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I have a project that turns a LED strip on and off using capacitive touch. The project uses an Arduino Micro as the brain and a 12v wall wart to power everything. I have the Arduino ground and negative from the power supply running to a common ground.

Everything works as expected during testing and the Arduino is connected via USB to my computer; however, when I unplug the USB and connect the circuit to the light installation, the lights just flicker on and off. When I touch my finger to the negative/ground (and presumably provide common ground), everything works as expected.

It seems obvious to me that I have a grounding issue, but really do not know how to proceed. Is there a specific technique for grounding from a wall wart? Do I need to somehow pull the ground/negative down? I feel like there is something obvious that I am missing.

Thank you in advance for any and all help.

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Seems like there is a problem in your wall wart, actually some people are having problems using wall wart with arduino. The problem is with the circuit of thre arduino regulator and with sime ripple variances with de wall wart, i sugest you to setup in your project a regulator (7805) and see if the problem is gone. This is by far problem not just with grounding, but with the power suply in general. I was used to make PIC projects and once it was happening exatly the same that happaned to you but the microcontroller started to work when i put my hand or finger close without even touching it :x. Crazy thing. If the power supply isn't well suited, the arduino enter in continous reset looping because of supply noise. So just put an external regulator, an maybe an extra capacitor (generally 10 nF) in arduino micro power supply rais (VCC-GND). Here's my tip. Good look man, tell me if it worked :)

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Yes, I agree with this and had exactly the same problem when migrating Arduino projects to PIC! If you're using a strip of Neopixels, there will be a lot of noise generated on the power rails, so use plenty of bypass capacitors and an LDO like @Matheus sugests. – Roger Rowland Feb 24 at 8:21
    
Here's an article describing how you can configure the simple regulator circuit: jumptuck.com/2011/11/15/voltage-regulation-friend-7805 – cortices Feb 27 at 22:54

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