Abstract: An end point quality control light circuit for a water demineralizer. The quality control light circuit comprises a power cord, a housing for the electronics, and a probe-light assembly. The probe-light assembly includes a pair of spaced apart A.C. energized probes, and applies approximately 12 volts to the water. The probes are transformer isolated from the power input, and the final drive to an indicating lamp is transistorized. The current through the probes, directly proportional to the quality of the water, develops a voltage that is compared to a threshold voltage. The indicating lamp is illuminated to indicate high water quality, and is switched off by the final drive transistor when the water quality becomes unacceptable. An optional switching circuit, isolated from the quality control circuit, enables the actuation or control of external devices in dependence upon water quality. Various techniques for minimizing leakage currents are also disclosed.