The silicon photo-multipliers (SiPMs) are commonly used in the construction of radiation detectors such as those used in high energy experiments and its applications, where an excellent time resolution is required for triggering. In most of this cases, the trigger systems electric charge information is discarded due to limitations in data acquisition. In this work we propose a method using a simple radiation detector based on an organic plastic scintillator $2times2times0.3$~cm$^3$ size, to estimate the electric charge obtained from the acquisition of the fast output signal of a SensL SiPM model C-60035-4P-EVB. Our results suggest a linear relation between the reconstructed electric charge from the fast output of the SiPM used with respect to the one reconstructed with its standard signal output. Using our electric charge reconstruction method, we compared the sensitivity of two plastic scintillators, BC404 and BC422Q, under the presence of Sr90, Cs137, Co60, and Na22 radiation sources.