Teaching fluid dynamics could be made easier if students were able to directly observe turbulent flows. Background Oriented Schlieren (BOS) could be a well-suited technique to serve that purpose because it does not require seeding or hazardous light source and it is easy to implement. To fully comply with educational constrains the experimental setup must be affordable. In this contribution we test the performances of a low-cost BOS system based on the combination of a smartphone camera together with comBOS, an open-source Matlab toolbox dedicated to performing BOS flow visualization. The flow on which those tools are tested is the thermal plume generated by natural convection above a heated square cylinder. In order to assess the accuracy of the new toolbox, the displacement maps computed by comBOS from images captured by a scientific camera are compared with those obtained by the commercial software Davis 10. Displacements in the range of ±0.15 pixel obtained from the images of the thermal plume are found nearly identical with both methods. Results only differ by the noise level that was found to be close to 0.008 pixel with comBOS, 33% higher than with Davis 10. The scientific camera is then replaced by three OPPO A94 smartphones allowing to visualize the plume from three different view angles. The qualitative analysis of the displacement maps obtained from the smartphone cameras and the scientific camera does not reveal any significant difference. Spectra on the fluctuations in time of the angular deflection measured with the two types of cameras are also agreeing well. The possibility of building a low-cost BOS apparatus is therefore demonstrated with a success that exceeds our expectations. Tests were extended by testing thirteenmore smartphones of different types. Nine of them were also well-suited for BOS, whereas four are producing non-usable images.