Im agree that PWM is the unique (good) solution.
If you want to do without any soft and without regulation (0/1):

a) Put the famous 555 working as oscillator and control it's reset pin

b) Get an square wave (from ... ?) and put one AND gate; one input to 8051 the other to the square wave and the output to the transistor.

With a little soft:
c) If you don't use the timers use one for this.

d) If you have a periodical interrupt invert the output each time i.e. with cpl instruction.