How to calculate simple transistor switch. How to light LED in the dark

I needed to make LED light in the dark, so I found this video, where Ben Krasnow explains how to make a simple transistor switch.

LED driver.

Driver is a thing which gives constant current regardless of what you will connect to it.

LED's work on current. They consume needed voltage without noticing it. If we connect LED to 12 volt DC, he consume needed (for example) 2 volts and spare voltage will go further. (if there is nothing else all 12 volts will be on LED and it will most likely burn).

LED matrix

I decided to play a little bit with LED matrix, so i will write what i found out.