Technology educators often provide activities where pupils design programmed technological solutions (PTS) with various programming materials for developing pupils’ technological knowledge related to PTS and digital technology. However, few studies have investigated how pupils experience these activities. To fill this gap in knowledge, this phenomenographic study explores how pupils experience designing a PTS using BBC Micro:bit and identifies the technological knowledge these pupils need, in terms of critical aspects (i.e., aspects necessary to discern), to successfully solve a real world task—the design and coding of a burglar alarm. The data were gathered from sketches, interviews, and video-recordings of pupils aged 10 and 14. This study shows that the pupils struggled with two intertwined phenomena during the activity: the dual nature of the PTS (i.e., the structure and function) and the BBC Micro:bit material. The findings indicated that the pupils needed to understand what components to use based on their function and how to organise these components so they interacted with a code that used feedback control. That is, the pupils needed to code a conditional statement by combining blocks in the BBC Micro:bit editor. To produce working code, the pupils needed to know what the blocks represent, where to find the blocks in the editor, and how to interpret the shapes of the blocks. The results have implications for teaching technological knowledge, suggesting an importance of addressing these phenomena and critical aspects with respect to developing pupils’ conceptual and procedural knowledge related to designing PTS with BBC Micro:bit.