A light-emitting diode (LED) based dual-wavelength detector has been constructed. The detector's optical parts include a flow cell, a dual-wavelength LED and a photodiode. Neither mirrors nor lenses are used. All the electronic components of the detector, including a signal amplifier, an A/D converter and a D/A converter, and an Intel 8031 single-chip microcomputer, are integrated on a small board. The light beams are modulated electronically under the control of the microcomputer. While a solution of absorbance 0.875 was measured repeatedly, an relative standard deviation of 0.24% was observed. Such a detector with red/yellow LED has been coupled with a flow injection analysis system for the determination of micromolar levels of cobalt.