now you would never know.

Finally a photon of blue light has more energy in it that a photon of any other colour so this gives a greater kick to the retina when on impacts on it.

@Groove, ok so i have wierd eyes that are extra sensitive to blue and purple, now that i’ve had cataracts removed i see the world in a whole new light so to speak including sensitivity to ultraviolet which is normally blocked by the human eye but not by the synthetic lenses they implant

@ GrumpyMike,
I remember the dolly blue factory (my granny’s shop used to sell it in little muslin twists) and the river below the factory also ran blue!
Difficult to imagine that level of obvious pollution being tolerated these days.

I once read that cataract patients were employed as spies to read morse from UV lamps from subs offshore.
(Don’t know how true that is, nor can I remember where I read it!)

I haven’t heard that one, alot of people who have had this done say colours are more vivid, im used to it now but to start with everything looked purple for a while even clear blue sky