Optical ion sensors normally have a relative narrow sensitive detection window. Here, based on multicolor light-emitting diodes (LEDs), we report on an electrochemical-to-optical signal transduction scheme under chronoamperometry control to convert the potentiometric response of ion-selective electrodes (ISEs) to optical output with tunable sensitivity and much wider response range. The sensing principle was demonstrated on K+, Ca2+, and Pb2+. LED light intensity was found to depend linearly on the concentration of monovalent ions. Optical signals could be captured with photomultiplier tubes or digital cameras, and a visual alarming system to monitor abnormal ion concentration was also developed from super-Nernstian electrodes.