Robots are often equipped with 2D laser-rangefinders (LRFs) and cameras since they complement well to each other. In order to correctly combine the measurements from both sensors, it is required to know their relative pose, that is, to solve their extrinsic calibration. In this paper we present a simple, quick and effective minimal solution for the extrinsic calibration problem. Our approach does not require any onpurpose calibration pattern: it bases on the observation of an orthogonal trihedron, which is profusely found as corners in human-made scenarios. The proposal is validated with synthetic and real experiments, showing better performance than existing alternatives. An implementation of our approach is made available as open-source software.