The NE555 timer can achieve a 50 percent duty cycle. The duty cycle adjustment range of this circuit is from 42 to 55 percent.

  Resistors R1 and R2 were selected first and then resistor R3 was selected to give the best control range based on measurements at the output of the timer.

  The major disadvantage of using the NE555 for this application is that the output frequency changes as the duty cycle changes