I installed a second light on our deck that could not be controlled by the main switch inside. So, one light by the door, and one on the other side of the deck on a completely different circuit. I wanted to pair these together using the Mirror App, and figured that I will need to put a smart switch in the wall with a smart bulb in the separate light. That may still be the long term plan.
That said, I had two smart bulbs laying around, and tried something that I was sure would not work. I put a smart bulb in the light controlled by the switch by the door, and a smart bulb in the light on the separate circuit. I mirrored the switched bulb to the separate bulb. To my amazement, when power is cut to the master bulb, the slave bulb turns off. When power is restored to the master bulb, the bulb turns on. So, not only does the mirror work for a master that is switched on and off by the hub, but also for a master that is electrically turned on and off.
Am I just lucky here, or is this designed/expected behavior? If I can count on this working, it really saves time, money and hassle in not having to install a smart switch in the wall that will be "always on." Perhaps everyone else was aware of this trick, but I, for one, was really surprised.
So if I'm understanding this right, you have a smart bulb on a "dumb" switch and a smart bulb on a separate circuit? Then when the smart bulb gets effectively turned off via the "dumb" switch (so power is totally cut), you notice that the second (smart but still powered) bulb also turns off?
If so, what must be happening is that the hub is getting an "off" message from the first bulb. This is pretty uncommon: after all, you're cutting power to it, and the Zigbee (or whatever) radio needs power to send a message back to the hub. However, this is something I've seen people say happens for some bulbs, specifically Sengleds, most of the time (the guess is that they are able to detect this loss of power and use what's left to send this message, succeeding most of the time). You can verify that this is really what's happening--just look at the first bulb's "Current States" on the device page as this happens (to see whether "switch: on" becomes "switch: off") or look at the event history after the fact.
I wouldn't count on this happening 100% of the time given the above, but it's a neat feature people have seen on at least the Sengled mentioned above. Long-term, either a smart switch with local control disabled (so power isn't actually cut) that still sends button ("Central Scene" in Z-Wave terms, as I think all of these switches currently are) events on taps of the paddle/toggle or replacing the switch with a button device (Hue Dimmer, Eria Remote, Pico Remote, or many others) would probably be a good choice to keep all of the offs "soft" and the both bulbs at full "smart" functionality, as you say, but glad this is working regardless!
You articulated exactly why I was surprised that it worked. These are indeed Sengled bulbs, and so far, this has worked without fail throughout significant testing. They do go to an "off" state when you cut power, and this is registered by the hub. I even reverse-mirrored the second bulb back to the first, so that I can turn the light off at that location and it will turn off the main bulb. I'll probably implement a more elegant and robust solution with a smart switch in the not-too-distant future, but, for now, this is a very pleasant surprise. Can turn two bulbs on/off from two locations, and can use google or my phone for dimming. Pretty cool!
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