Thanks all for the feedback its much appreciated.
Just to make things clear, this is part of an actual circuit that has been in use for 20 years or so. I have come across it recently doing some charity work and im trying to understand it as well as i can.
Your replys give me confidence that im roughly on the right track. I agree that its not the tidyest of circuits and could probably have been implemented better but this is whats there.
The bit i cant fathom is a formula for V out in terms of the control and sensor variable resistor values but thats my problem.
Code:
The trick is to calculate the voltages with the Op amp disconnected
and then calculate what the Op amp will do when the connections are restored.
It is easier to do the calculations if the resistors going to the + & - inputs
of the Op amp are at least 10 times greater than the pot resistances.
Then the loading effect can be ignored to a reasonable
dergee of accuracy.
I realise the capacitors shown are really there just to deal with transients when the variable resistors are moved.
The comparitor circuits (after V out) compare V out with 2 and 3 volts (more resistor voltage dividers) to create either an up or down or no change signal.
Maybe looking at V out in terms of the control and sensor variable resistor values isnt the way to go.
Im glad its not an AC circuit cos then i would be into poles and zeros and well stretched beyond what i can remember about mr Laplace
Thanks all prh