I just stumbled upon this site, where fred27 have disassembled a White E27 bulb.
Part of his findings (everything below is his):
Onboard you’ll find the largest IC is a SAM21R21E18A. This is a nice 32-bit ARM microcontroller with built-in Zigbee. It’s the brains and communications for the device.
There are a number of test point accessible on the underside of the PCB. It took a fair bit of following traces, checking continuity with a multimeter and probing with an oscilloscope, but I managed to work out what many of these were for. This is what I found:
TP1 – Ground
TP2 – SWCLK
TP3 – SWDIO
TP4 – Serial TX
TP5 – Serial RX
TP6 – LED output (about 32V)
TP7 – Regulated 3.3V power to microcontroller
TP8 – RESET
TP11 – Ground
TP25 – power supply to LM2204 regulator
So there a number of interesting signal here. TP1 and TP11 are the signal ground for the microcontroller and logic circuitry. TP7 is the regulated supply voltage.
SWCLK (TP2), SWDIO (TP3) and RESET (TP8) are interesting. You’d need these if you want to load new firmware onto the microcontroller or debug it. Interesting stuff, but this is arguably starting to head over the ethical line of hacking. It’s more complicated than we need anyway.
The serial port (TP4 and TP5) are also interesting. You see some debug output on TP4 as the device powers up. Nothing as you use the device and switch the bulb on and off so not much use to us. Once again, more complicated than we need. If you want to take a look, it’s 3.3V TTL 115200 8N1.
I told you it was possible to play around with the device safely, and it is. NEVER power your device from the mains whilst poking around. If you have a clean 3.3V supply you can connect that to TP7 and power the device that way. Better still is to supply a slightly higher voltage to TP25. It’s usually at about 24V but even 5V (e.g. from a USB charger) will do the job. You might not be able to light up the LED board but it will connect to you Hue hub. At this voltage but you will be able to look at signals, but most importantly you’ll be able to do it safely.