[BasilFX] wanted to shoehorn custom firmware onto his IKEA Trådfri light bulb. The product consists of a GU10-size light bulb with a LED driver as well as IKEA’s custom ZigBee module controlling it all. A diffuser, enclosure shell, and Edison-screw base give the whole thing the same form factor as a standard A-series bulb. The Trådfri module, which ties together IKEA’s home automation products, consists of an ARM Cortex M4 MCU with integrated 2.4Ghz radio and 256 Kb of flash — not bad for 7 euros!
Coincidentally, [BasilFX] had just contributed EFM32 support to RIOT-OS (“the friendly OS for IoT”) so he was already halfway there. He used a JTAG/SWD-compatible debugger to flash the chip on the light bulb while the chip was still attached.
[BasilFX] admits the whole project is a proof of concept with no real use yet, though he has turned his eye toward getting the radio to work, with a goal of creating a network of light bulbs. You can find more info on his code repository.