Step 2: wind the transformer
this transformer is the crucial element to make joule thief work with lemon. it requires much more winding then in the normal joule thief design and only plain ferrite works - all colored rings failed.
1. take about 1.5-2 meters of copper wire, bend it in half and wind the coil with it. in this case it took around 50-60 windings arranged in two layers.
2. after the winding is done - cut the double end, so that you have 4 loose single tips.
3. clean the tips from the isolation lacquer with sanding paper
4. with a multimeter set on conductivity test, find two separate windings
5. twist two opposing ends of different windings into one contact
6. pre-solder the tips of the transformer.
your transformer is ready!