The first of these was the choice of microcontroller; although he used a Raspberry Pi Pico in the prototyping stage, the RP2040 was just too big to incorporate in the final build, so he used the snappily named STM 32 UO0 83KCU6 instead. The other big challenge is that at full brightness, the watch uses about an amp of current, which drains the battery (salvaged from a disposable vape) in about half an hour. To get round this, the default state of the watch is to turn the display off, with the wearer pressing a button to get the time and the cool Matrix-style rainfall animation. Through-hole-soldering components on to a flexible PCB worn next to soft human skin brings its own problems. Last but not least, the cost of the displays – never mind the microcontroller and charging circuitry – came to $420, though Sahko recommends the similar HCMS3901 display as an alternative, as it’s more easily available and plays more nicely with 3.3V.