The Swiss plate geophone system has proven to be a robust method for bedload transport monitoring in steep gravel-bed streams and mountain rivers. It has recently been demonstrated that the maximum amplitude of the Swiss plate geophone signal contains information about the maximum size of the transported particles. In this study we go a step further and extract grain size distribution information from the recorded raw signal. A method based in the amplitude of the signal has been developed: the number of impulses registered at different threshold-amplitudes (the so-called amplitude histograms) from the geophone’s signal were computed. These amplitude histograms are related to the measured grain size distribution of the transported bedload particles. 45 direct bedload samples caught by the automatic basket samplers at the Erlenbach are analysed. The measured grain size distribution curve could be successfully reproduced with the computed amplitude histograms. The presented results lead to the conclusion that a method based only on the amplitude of the geophones signal may be appropriate to extract grain size information from the Swiss plate geophone system.