I am currently working on a simple circuit which includes creating duty cycle from DC. Also, I need to change it's pulse width with time. I have decided to use op-amp relaxation oscillator to produce square waves but I have no idea how to chance it's width. I am allowed to use resistors, capacitors, inductors, diodes, OPAPs, transistors and relays.
My question is how can I create a circuit which produces duty cycle and it's pulse width is changing with time as shown at the picture above.