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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.
BootySnorkeler
@AkosLukacs
With a regular diode like 1N4007, you have about 0.6-0.7V voltage drop. So with a 3V coin cell you have ~2.3V for the LEDs, with a schottky you would have ~2.7V.
It will be better with a schottky, but several LEDs can be a pretty heavy load for a coin cell...
What color LEDs do you have? (forward voltage depends on color). Do you have any current limiting resistors for the LEDs? Maybe you can can change the current limiting resistors to lower value ones. But do the maths, so you don't burn the LEDs if you power it from 3.3v
Or just remember to only power your board from 3V source (just use 2*AA batteries if you don't have a variable power supply)