I have found that three FSRs in parallel will relaibly trigger an endstop pin on the smoothieboard without any other circuitry. I have them mounted under my printbed on a Kossel mini. I did have to experiment a bit with the physical mounting to get it right (varying the size of the contact patch between FSR and bed)
The utilibots FSR kit with the driver board works great.
connect all fur’s to the driver board, and the driver board to the z-endstop.
probe triggers at 0mm.
done deal.