Opto-electric characterization of pH test strip based on optical absorbance using tri-chromatic LED and phototransistor

Optical pH measurement commonly uses a strip of paper with embedded indicator. A pH test strip which has four (4) test pads changes its color when it is dipped in a sample solution. The color change is compared to the color chart to identify the pH value of the sample solution. The purpose of this study is to extract the parameter of an optoelectronic pH sensor based on the hue absorbance of the pH test strip and be implemented on Simulation Program with Integrated Circuit Emphasis (SPICE). Through experiments using a devised sensor module, highest linearity is obtained when the ILED is 20 mA. The sensitivity of the device at pad 1 is 0.4217 mV/pH with a correlation coefficient of 0.8177, pad 2 is 0.3667 V/pH with a correlation coefficient of 0.9597, pad 3 is 0.2659 V/pH with a correlation coefficient of 0.0.9923, and pad 4 is 0.0347 V/pH with a correlation coefficient of 0.9948. It outputs the RGB Hue levels (in voltage) of the pH test strip. Optoelectric characteristics of the pH sensor are described by the result acquired in the experiments on parameter extraction. The transconductance of the device is derived based on the phototransistor current and pH-dependent voltage. The extracted parameters used on the equivalent circuit of the optoelectronic pH sensor is simulated on SPICE. The percent error between the responses on experimental and simulation of the test pads 1, 2, 3 and 4 are 10.28%, 0.01%, 0.34% and 0.86% respectively. Based on these results, an optoelectronic pH sensor model is developed.