PWM controller with manual or LFO-driven sweep

[earthtonesaudio]

This is an unverified work in progress:



For normal operation, if U1c is oscillating, its frequency is much lower than that of U1d.  The comparator then outputs a pulse train with a frequency determined by U1d and pulse width determined by U1c. 
S2 allows U1c to be either fast, slow, or stopped (manual).  The R, 3R, 2R ratios ensure the "manual" voltage is between 1/3 and 2/3 of the supply, the same thresholds as the inputs to the CD4093.
Because the voltage at each comparator input is between 1/3 and 2/3 supply, the duty cycle varies between 0 and 100 percent.

Finally, the toggle flip flop of U1a and its inverted output at U1b stop the oscillators so if this is built into an effect, the bypass mode will be quiet.  The pull-up resistor (Rp) for the comparator is tied to the output of U1b so that the control inputs to U2b, U2c, and U2d go low during "bypass" mode, regardless of the output of the comparator.

For normal PWM-variable resistor applications, I would say you'd want U1d's frequency to be fixed and fast, so C2 could be omitted and Rb could be replaced with a wire.

[earthtonesaudio]

I changed U2a to a JFET switch for no particular reason, and added a gating circuit so the oscillators stop when the input falls below a certain average amplitude:



Note, the comparators must have open drain/collector outputs, and the window comparator circuit in the upper left is from here.  The hysteresis control acts as a threshold for gating the oscillators.