Hello Kai,
Having gone through the circuit i could arrive at the following description.

Recalling the circuit here,



The LM324 (with o/p Usig) forms the differential amplifier.

The rest of LM324's connects to form an instrumentation amplifier of gain 1, 10, 100 to amplify the drop accross the Reed Switch contact(RCont).
But this is very general and hence to have a detailed analysis i would require the following details:

i could not understand why i should toggle BC337 every 900uSec.
And the equation given to calculate the CR is not clear to me. can i get some details on that?

Mainly, it would be of much help to me to work with the values you have selected for the components, and
thus compare its performance with my application if you could describe 'how the circuit operates'.
Since, this could make me also to know about the issues that should be concentrated in the intended design and thereby knowing the steps that should be adopted to overcome it!

Requiring your assistance

Thanking you,
Suresh.