To be honest, my recent simple relay hack wasn’t really all that great. It just used the high power constant current output to drive a SSR. It wasn’t ideal, but it worked. I decided that it was worth the effort to track down some more useful outputs and properly detect the desired state of the bulb.
All it took was a little bit of poking around and probing the pins of the SAM R21 microcontroller with an oscilloscope. It wasn’t actually that hard. On the B22 bayonet fitting version of the bulb I found some.
PA16 (pin 17 of the microcontroller) has a simple on/off output. It’s high (3.3V) if the light is on at all and low if not. This is the perfect output to use to drive a simple output for a relay. It can be run straight to a SSR if that’s appropriate for your load. Or it can be used as an input to another microcontroller, a transistor to drive a relay, etc. This is probably the easiest one to use for a simple load. A slightly easier place to access it is on the end of diode D3 nearest the micro. It’s still some fiddly soldering, but it’s easier that a QFN pad on the SAM R21 itself.
PA17 (pin 18 of the microcontroller) has a decent PWM output. Unlike the one I found on TP5 of the E27 bulb, this stays high when the bulb is 100% on. It’s a 3.3V signal. It doesn’t seem to be quite linear – i.e. 50% brightness isn’t a 50% duty cycle but this may be deliberate to compensate for how bright it actually looks.
TP25 has a DC voltage of about 14V which is fed to the LM2204 3.3V regulator. This seemed handy for driving the coil of a 12V relay.
I rigged up a simple relay circuit – using the 3.3V PA16 output to drive a NPN transistor that can then drive the relay. Add the usual flyback diode (and a MOV across the relay contacts for the benefit of inductive loads) and all is g0od!