Schematic:
http://img65.imageshack.us/img65/2761/resistorreednfru7.png

Objective, it find worthy group of resistors.

Condition: The total or partial sum of the values of R never must equalize.

Sample 3 elements:
R1+R2+R3 result not equal R1, R2, R3 or R1+R2 or R1+R3 or R2+R3

For know values resistor check class resistor this page
http://www.logwell.com/tech/components/resistor_values.html

For make group use Class E24 5% or E48 2%. Not mix class.

So far, get 5 elements for 5% E24 class...,possible get more?

Not is combination 2R or R-2R values. (Traditional A/D system)

This for replace the electric connection (traditional) of electric switches of a house, through circuit. Also, level meter function, with floats magnetic

Tranks help.... link tool for calculation

 

Have the resistors be binary weighted.
start with a value - say, 100 Ohms.
The next resistor should be double that resistance. 200 Ohms is a standard value.
The next resistor should be double the prior value. 390 is likely close enough.
Double again; 820 is the closest.
The next would be 1.6k.
So, no matter what combination of switches you had open or closed, the resistance across the network would result in a unique value.