I have just completed Project #4 from the Arduino Projects Book, and I'm confused as to why the green LED seems so much weaker than the red and blue LEDs.
This has been discussed in other threads on the forum, which all seem to agree that the green LED just isn't as bright, and requires a much higher input (100x) than the red and blue to be visible:
Project 04 - The Green light of the multicolored led is very weak Project 04:
"I manually set the values to see what different ratios gave and the only way I could get a good green led was to set the r-g-b to 3-255-3. Even setting the red and blue to 5 made the light noticeably non-green."
I've even added another 220ohm resistor in parallel with the green anode to make up for its higher forward voltage drop, but it still wasn't visible with blue or red levels above 3.
The highest perception of brightness experienced by the human eye is generated with green light on the 555 nm wavelength. So you would expect, all esle being equal, the green led to appear brightest.
But all else ISNT equal;
Green LEDs are made in many different ways; the easiest (cheapest) are very inefficient.
The most efficient are a complex structure with different layers, so more expensive. You get what you pay for.
This table you can see the "cheap" LEDs made with Gallium Phosphide (GaP); they all have a forward voltage of about 2V. Notice for the green LEDs other types such as AlGaP have a higher forward voltage. They are more efficient.
All this can give a difference in brightness by 2-3 times, but not a ratio of 3 to 255 for R and G LEDs, as they say at the link
I think it's just a bad LED.
Undo that, you're just bypassing the LED current. As mentioned above, different color LEDs have different forward voltage characteristics which means you may need to use different value resistors to get the same current through them. Not sure if this is what you're seeing, but it's something you should be aware of.
According to the datasheet the forward voltage of the green LED was supposed to be 3.0-3.5V, which suggested to me that it was of higher quality. However, after your post, I measured the voltage drop and it was never more than 2.03V. Seems the starter kit dataseheets are outdated, or my kit had a component mixup.
It's not just the forward drop. It's also the efficiency (ratio of luminous output to current input). Voltage drop measurements aren't very meaningful unless specified at a typical current.
I just measured a 2.08V drop across the green LED with a forward current of 19.6mA (achieved with two parallel 220ohm resistors). Unfortunately I don't have the right datasheet, but doesn't this support the idea that my LED is an inefficient GaP?
Yes, it does support that. But there are now many different formulations out there, I wouldn't consider forward voltage as any definite indication of semiconductor type.
What is actually more indicative, is the luminance.
yes, it does "support" it.
The band gap of GaP is 2.24eV which corresponds to a wavelength of ABOUT 550nm (ie green) and requires around 2V of excitiation.
The band gap of GaN is 3.4eV which requires around 3.5V of excitation; but the corresponding wavelength is 360nm - which is in the ultra-violet. A multi-layer structure or clever use of a green phosphor allows it to emit in the green region.
