When it comes to catching some rays, it can be tough to strike the right balance. Too little sun can lead to a deficiency of vitamin D and disrupted sleep, whereas too much can leave you with a nasty sunburn and a higher risk of developing skin cancer.

Anyone wishing to measure their ultraviolet exposure must currently rely on devices known as dosimeters, which are the size of a name tag or a wristwatch and use a light-sensitive material to calculate exposure to different types of radiation, including sunlight. But these are often expensive, rely on battery packs sensitive to water damage, and must be fastened to clothing with straps or clips, making them cumbersome for a day at the beach.

Enter the minidosimeter. The device—which looks like a button—sticks directly to skin or clothes, even when they’re wet. It uses a photodiode, a kind of semiconductor that converts light into electricity, to not only measure the sunlight to which it’s been exposed, but for power, eliminating the need for batteries. A tiny antenna sends measurements wirelessly to the user’s smartphone.

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