The controller is a simple piece of perf board, with a Wemos D1 mini ESP32 module flanked by some passives, a barrel socket for power, and the usual DIN connector for connecting the MIDI instrument.

wemos mini d1 4 Articles . Battery-Powered ESP8266 Sensor? Never Been Simpler. December 31, 2022 by Arya Voronova 23 Comments ... You have an ESP8266 board like the Wemos D1, ...