high power LED gradual turn on in a single arduino pin

Hi, I am not an electronics expert, but I want to make a LIGHTSABER and my idea is to make the blade turn on gradually (not scrolling up and down but from OFF to ON gradually along 1 second maybe?)

The problem is that this lightsaber blade is going to have up to 12W of power (4A x 3v) and as long as I know, Arduino pins cannot provide more than 40mA of current, To give this amount of power through an Arduino pin I would need a transistor working arround.

But what about the gradual turn on? does it work with a transistor too? or do I need any kind of electronic component that I don't know? Some schematics would be super helpfull!

thanks!!!

PD: I am using Arduino pro mini 3.3v (because you know, tiny space for electronics = tiny electronics) is that an issue for this project?

You use a PWM pin and fade up the LED.

However, you need a dimable constant current driver if you have such a high power LED.

I am using Arduino pro mini 3.3v (because you know, tiny space for electronics = tiny electronics) is that an issue for this project?

Yes it means you only have 3V3 to control the constant current drive.

This may get your juices flowing:

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I suppose you propose to power this from a 3.5V Li-Poly battery?

If so, you are going to need some pretty serious electronics just to power the LED, let alone perform PWM.