A recent discussion on a circuit with which frequency and duty cycle can be varied independent of each other was the inspiration for the attached circuit. R10 and R15 are intended to model a variable resistor with the input connected to the wiper. This controls the duty cycle. Frequency is controlled independently via R9. In the simulation, two discreet frequencies are chosen by R9, and duty cycle is set to approximately 66%. The duty cycle is visably unaffected by the frequency.
The advantage of this curcuit is the output is taken from the timer, and exploits the excellent characteristics of the timer's output stage. Another point is an application might requre the variable duty cycle period start coincedent with the timer's period. That can not be achieved by using a comparator connected to the timer's capacitor, as in other designs.
The advantage of this curcuit is the output is taken from the timer, and exploits the excellent characteristics of the timer's output stage. Another point is an application might requre the variable duty cycle period start coincedent with the timer's period. That can not be achieved by using a comparator connected to the timer's capacitor, as in other designs.