This article reports the development of a two-tier simple electric circuits test (2T-SEC Test). Simple electric circuits are introduced to students early in their school years and are an important part of the physics curriculum all the way to the university level. However, many students do not develop an adequate conceptual understanding of simple electric circuits and especially of electrical voltage. For this reason, six physics education research groups from Germany and Austria started a large study to evaluate and improve middle-school students' conceptual understanding of simple electric circuits. As a first step, we developed the test instrument presented in this article. First, nine semi-structured interviews were conducted with middle-school students in Austria. The interviews were transcribed, analyzed and (common) student difficulties with simple electric circuits were identified. Second, based on the results from the interviews and previous research on this topic, an instrument consisting of 25 two-tier multiple-choice items was developed and administered to N=1568 middle-school students in Austria and Germany. Third, the data were evaluated using Rasch analysis. The item reliability of the test was 0.99, while the person reliability of paired scoring was 0.62 and the reliability of separate scoring was 0.75. The average difficulties of the main concepts regarding simple electric circuits (open and closed circuits, electric current, resistance, series and parallel circuits, and electrical voltage) were calculated and compared. The analysis suggests that the concept of voltage is most difficult for students while distinguishing between open and closed circuits is the easiest.