Establishing optimal challenge enhances intrinsic motivation, interest, and the probability of success in the learning activity. In K-12 problem-based learning (PBL), students may struggle to address associated tasks that are beyond their current ability levels. This paper suggested learner-centered scaffolding systems (LSS) to improve K-12 students' perception of optimal challenge by addressing their learning issues in PBL. LSS enhances students' experience in autonomy and competence by providing multiple types of scaffolding in accordance with students' different needs and difficulties in PBL. Students can control the nature and frequency of scaffolding by themselves according to their needs and ability, and it plays a role in improving their self-directed learning skills. Last, peer scaffolding between students with similar abilities satisfies students' needs for relatedness.