I need to control the duty cycle of a Pulse Width Modulation (PWM) circuit from a DC voltage input. To do this, I've been looking into 555-timer circuits - but every circuit I've come across involves controlling the duty cycle by using a variable resistor/changing resistor values (e.g. changing the value of Rb in the diaram shown below).
I've researched and thought about it a lot, but I can't find a way to use a certain input voltage to give a certain duty cycle in a PWM cicruit with the 555-timer, so any suggestions I may not have thought of or letting me know it can/can't be done would be appreciated.
Thanks
As a note: The requirement of controlling a PWM circuit duty cycle with an input voltage and not by other means is necessary for what I'm doing... I'm not trying to make my life difficult
Also, I forgot to mention I'm also not allowed to use microcontrollers!