Yes ajb, you're right. That was my first instinct as well, just to amplify the difference between the Set Point and the Actual value and feed that into the 555 control pin. You can see that I tried to implement that in my original post I attached the diff amps I was trying to use, as you said an opamp and 4 resistors. Since I only have single power supply I thought of having one diff amp for the case when the sensed value is greater than the set point, and another diff amp for when the sensed value is lower than the set point, that's precisely the point where I got stuck and decided to post in here. And since everything is pulsing, yes I do need a bunch of low pass filtering in order to try and get DC voltages out of the pulsing signaling, which is not that big of a deal really.
So what would be the next step? I can see now that I was on the right track, how would you make this work? Thanks again for the help. I'm re attaching the diff amps screen shot that I posted in my original post.
Seekonk, you are right that they are not designers, they're not even engineers, they just bought a bunch of cheapo PWMs online to drive the load, and those PWMs happened to be implemented with a 555. Is not that they are making me use a 555, is just simply that that's what we are stuck with. And even though the PWMs are cheap and nasty, they do get the work done for our application, all I need is to generate that damn control signal and I'll be set. Thanks for your comments.