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Knowing how and when people interact with their surroundings is crucial for constructing dynamic and intelligent environments. Despite the importance of this problem, an attainable and simple solution is still lacking. Current solutions often require powered sensors on monitored objects or users themselves. Many such systems use batteries [1-3], which are costly and time consuming to replace. Some powered systems connect to the grid, which may save swapping batteries, but at the price of restricted placement options. Other solutions use passive tags on monitored objects or require no tags at all, but many of these systems have prohibitive characteristics. For instance, camera-based systems [4,5] generally will not work if their view is occluded. Many other systems that rely on passive tags or do not use tags require direct line-of-sight or close proximity to work. As such, our goal was to design and develop small, cheap, easy-to-install tags that do not require any batteries, silicon chips or discrete electronic components, which can be monitored without direct line-of-sight.