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.

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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.

  • \$\begingroup\$ Not that your question isn't perfectly valid, but why in the world would you eliminate the IC that's probably most suitable for this task? \$\endgroup\$ Dec 12, 2014 at 21:46
  • \$\begingroup\$ Because that's what his homework says :P \$\endgroup\$
    – Majenko
    Dec 12, 2014 at 21:52
  • 5
    \$\begingroup\$ Just design it with a 555, then delete the 555 and replace it with the 555's internal schematic :D \$\endgroup\$
    – Majenko
    Dec 12, 2014 at 21:55
  • 1
    \$\begingroup\$ Look at a 555 equivalent diagram and see how it achieves duty cycle control using a two level comparator and charge and discharge circuitry. Go and do functionally likewise. That's the other end of the approach that Majenko suggests - destination should be the ame. \$\endgroup\$
    – Russell McMahon
    Dec 12, 2014 at 23:20
  • \$\begingroup\$ @Majeko is right that's my homework's restriction \$\endgroup\$
    – sercand
    Dec 12, 2014 at 23:30

1 Answer 1


Use the relaxation oscillator to make a ramp or triangle wave and then compare that wave's voltage to a DC voltage that you can control (for example with a potentiometer).


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