Building a touch-controlled RGB table lamp inside the leftover shell of a Hatchimals Alive egg, with an ESP32-WROOM-32, a 12-LED WS2812 ring and WLED presets bound to capacitive touch. Covers the WS2812 0.7 × VDD logic-level threshold and the silicon-diode fix for it, and the wiring.
What an optocoupler is and how electrical isolation works, built from a discrete LED and phototransistor on a breadboard with measurements, then replaced with a real 4N35 chip, and where optocouplers show up in actual circuits.
Building a DIY Bluetooth page-turner remote for a Kobo e-reader by making an ESP32-S3 Lilka device act as a BLE HID keyboard, including BLE debugging on Debian, testing on the Kobo Clara 2E, and the full project code.