Solid state pH sensor based on light emitting diodes (LED) as detector platform

King Tong Lau*, R. Shepherd, Danny Diamond, Dermot Diamond

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

31 Citations (Scopus)


A low-power, high sensitivity, very low-cost light emitting diode (LED)-based device developed for low-cost sensor networks was modified with bromocresol green membrane to work as a solid-state pH sensor. 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 microsecond) 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. A light dependent resistor (LDR) modified with similar sensor membrane was also used as a comparison method. Both the LED sensor and the LDR sensor responded to various pH buffer solutions in a similar way to obtain sigmoidal curves expected of the dye. The pKa value obtained for the sensors was found to agree with the literature value.

Original languageEnglish
Pages (from-to)848-859
Number of pages12
Issue number8
Publication statusPublished - Aug 2006
Externally publishedYes


  • Colorimetric sensor
  • Light dependent resistor
  • Light emitting diode
  • Optical sensing
  • Solid-state pH sensor
  • pH sensing

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