Hello,
Thank you all for the answers!
After cutting, I adjusted the number of leds in my program, but without results.
I powered the leds by the 5V and gnd on the arduino Nano, and powered the arduino by a usb cable.
What happened in the first image is that I connected the data to the 5V input, and then I think I broke my 3 first leds. This also happened when I put 8V to the leds; some shine a lot brighter afterwards, also on 5V.
I saw that a 470 Ohm resistor should be added to the Din, that's why I added instead a potentiometer. Why does this make no sense?
Yes before cutting I disconnected everything. The power supply was in this case a simple transformer, which isn't stabilized so I was quite happy to get it on 5.04V with the Voltmeter.
Sorry for the image detail, I actually had a .jiff file which I couldn't upload, so that's why I transformed it to .png by snipping tool.
The soldering seems like a good idea, so I will try this. Are there some other things I could try?
Thanks in advance!!