This is getting a bit far from the topic of Raspberry Pi and more towards the topic of electronics in general, but I decided to share some info anyway (related to this project), somebody might find it really useful
This morning I decided to experiment a bit with white, blue and orange LEDs, to see how far I can starve them and still get acceptable light from them (how little current is enough). The results are very surprising (at least for me). As you know the current recommended by the manufacturer for standard LEDs like these is 20 mA. Well, I found that giving them 20 mA is a huge waste
At 5 mA their light is barely less than at 20 mA (at 5 mA their light is still so strong that it hurts to look right into them). But what is even more surprising is that their light starts to decrease noticeably only betwen 1-2 mA and they still provide quite acceptable light at only 0.5 mA!!! I did nt starve them even more, but I'm guessing the limits can be pushed even more.
It seems that the blue LEDs give the strongest light at very low currents, the oranges are next in line and the white ones lose most of their light at very low current values. Well this is true for the 3 LEDs I experimented with, perhaps the differences come not from the color, but from tiny manufacturing differences betwen these 3 particular LEDs. Perhaps such differences exist among different instances of the same color.
Hopefully my multimeter is accurate and has not fooled me. Also, I did these measurements with a power supply that has an open circuit voltage of 8V (which surprises me quite much because it's a Nokia phone charger and it's clearly labeled with 5V @ 350 mA). Hopefully at the 5V coming from the Pi the behavior of the LEDs won't change.
I decided to add individual adjustable 10K resistors to each LED (in line with a regular one calculated to let 5mA through), so that I can adjust their brightness from 5mA to less than 0.5mA because I intend to run the led clock all the time and at night it can be quite disturbing when 17 LEDs give a strong light and you're trying to sleep in the same room