Generate pwm using ic 555

Introduction

IC 555

IC 555 is an astable multivibrator (oscillator).

Frequency: The ON time is defined by the time taken to its capacitor to charge to 2/3 (1/e??) level through pin7 resistor, and the OFF time is the discharging time of the capacitor through pin7, 1/3.

Method 1

The capacitor is charging through $R_{1}$ and $R_{2}$ but discharges only through $R_{2}$ using IC 555. Thus $R_{2}$ should be a potentiometer.

The $0.01\mu$F capacitor is to ensure that the CTRL and GND stays on the same voltage level. The CTRL pin of the 555 is to level out any fluctuations in the power supply voltage that might affect the operation.

${\begin{aligned}T_{\text{on}}&=0.693(R_{1}+R_{2})C\\T_{\text{off}}&=0.693R_{2}C\\f&={\frac {1.44}{(R_{1}+2R_{2})C}}\end{aligned}}$

555 internal design and theory

See https://www.electronicshub.org/555-timer-pwm/

and the internal circuit looks like below:

References

https://www.homemade-circuits.com/how-to-use-ic-555-for-generating-pwm/

https://www.electronicshub.org/555-timer-pwm/