Blue (or Red) Pill
These boards have been available for quite some time from eBay and AliExpress.
Just search for “STM32F103C8T6 board” on those sites.
(this diagram is from an excellent page about the Blue
The LED is on PC13, “0” is on.
- There are several different variants of this board out in the wild, different
colours, slightly different USB jack details (all Micro-USB), slightly
different placement of the 2x3 BOOT0/BOOT1 jumper block.
Schematic: as PDF (copied from the same STM32duino wiki as the diagram).