The “Blue Pill” is a small development board with an STM32F103 32-bit ARM µC on it, plus essential supporting logic. It’s widely available and very low-cost. See this wiki page on the STM32duino website for a wealth of useful information.
The tricky part is getting over the initial hurdle of uploading firmware to this little board. There are two ways to do it, but both of them require some care and tinkering:
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