Relaxation oscillator, how to change the duty cycle
Charge the capacitor faster (or slower) than you discharge it.

For example, replace R1 with this:







simulate this circuit &ndash; Schematic created using CircuitLab

Now, when discharging (Vout low), D1 conducts putting R1 and R2 in parallel. This will reduce the period, but also increase the duty cycle (by decreasing the low period). If you wanted to reduce teh duty cycle, reverse the diode.