Figure 1 and 2
The circuit consists off three parts:
- A-stable multivibrator: IC1 (red)
- A non linear sawtooth circuit: T2 en T1 (green)
- Linear amplifier: IC3 (blue)
The non linearity of the sawtooth signal compensates a little the non linearity of the light intensity of the LEDs.
The frequency can be changed by changing the values of cap C3 and the duty cycle can be changed by P1.
Changing one off above parameters, the value of capacitor C5 should also be changed.