Occasionally you run across a product that you just know is simply too good to be true. You might not know why, but you’ve got a hunch that what the bombastic phrasing on the package is telling you just doesn’t quite align with reality. That’s the feeling I got recently when I spotted the “LED intellibulb Battery Backup” bulb by Feit Electric. For around $12 USD at Home Depot, the box promises the purchaser will “Never be in the dark again”, and that the bulb will continue to work normally for up to 3.5 hours when the power is out. If I could repurpose that to make a tiny UPS for a microcontroller project of my own, it could be even more useful.
Now an LED light bulb with a battery in the base isn’t exactly rocket science, we can understand the product conceptually at a glance. But as they say, the devil is in the details. The box claims the bulb consumes 8.5 watts, but a battery with enough capacity to run such a load for 3.5 hours would be far too large to fit inside of a light bulb. Obviously there’s more to the story.
On the side of the box, in the smallest font used on the whole package, we get our clue. The bulb drops down to 200 lumens when in battery backup mode, or roughly as bright as a cheap LED flashlight. Now things are starting to come together. Without even opening the device, we can be fairly sure it will contain two separate arrays of LEDs: one low set for battery, and a brighter set to run when the bulb has AC power.
Still, I tend to be of the opinion that anything less than $20 or so is worth cracking open to see what makes it tick. Even if the product itself is underwhelming, there’s a chance the internal components could be useful or interesting. With that in mind, let’s see what’s inside a battery backup light bulb, and what we might be able to do with it.
I fully expected to have to cut the bulb open, but was pleasantly surprised you can disassemble it non-destructively. Not that it was the intent during manufacturing, of course, but it’s a nice side effect of the fact that some poor soul probably had to assemble these things by hand.
If you squeeze the frosted plastic dome, eventually the glue used to secure it will pop, and from there it just takes some light prying to release. Once the dome is off, you’ll be presented with the LED array. Three screws around the outside of the LEDs will allow you to pull all the electronics out of the bulb. Two wires run down into the base, which unfortunately appears to be pressed into the plastic permanently. So to extract the electronics you either need to snip the wires or desolder them from the board.
As expected, there are two concentric rings of LEDs in the array which turn on or off depending on whether the bulb is on AC or DC power. The inner ring of LEDs as well as the five in the center light up when on DC power, and when AC is available the outer LEDs come on. It’s worth noting that the center LEDs don’t get any brighter on AC versus DC, but that the outer LEDs are much brighter than the inner.
This makes sense given the information on the box: if the total output of the bulb is 600 lumens, but only 200 lumens on battery power, we know that the outer LED ring must output roughly 400 lumens on its own.
The array looks reasonably well made, and is attached to a rather nice circular aluminum heatsink. While the wires aren’t labeled, it isn’t hard to figure out that the center wire is negative and the two outer wires correspond to the two LED rings. This module would be very easy to reuse in a project where you might want variable brightness without having to bother with PWM.
The board should look pretty familiar if you’ve ever seen the inside of an LED bulb before. It’s a double-sided PCB with a fairly simple layout: transformer and capacitors on the top side for AC to DC conversion, with the flip side featuring the brains of the operation. The white connector on the top side of the board connects to the 3.7V 2000mAh battery, which incidentally takes up most of the internal volume of the bulb.
Theory of Operation
The thing to remember about this bulb is that it isn’t like an emergency light; it doesn’t just automatically turn on when the power is cut. It’s a light bulb, after all, and it needs to turn off when you flip the switch or unscrew it.
Instead the bulb detects when power is lost in the circuit it’s connected to. It does this by testing the resistance between its AC terminals when it loses power. If there is “infinite” resistance, it knows that it has been switched off or unplugged.
Interestingly, the circuitry in the bulb is sensitive enough that if you hold the bulb in your bare hand it will see it as an unpowered circuit and light up. Depending on your sense of humor, that might be worth the $12 alone.
I wanted to clarify how the bulb works, because I think it opens up some interesting possibilities for reusing the hardware. If the LEDs operate at 3 V, and the bulb’s circuitry is able to maintain that voltage no matter if it’s connected to AC power or not, we essentially have a low voltage uninterruptible power supply (UPS) on our hands.
Proof of Concept
The power is probably pretty “dirty”, and spikes when coming on and off of AC seem likely. You should have a big capacitor across the DC side of this board at the very least. But as a quick demonstration, I was able to take the LED wires and stick them right into the 3.3 V side of a Wemos D1. Again, this isn’t a great idea, but shows the basic premise is workable.
If we assume a consumption of 250 mA for the D1, this little hacked UPS should be able to run it for at least 5 hours or so. Given the advertised 3.5 hour run time figure, as well as the 2000 mAh capacity printed on the battery, this board should be able to supply at least 400 mA at 3 V. With a boost converter you could get 5 V out of it, but probably not with enough current to run much.
A lot of people are hesitant to fiddle with AC circuitry, so the fact that this offers a turn-key solution and lets you focus on the DC side is a big plus. With the addition of an enclosure and lamp cord for the AC side, this could be an interesting “Poor Man’s UPS” for microcontroller projects constructed entirely from parts available at Home Depot. There’s a certain MacGyver element to being able to set something like this up without having to leave your local big box store.
There’s some decent hardware in the bulb that might be worth the cost of admission, especially when these bulbs invariably hit the clearance section for $6 or so. The dual-brightness LED array and relatively beefy battery can be easily repurposed, for a start. It’s not the best salvage deal we’ve seen from the Home Depot, but you could do worse.
But personally the idea of using these bulbs as a cheap DC UPS is the most appealing to me. It certainly warrants further investigation, and it would be interesting to see what the community can come up with in terms of appropriate loads to swap the LED array out for. I’ll be doing some more work with this concept, so keep an eye out for a future post on the subject.