I have had a rough time trying to understand why this isn't working correctly. Quick background: I set up a system to pop open car doors using touch sensors. I heard that powering the Arduino Uno R3 straight off the car battery is not a good idea, so I bought a buck converter. I wired it up and calibrated the output on the converter for 5V. When I hooked it up to the Arduino (via the barrel jack), I experienced some odd effects. The output pins and the 5v power pins were only getting 3.5V. I had to adjust the buck converter up to about over 7V in order to get 5v on the Uno.
Then the sensors started acting oddly. Auto triggering and alternately not working. Anyone know what I am doing wrong?