This paper proposes a novel adaptive-gain second order sliding mode controller. The proposed controller is based on discontinous-impulsive action and only uses information of the output, no differentiation is needed. A practical second order sliding mode is ensured in spite of bounded perturbations with unknown bound. The gain adaptation is based on particular con- ditions attainable on sliding mode which can be verified with the only information of the output. Both, the convergence conditions and adaptation proposal are tested through simulations.