This work incorporates an open-ended design experience into an introductory circuits laboratory with the intended outcome of increasing self-efficacy for circuit prototyping and design. The authors have implemented a tinkering-based laboratory, in which students spend each lab period building a component of an inexpensive robot. The course culminates in a four week open-ended final hack that adds functionality to the finished robot. This design project prompts students to make connections across disciplines and exercise design thinking in a low-stakes environment. To determine the impact of this design experience on student learning, self-efficacy was measured through optional surveys administered both before and after the final hack. The design project resulted in a significant increase in design self-efficacy for students with some prior electronics experience, as well as an increase in prototyping self-efficacy in less experienced students, indicating that the design requirement had a positive impact on self-efficacy overall. It also showed that undergraduates in this course were ready to engage in a structured open-ended design experience even though they did not have a classical foundation in all the relevant theory, a common justification for the omission of design projects from intermediate-level engineering curricula.