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Originally Posted by Vector86
I breadboarded it up with an lm339 but I couldn't get it to work the way I want it to. Is there anything I should look closely at?
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Can you post more details? What component values are you using? How is it working that is different from what you want?
The output of the comparator should go high when Vc is lower than Vr and Low when Vc is higher than Vr. You need to use a value for the resistors and capacitor that makes it take long enough to charge/discharge to the triggering voltage that you set. You would probably want to use a cap that is a few uF instead of something in the nF or pF range. Small capacitors are more easily influenced by noise, and the leakage current from the diode and comparator would mess up your timing calculations. You may want to look up the formulas for RC circuits if you haven't already.
One more note: with this circuit as is, if the triggering voltage Vs varies, it will change the timing.