A gas generator for an airbag includes a first gas reserve with a first orifice, a first mechanism arranged for opening the first gas reserve, and a diffusion chamber arranged for diffusing the discharge gases. The generator additionally includes a mixing chamber comprising at least one first communication surface with the diffusion chamber. The gas generator further including a second mechanism comprising a second actuator and a mobile obturator between a first obstruction position, in which it obstructs a second communication surface with a diffusion chamber and a second passage position, in which the second communication surface is free.