A wearable device category that sometimes gets overlooked by DIY builders is the smartwatch. While commercially produced devices may feature a compact size, they lack hackability. However, a maker known as rahmanshaber has solved both issues with the open source MutantW smartwatch design.
The 44-millimeter footprint is similar in size to commercial smartwatches. However, you can build it with 3D-printed parts, off-the-shelf components, and a small custom PCB.
An ESP32-S1 module powers the MutantW. Output comes from the prominent 1.7-inch 240x280 RGB LCD on the face of the watch. The display rahmanshaber uses a flex ribbon cable, which is why the custom PCB is helpful. It connects the ESP32 and SPI-based screen.
Along with the TFT LCD, feedback comes in two forms. First, a WS2812B RGB LED, also known as a NeoPixel, provides visual indications. We imagine it would help communicate the charing state with the 4-pin docking connector. The other feedback mechanism is tactile with a vibration motor. In addition, users can use two SMD pushbuttons to interact with the watch since the screen does not have a touch interface.
The watch can communicate with other devices with either WiFi or Bluetooth for connectivity. Related, since you will likely be on the go with MutantW, it supports (re-)programming over-the-air--eliminating the need for a USB cable during development. (Although the 4-pin charging cable does support serial communication.)