Difference between Arduino 5V power and output of 7805 5V power

Be very careful using a potentiometer to control the LED, if you turn it too far you can end up having much less resistance in series with the LED as you think = BANG.

Also know that the 10k pot may not be linear. If for example it is a log pot, then half way is not necessarily 5k, in fact it is usually only around 1k.