14 volts and PWM with Atmega8
Here is a simple circuit for controlling a LED from a 5V digital output while powering it from a higher voltage:



When the digital output is low, the transistor is off and the LED unlit.  When the digital output is high, the base of Q1 is held at 5V.  With the emitter resistor, this causes Q1 to act like a reasonably constant current sink.  If the B-E junction drops 700mV, then there will be 4.3V accross R1, which makes the current close to 10mA.  At long as there is sufficient voltage for the transistor to sink this current, the current will be largely independent of the supply voltage.  This also a good design if the supply has a lot of ripple but you want stead LED light output.

Let's say you are using a green LED that drops a little over 2V.  The collector of Q1 can go down to a little below 5V and still act like a current sink.  That means as long as you have around 7V or more on the anode of the LED, it will be driven with 10mA when the digital output is high.