Optical liquid sensor The optical liquid sensor can be used to test for any liquid. Unlike conventional cable testers, there is no need for the liquids to be conductive. It has a digital output with end termination option to constantly monitor the wiring between the sensor and the monitoring system. The sensor can be connected directly to one of the four inputs on the back of a VizioGuard system (optionally to an ePowerSwitch Master via the digital input module). Power is supplied via the floating connection terminal.