We present results of an experimental study of stochastic resonance in an electronic Chua s circuit whose dynamics switches between two different stable chaotic attractors when it is driven by a periodic signal and a Gaussian white noise. Due to the internal dynamics of the attractors the minimum amplitude for the external forcing to induce jumps strongly depends on the external frequency. We determine from the Fourier transform of the output signal the amplification factor of the input signal and study its dependence on the external frequency and the noise intensity. We show that the envelope of the distribution of switching times follows a gamma distribution, typical from bistable systems, and that the mean switching time decays exponentially with the noise intensity. We propose a simple method for obtaining the optimal noise intensity from the residence and switching times probability distributions and show that it coincides with the value obtained from the maximum of the amplification factor.