Broadening citizen participation in scientific knowledge production has become a priority of national and supra-national institutions and research agencies. Out of their interest for scientific research and public learning, citizen science projects are often presented as offering a unique opportunity to involve more directly the public in policy-making. However, despite twenty years of flourishment of participative scientific research, making citizen science a tool to foster participatory democracy remains a challenge. Political outputs of citizen science are indeed often restricted to its role in the production of data to inform policy making processes. In this paper, we propose an innovative theoretical model of democratization through citizen science, in which participative data collection is associated with public online deliberation. Drawing both on online political deliberation research and citizen science literature, we argue that citizens' engagement in contributory science could help create the conditions of good-quality public deliberations. We then present a technical device (an online platform) that put this model into practice in the context of the regulation of public lighting in two French municipalities.