Not sure if a device like this has been covered before here - I couldn't turn anything up, at least not with the Sengled brand name.
A few weeks ago I heard about a LED light bulb with a backup battery in it. Yesterday I went to the local hardware store, and lo and behold, they happened to be selling the exact things for half price. You guessed it, I bought one. I have seen various lights that charge on the mains, but this one is different to anything I have encounter before.
This is what it is like
https://us.sengled.com/products/sengled-led-with-battery-back-up-a19-bulbThe one I got is a bayonet fitting and rated for 220-240V, and it looks pretty much like a normal LED bulb, perhaps a teeny bit longer, and definitely a lot heavier.
This is what they say about it on that site: "Everbright automatically switches on when the power goes out but can be turned on and off as normal during a power outage." That sounds pretty normal, but there are no switches or buttons on it, so how do you turn it on and off when the power is out? Well, you just use the normal wall switch, or for my test, a switch on the power board that I plugged a desk lamp holder into!
If you look at the reviews on the site linked to above, some buyers tried to test that the bulbs would come on if the power fails, and one returned his one when it failed this test. Another tried every possible way he could think of to simulate a power failure, but never succeeded - he was then told that the lamp was smart enough to be able to know a real power failure from a test!
I found I could get it to work as advertised, by turning off the circuit it was plugged into at the switchboard. In this state, yes, it could be turned on and off as if the power was still on, using the switch on the power board, or using the switch on the outlet that the power board was plugged into. I also found, that when the power was out, and the bulb was turned on, removing it from the socket caused it to turn off. Another observation is that whenever you turn it on or off, there is a slight delay before it responds.
I tried another test, where I plugged the desk lamp holder into a long extension cord, around 30 meters worth, and no, in this situation I could not trick it into believing the power was off.
There is a panel on the side of the bulb that looks like it could slide out, but it appears to be held in place with a pin or screw, and I have not tried to prise it out.
I found that there is around 1.7VDC across the pins on the bulb when it is unplugged. I also discovered that connecting a 390K resistor across the pins causes it to turn on.
Whatever it is doing, it does not seem to be perfect. One reviewer on that site returned his one when he found that he could not turn it off.
So, what black magic is employed? How does it tell the difference between the power being turned of by the normal or "intended" switch, from losing power due to a tree falling on the power line down the street (of the power being cut at the switchboard)?