So, the batteries have arrived. Just in case, I have ordered some smaller ones, as well as the mighty 302040.

The wires were soldered (bat+ to PCB+, bat- to PCB-), and I tried to put back the device.

Here comes the annoying bit: the sensory switch. Or One-Wire Touch Detector to be more precise. The thing consists of some magic on the PCB, a wire, and a copper plate. Touching the wire works, so does touching the PCB. If you mess with it once the good battery is attached, you'll be blinded quite fast.

To assemble the device without being blinded, use some electrical tape to cover the LEDs. To make the thing easier to assemble, use an audio wire instead of a stock one for the switch. A touch of CA glue (or epoxy in my case), and the light is as good as new (or even better).