This master thesis deals with design and construction of device for optic biosensor. The device are using for detected halogen compound in the water. For excitation testing sample is used LED diode about wave length 590 nm. Photocounter is used for detection of the emitted light after fluorescence Such as fluorescent marker are 5(6)-carboxynaphthofluorescein. The thesis is divided into three head parts. The first part includes the theory needed for realization; the second part takes in detail description of the construction and function of the device and the last part describes measurements with the device.