Adding a second resistor Rb to the 555 timer and connecting the trigger input to the threshold input causes the timer to self-trigger and run as a multivibrator. The capacitor C will charge through Ra & Rb and discharge through Rb only. The duty cycle can be controlled by changing the values of Ra and Rb. Input Ra, Rb (in kOhms) & C (in uFarads) then push the Calculate button to get the values for the output high-level duration tH and low-level duration tL and frequency. Output driver and output waveform duty cycles are also determined.