YES, my first is working. More to make. In my home now: "Big brother is watching you" Placed my first at the door of the refrigerator ;-)

@GertSanders, you write:
powering something with low consumption
You can set both pins D2 and D3 of the processor to OUTPUT, do not mount the pull up resistors, do not activate the internal PullUPs, and by setting one of the pins to HIGH and the other to LOW you can power something. You will need to take care that a maximum of 20mA is consumed, as this is the recommended maximum amperage the pins of the atmega328 can drive or source.

Sure not to mount the pull up resistors. But how do de-activate the internal PullUPs, and by setting one of the pins to HIGH and the other to LOW.

Can you explain in "jip en Janneke/suske en wiske" language how to. With the help of this site I made my first one:
https://forum.mysensors.org/topic/3018/tutorial-how-to-burn-1mhz-8mhz-bootloader-using-arduino-ide-1-6-5-r5

For me here the same.

When using a sensor ( door, PIR, temp) no problem. When using a relay I get the same as above.

Log from my gateway ( connected via Pimatic):

debug [pimatic-mysensors]: <- MySensorSwitch { sender: 7, sensor: 1, type: 2, value: '0' }
22:01:52debug [pimatic-mysensors]: <- I_LOG_MESSAGE 0;0;3;0;9;read: 7-7-0 s=1,c=1,t=2,pt=0,l=1:0
22:01:52debug [pimatic-mysensors]: <- MySensorSwitch { sender: 7, sensor: 1, type: 2, value: '0' }
22:01:52debug [pimatic-mysensors]: <- I_LOG_MESSAGE 0;0;3;0;9;read: 7-7-0 s=1,c=1,t=2,pt=0,l=1:0
22:01:52debug [pimatic-mysensors]: <- MySensorSwitch { sender: 7, sensor: 1, type: 2, value: '0' }
22:01:52debug [pimatic-mysensors]: <- I_LOG_MESSAGE 0;0;3;0;9;read: 7-7-0 s=1,c=1,t=2,pt=0,l=1:0

etc... etc....

and from my Relay:

read: 0-1-7 s=1,c=1,t=2,pt=0,l=1,sg=0:0
send: 7-7-0-0 s=1,c=1,t=2,pt=0,l=1,sg=0,st=ok:0
read: 0-1-7 s=1,c=1,t=2,pt=0,l=1,sg=0:0
send: 7-7-0-0 s=1,c=1,t=2,pt=0,l=1,sg=0,st=ok:0
read: 0-1-7 s=1,c=1,t=2,pt=0,l=1,sg=0:0
send: 7-7-0-0 s=1,c=1,t=2,pt=0,l=1,sg=0,st=ok:0

I am a newbee, but what can I do to help ?