Electronic – Safe LED Operating Current

Power seems to be an important aspect in your design, so you might consider using different LEDs. Low-current LEDs for instance take 2mA instead of 20mA.

Or maybe 5mA through your existing LEDs will produce enough light for your taste.

Another angle would be to design your LED drive circuit for as low a voltage as you can. If you can design it for 3V and you use a switched power supply to create this 3V from 5V (or maybe 12V), you have saved 40% energy. This is especially effective when you use a battry, which produces a lower voltage over time.

I guess that when you want to iluminate (nearly) all LEDs you can get by with illuminating them each a bit less. This logic could be incorporated in your software, so the worst case current would be reduced.

You must design your circuitry for the worst case, but for battery life it might be more realistic to calculate with averaged cases. So get some more info on what you want to show when you need to know how a given battery will last.

As often, your one good question results in an avelange of questions, and some questioning of your basic assumptions. That's system design :)

A PIC output does not behave like a direct connection to a power supply.

The output of a PIC operated from 3.3V would behave (roughly) something like a 30-150 ohm resistor, depending on a number of factors.

The shape will be similar to this (5V) but the numbers will likely be quite different:

The LED has a voltage-current curve like this:-

As you can see, with 3.3V directly across the LED, the current will be very high (it will behave a bit differently off the graph).

100mA will damage most small LEDs fairly quickly. You'll also likely see the color change as the chip overheats (it will move in the direction of longer wavelength as the chip heats). The opposite of when you dip an LED in LN2.

On the other hand, when you connect it directly to an output, the output acts sort of like a current limiting resistor (though not a very predictable one), and it probably will not see a destructive current (not guaranteed).

Bottom line, if you want a predictable and safe current, you should use a current limiting element of some kind- a resistor is usually a reasonable choice.