I wonder if someone can help on this matter, as I can't find a clear answer.
Summing up: I am using a Teensy LC connected with 4x capacity sensors and 1x Neopixel LED ring (12 pixels). By touching each sensor, the LED ring should change its color and one switch on a 8-Relay-Module should be triggered. On the high voltage side are 4x micro pumps (each needs 12V) located.
My wiring is as follows:
5V USB transfer going to Teensy LC
12V power supply to power the pumps on the high voltage side
LED-Ring and VCC low voltage side of the relay are connected to 5V Output Teensy
Each component independently (LED-Ring, Relay, Pumps) works perfectly. When the high voltage side of the relay is not connected to a power supply, then the LED-Ring works as it's supposed to. As soon as the 12V is powering the pumps, the LED-Ring doesn't work (it emits light, but it doesn't change anymore) while the pumps are running. In my initial wiring, I had two different power supplies for each (1) Relay (high voltage side) and (2) Teensy LC (LED-Ring and Relay low voltage side is connected to it)
What I have already tried:
Having a common ground by connecting the Teensy LC to the ground of the 12V power supply / also tried it with connecting the Ground of the relays low voltage side to the same ground as the high voltage side
Isolating the Board from the relay by connecting the jumper of the relay to an external power source
Connecting the Teensy LC (5V), the LED-Ring (5V) and the low voltage side of the relay (5V) to the same power supply as the relays high voltage side (12V) by putting a step down converter (12V to 5V) inbetween
Get a blank sheet of printer paper and a black ball point pen and draw a schematic of the circuit.
Nobody wants to try to trace lines in a Fritzing. It is counter productive.
List all you loads numerically with there voltage and current. (I don't see any pump current or Neopixel current info) . If you don't know post links for everything except the Teensy.
It finally works! Powering the 5V parts in parallel (Teensy LC + LED-Ring + Relay) didn't work well -> the LED-Ring didn't interact as supposed to. It seemed more stable when the Teensy receives power (in my case by a 5V powerbank) and then the LED-Ring and Relay are both powered in parallel by the board.
But the biggest mistake was running 12V through a breadboard thanks @J-M-L !