MITSUBISHI ELECTRIC RESEARCH LABORATORIES http://www.merl.com A Low-cost Optical Sensing Device Based on Paired Emitter-detector Light Emitting Diodes King-Tong Lau, Susan Baldwin, Martina O’Toole, Roderick Shepherd, William J. Yerazunis, Shinichi Izuo, Satoshi Ueyama, Dermont Diamond TR2005-098 December 2005 Abstract A low power, high sensitivity, very low cost light emitting diode (LED) based device for intensity based light measurements is described. In this approach, a reverse-biased LED functioning as a photodiode, is coupled with a second LED conﬁgured in conventional emission mode. A simple timer circuit measures how long (in us) it takes for the photocurrent generated on the detector LED to discharge its capacitance from logic 1(+5 V) to logic 0 (+1.7 V). The entire instrument provides an inherently digital output of light intensity measurements for a few cents. this light intensity dependent discharge process has been applied to measuring concentrations of coloured solutions and a mathematical model developed based on the Beer-Lambert Law. Anal. Chim. Acta This work may not be copied or reproduced in whole or in part for any commercial purpose. Permission to copy in whole or in part without payment of fee is granted for nonproﬁt educational and research purposes provided that all such whole or partial copies include the following: a notice that such copying is by permission of Mitsubishi Electric Research Laboratories, Inc.; an acknowledgment of the authors and individual contributions to the work; and all applicable portions of the copyright notice. Copying, reproduction, or republishing for any other purpose shall require a license with payment of fee to Mitsubishi Electric Research Laboratories, Inc. All rights reserved. Copyright c  Mitsubishi Electric Research Laboratories, Inc., 2005 201 Broadway, Cambridge, Massachusetts 02139