The Theory of Threshold Concepts (TCs), first articulated by Land and Meyer in 2003, provides educators in many disciplines with a tool to identify those special ideas that both define the characteristic ways of thinking of expert practitioners, and cause the greatest learning difficulties for students. Concept inventories are popular assessment tools, epitomized by the widely-accepted Force Concept Inventory of Hestenes et al., introduced circa 1992. It is a natural marriage to bring these two thrusts together to produce “Threshold-Concept Inventories”. We report ongoing work to develop and verify such a TC-inspired inventory assessment tool in the field of electronics and simple circuit theory. We identify the difficulty in the development of questions targeted at assessing understanding of single threshold concepts and present results in support of a strategy to deal with this.