I wanted to solder an ON/OFF switch for my arduino. So I bought some switches: HERE (https://www.amazon.de/gp/product/B071X935QN/ref=ppx_od_dt_b_asin_title_s01?ie=UTF8&psc=1)
It works, but there seems to be some power drop since the lcd display which I connected to my arduino will no longer work properly (but connecting it to the battery directly without my soldered switch works). See attached image of my soldered switch.
I assume the poorly made connections between cable and switch is causing this.
Are there some kind of adapters which I can use to connect pin cables with power jacks/switches etc?
There are both switches and barrel jacks with screw terminals :)
But if you want to do more, just spend some time learning to solder better. Seems like you don't really let the solder flow.
Also, those jumper wires you used are usually really really thin. Little bit bigger is better and probably easier to solder as well.