LED's dimming mysterious behavior with Nano

Ah, you want to fix it yourself! Well, I can't argue with that. It's commendable.

But if you change your mind,
"The language of electronics is a schematic or circuit diagram. Long descriptions of what is connected to what are generally useless. A schematic is a drawing of what connects to what; please make one and photograph it. We don't mind if it's hand drawn, scruffy and does not use the correct symbols. Please don't post Fritzing diagrams. They may look nice but they are very easy to misinterpret"

https://forum.arduino.cc/t/how-to-get-the-best-out-of-this-forum/679966#schematics-or-circuit-diagrams

Your hardware sounds a little off the beaten path, so images would help. Actually in this case, it would probably cut to the chase.