I did some calculations with R, 2R, 3R, ... (parallel resistors) and I got acceptable voltage thresholds.
I'll run some calculations with R, 4R, 8R, ... (by example) to check for any improvements.
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Btw, to clarify this: every motion sensors could activate independently and I need to know exactly which one (or ones) becomes active at any moment.
For the series topology, there should be no problems with 6 binary resistors (1K, 2K, 4K, 8K, 16K and 32K, by example).