Tank Sensor theory (poll)

I have read a lot of the topics and suggestions on tank cleaning and the reasons why the sensors show full most of the times and how to deal with it. However, what I am missing and would like to know is the ELECTRONIC THEORY describing the design of the LED light method to show tank usage. I.E. Is the supply voltage applied to the lights on the panel and the sensors complete the circuit to ground causing the lights to light? If so then it would seem that there should be 4 lines running the signals back to the control panel? Looking at mine, I have 4 sensors each with a wire and the 3 for 1/3 2/3 and Full being in a "Wire Or" configuration with the white ( ground most likely) so 4 wires end up being 2 wires going back to the panel? One person mentioned adjusting "POTS" which suggests a set of voltage comparators to adjust sensor sensitivity. Not trying to rehash what has already been said - I have read all of them but none address the actually "current flow theory" which I am interested in - any Engineers out there or design buffs; your comments would be greatly appreciated.