Finally!! I got it to work :)

Using "Continuity mode" on my multimeter, I found out that two connections didn't work. I probably damaged my board during soldering. Using two wires I was able to get it working

One step closer! Thanks again @sundberg84 and @scalz

Hey @Nca78
Thanks for the suggestion... I just watched this video from sparkfun: https://www.youtube.com/watch?v=f95i88OSWB4
Wow.... I was doing it soooo wrong haha. I'm gonna practice a bit to get my "balls" look more like "volcanos" (yes, still talking about electronics :P )

Thanks for the fast replies! The flappy thing isn't really a flappy thing :) It's a fixed panel which pushes the mail downwards into the bag and prevents burglars using a wire to open the door. The mailbox itself has brushes, so there actually no moving parts.

@mfalkvidd you were right!! I used a bit of force to get all pins connected and it worked :) I'll do some soldering tomorrow! Thanks again!

I noticed the build guide says to use data pin 3, however, the sketch from the examples (DhtTemperatureAndHumiditySensor.ino) uses pin 2. So maybe you got a mismatch there as well.

Again: thanks for the fast replies. Don't guys to work or something? :P

1. Connecting the GND to the FTDI made my monitor work again
2. Using the multimeter I see the radio gets 3.3 V, so I guess the problem is somewhere in the data pins.
3. @scalz Thanks for the link! I was looking for something like that. I'm gonna read it and I'll return with some questions, or answers ;)