I have just been amusing myself by making a colour LED strip display for the voltage output from a pot; blue green yellow red, 4 LEDs, simple logic.
It all works fine, except that for some reason the green LED is much dimmer than the rest.
I have
a) swapped in another green LED in the same spot. it is also very dim. so it is not an issue with that particular, individual LED.
b) swapped the green LED with its blue neighbour. it remains dim in the new spot, where the blue LED was nice and bright. and the blue LED is nice and bright in the spot where the dim green one was.
I therefore conclude that it's neither a low voltage on the output port nor a wiring problem with the green LED's slot in the breadboard, but something about the green LEDs themselves. could this just be a bad mfr run? I know LEDs are cheap as dirt so I'm not particularly upset, just curious as to why one colour should be so dim compared to the 3 other colours.
If you buy the cheapest one you can find you may not get a manufacturer's part number so you can't check the datasheet.
LED light output(for regular "small" LEDs) is specified in mcd (millicandelas) with a given current (mA), and if you check the datasheet and buy different colors from the same manufacturer's part-series, they should be better-matched.
Different colors have different voltage-drops, so you may have to adjust the series resistor value to get the same current through different colors.
Interesting. If it is a duff LED then more than one of my green ones is weak, 'cos I did try swapping in another one.
I used the same pulldown resistor on all the LEDs per the instructions, 220 ohm I think. I don't think I have any lower value resistors to try (haven't really tooled up for Arduino fun and games yet).
I've just been googling this subject, since I have found exactly the same phenomenon with one of my projects - the green LED's are dimmer than either the red, yellow or amber.
Putting a much lower resistor inline with the green's does make a difference, but even so the green is dimmer at 8ma than the other colours at 5ma.
I've tried it with 5mm, 3mm, and other different shapes of LED bought at different times (so should be from different batches) and of course swopping the LEDs around in case the wiring or resistor is causing the problem, but the results are the same.
Since the original post, has anyone come up with a reason for the phenomenon?
The answer you are looking for, is in reply 2 by DVDdoug and in the last line.
You need to get datasheets for each LED, so you can find out what voltages and currents generate the same luminosity.
There will be some tolerances you might want to try to compensate too.
Someone's eye might register different luminosity at different colours compared to the person next to them.
So just focussing on current won't help you much.
This is also why it isn't real easy to create a large pixel display that will display an even coloured surface when lit by separate LEDs per pixel.
it seems that the colors on the upper end of the magnetic spectrum require more power. Green, Blue, UV
(I think I read that somewhere)
also LED's can have "lens focus"... that can make one look brighter than another when view at a different angle
Thanks MAS3, I should have mentioned that I do have the datasheets for the LED's (or at least an extract from them from the suppliers) and the different colours do, indeed, have differing forward voltages and power (the MCD), these I have been juggling with to see if I can get the greens up to the brightness of the others.
I've attached a spreadsheet that I prepared using the datasheets. I would have expected the greens and yellows to have similar brightness, but they are nowhere near!
LandonW - quite right, when viewed from the side the LED's do differ, so I've focused at looking at them from above. The 3mm Reds are vastly differ in brightness when viewed from the top compared to the side.
I even tried removing my specs (which have 100% UV protection) to see if that made a difference, but alas not!
Ho hum, just have to experiment a little more, but it is a curiosity.
Please don't hate for adding to an older thread. And I am aware that biological sensor arrays will vary according to the operator. This topic still seems relevant as I have recently begun to delve into the mysteries of the Arduino way. I too was wanting to know what I was doing wrong for the green and yellow led's to be so noticeably dimmer than the rest. Turns out, nothing is wrong. Every different color of led from every different manufacturer and from each individual product run will have varying levels of input, ouput, minimums, maximums and input/output curves. This of course has already been stated. On to the fun stuff:
As soon as I learned how to script with the pwm side of things I wrote a sketch to play with the rgbLed. And of course noted that it too had varying amounts of output to each of the colors. So, I diddled with the upper threshhold for each channel until they had seemingly equal outputs at peak. Here is where biologic array comes in to play. Keeping my eyes a similar distance from the Led at a proper angle so that all outputs were in view (the die and lense are not a perfect match so you'll have to play with the thing to find the right angle and distance), I changed the upper limit until each junction sturated my retinas with a relative diameter that seemed to be about 2mm. Actual size of saturation is moot, so long as each color reached the same saturation point and appeared to be the same brightness.
Anyone can do these things. When all else fails, just chase each trail you come across 'till you arrive at the result you're looking for. Hey You! Staring into led's is a bit reckless. Don't run the thing full throttle right off or you could hurt yourself. And definitely don't stare into lasers. ZipZap! How'd you get that hole in your brain?
If there isn't enough description in the sketch, give a shout.
Side Note: I am thoroughly enjoying the Arduino starter kit and learning how to program. The purple thing is a rabbit trail. A way to incorporate something a bit off while maintaining the integrity of the rest of the sketch and circuit. Kind of a brain game for me.