Peter Holzhauser has published Etherbulb on GitHub, open source firmware that turns an inexpensive ESP32-C3 smart bulb into a receive-only radio. The bulb listens to the 2.4 GHz band and the mobile bands nearby, classifies what it hears, and colors the room by signal type. Blue for Bluetooth, green for WiFi, and a color of your choice for anything else you pick. A stronger signal makes a brighter light.

There is no app, no cloud, and no account. The bulb doesn’t decode content and it doesn’t transmit on the bands it watches. It serves its own JSON API and dashboard, including endpoints for status, configuration, a log, and a raw snapshot of what it is hearing. The firmware locks out running the RGB and white channels at the same time, with a hardware check on the PWM duty cycles.

The receiver path comes from the ESPARGOS project’s esp-sdr by Florian Euchner, Zoltan Doczi and contributors, with the signal and mobile band tables from the ESPsoup app.…