I just wired a CD74HC4067 analog/digital multiplexer to a matrix of leds. The multiplexer is wired to control the LEDs, and is connected to an arduino board, which is connected via USB to my macbook air. Then, I noted something odd: when I approximate my hand to the multiplexer, some leds light up, even if I'm not sending any signals to the multiplexer. I can only assume that my hand is interfering with the signal pins of the multiplexer, which are four. Then, I noticed that when I unplug the AC charger from the macbook, the interference disappears.
I've searched about this kind of interference, and it appears that what is happening is some kind of AC hum touch switch like the one described in this article. It appears that a near by AC current propagates through the room and makes me induce some electric current on things I touch. However, why is it that the interference only works when the macbook is plugged to the AC charger? First of all, the AC part of the charger is far away, the cable that is connected to the macbook is already DC. If the problem was about a near AC current, it'd happen no matter if the cable was connected to the macbook.
If I touch the macbook and approximate the hands to the multiplexer, the interference also disappears completely. I tried to take my macbook away from the multiplexer to see if the AC signal was being generated inside the macbook but the interference appeared to stay the same.