Thank you for the explanation! Perhaps (most likely) I have wired it wrong?...
I'm using a 10k resistor.
I rewired it again, and I must have made a mistake before. I have it like in the last diagram and everything works perfectly! 
I do so love it when everything works as it should! It's like a little piece of magic! 
Thanks again wvmarle and Paul___B!
Now I have pull-down resistors and diodes in my arsenal for future projects. And the TTP223s ofcourse, they really are great little sensors.
Janezek:
Thank you for the explanation! Perhaps (most likely) I have wired it wrong?...
The diodes are the wrong way around in that image.
Janezek, how have these sensors held up for you over time?
They worked okay while I used them, however I had one issue with the way they work: they stop being triggered after about 5 seconds, so you cannot use them for sensing continuous touch longer than that - after 5 seconds you have to remove your hand/finger, and trigger them again.
I didn't find a way to get around that issue, next time I'll use them I'll look around if there are some models which do not have this limit...
But if you only need them for short touch triggering, then they worked ok for me.
It's only if you want them to be triggered for as long as there is touch present they seem to have an about 5 second limit.
There is no connection across the breadboardâs center. So the resistor in the pic is not connected to ground.
Janezek:
They worked okay while I used them, however I had one issue with the way they work: they stop being triggered after about 5 seconds, so you cannot use them for sensing continuous touch longer than that - after 5 seconds you have to remove your hand/finger, and trigger them again.
That is most likely a result of the sensor auto-calibrating, which is a feature rather than a bug.
Hi Janezek,
any chance you would post a picture of your wood and LED project ?
Sounds interresting !
Regards, and merry christmas!
Hi Niko,
sure, I've uploaded a video which shows how the wooden touch sensing part is made:
When touched, a few WS2812B LED strips start pulsating, nothing special.
Merry Christmas!