Single transistor LED switch

I couldn't find anything really useful quickly about picking resistor values and transistors to drive LEDs at the right current so I put together a blog post with a simple circuit and explanations of the various current and resistor calculations to help non-engineers.

Arduino:figuring out transistors and associated resistors

Hope this is helpful to someone. I'll be building this up layer by layer as time goes on.

This is very helpful - thanks!

I printed it out and stuck it on my wall.
Thankyou for the great explanation.