I'm trying to design a 555 circuit that outputs a low frequency PWM signal (1-10 Hz) and is adjustable in both frequency and duty cycle (0-100%). I've come across this design and it seems to work pretty well assuming I use two potentiometers for the resistors. Basically the output time(sec) values Ton and Toff are adjusted using the below equations:
Ton = .67 * R1 * C
Toff = .67 * R2 * C
Ton = Duty/Freq
Toff = (1-Duty)/Freq
When I've been simulating this (shown below) I'm finding that the actual duty cycle and frequency have some error compared to the "calculated" values. How can I account for this difference between calculated and actual?
***Update to illustrate the error further, to get to 1Hz 50% duty i needed to adjust the values to the below. Right now i just "guess and checked" but would like to understand it better and be able to have equations that more accurately describe this behavior.