Overall goal: Modify a set of decorative lights to run on mains power in a way that's safe enough to keep in my daughter's room.

The decorative lights used three AA batteries, so I wanted a 4 to 5 volt DC power supply. And the cheapest and easiest source for that? USB phone chargers. I went to Five Below and came back with an "Xtreme" branded "Xtra Power" charger that looked like it would do the job.

This charger has two press fitted side covers, so it looked easy enough to open. If you find one like this, remove the side with the "Xtreme" logo on it first - the other side has two screws holding it to the PCB. It pried open with two screwdrivers.

This thing isn't UL listed, but it isn't as scary as some chargers Hackaday has featured. It has a transformer and a cutout running down the center of the board that appears intended to separate the 5 volt and 110 volt sides of the PCB.

The large electrolytic capacitor across the USB connections made it easy enough to identify which traces on the 5 volt plane are positive and negative. I soldered a wire to each.

Next step is to drill holes in the lower cover on the charger and in the battery case.

I didn't want to add any pieces of metal to the charger case that could cause a short, so I opted to glue the cases together. We'll see how well this holds up.

Run the wires through the holes...

...and solder them to the positive and negative terminals.

Then all that's left to do is put the covers back on, plug it in, and test it

Mission accomplished. One set of decorative LEDs, zero batteries required. And this can double as a USB charger, as a bonus.