Please. No resistors voltage divider.
May be it works (I doubt it) it is a heat focus and PWM is not difficult.

The PWM soft:

pseudo code:

in the main:
program the vectors
Program a timer
activate the timer interrupt
...
...
;the condition to activate the motor
setb bit ;An internal bit not the pin port
...


;interrupt service
reload the timer
jnb bit,next
cpl bit
mov c, bit
mov px.x,bit
next:
...
... ;the rest of the interrupt routine
... ;in example the famous 1 second timer
...
reti

That's all. You only need the motor the transistor, one diode and one resistor. (i prefer and optocopler)