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Ah, ok. I meant the 2*AA suggestion while you are developing, so you don't waste a lot of coin cells. Blue LEDs forward voltage is right around 3V or so,
Gordon's circuit maybe the easiest way forward, as there is no diode waste running from the battery. And no different voltage levels when powered from USB or battery.
AkosLukacs
Hey @AkosLukacs, I have two multiplexed 7-segment LEDs and five more LEDs each with their own GPIO pin, all colored blue. All of them are tied to a 330 ohm resistor in series. The microcontroller I'm using operates on 3.3v logic so this might not be necessary, I just tossed the resistors in for safety. The LEDs are fairly visible during normal use, although not at their peak brightness. My circuit is a wristwatch, so I don't have the real estate for two AA batteries.