What value resistor do I need for a transistor base?
Hello, I am trying to work out what resistor I need to use on the base of a 2n3904 base. I am using a 555 timer to flash some leds all powered by a 12v power supply. There are 4 leds in series each with a forward voltage of 2.1v and they require 50ma. I have found that I need a 82 ohm resistor to protect the leds using a online calculator.
Pin 3 is driving the transistor base and the transistor controls the leds but I do not know what resistor I need on the base.
Any help is appreciated!
Edit: here is the finished circuit. http://vimeo.com/33513619