The given task is about finding a proper way of detecting the mixing ratio of diesel with biodiesel, gasoline, fuel oil, cracked gas oil and kerosene. The development of the sensor was limited by given boundary conditions, because the device should be applicable in an automobile. This thesis starts with the evaluation of possible ways to realize the detector: - Fluorescence spectroscopy: Luminescence is defined as the radiation, or light, emitted by an atom or molecule after it has absorbed energy. Radiation is detected. -Raman spectroscopy: Generally Raman spectroscopy works in the same way as fluorescence spectroscopy. The main difference is that the electrons are not excited to an excited state, but to a virtual state. Radiation is detected. -Permittivity measurement: Every substance located between the plates of a capacitor has a specific permittivity, which affects the capacity itself. Measurement of the permittivity by the change of the capacity. Absorption behavior: Every transparent material has a specific absorption behavior of light, which results in a reduced transmitted light intensity. Transmitted light intensity is detected. After the comparison of the named options, it was decided to realize the fluorescence spectroscopy option, because it had the biggest chance of success. The next step was the design of the test setup, with the selection of the optical components (light source: LED, photo detector: photodiode) and the design of the amplification circuit. This circuit is comprised of a low-pass filter, which reduces the AC component, and a non-inverting amplifier. After the first test runs, it became clear that the fluorescence signal was not detectable. Since measurements of the transmitted light were also possible with the existing test setup, several test runs were carried out. These measurements then provided good results. By the measurement of the transmitted light, it was possible to determine the fuel mixing ratios with an accuracy of a few percent. Because of the good results, the given task was assumed to be solved.