Here are some of the example circuits and tutorials for generating pwm using 555 timer ic and op amp ic http://www.maxim-ic.com/app-notes/index.mvp/id/3201 http://www.instructables.com/id/OP-AMP-PWM/ http://www.instructables.com/id/Simple-and-dirty-Pulse-Width-Modulation-PWM-Wi/ http://www.dprg.org/tutorials/2005-11a/index.html http://www.williamson-labs.com/555-circuits.htm