Let's consider the following schematics- a switch connected to microcontroller's input with internal 50kOhm pullup resistor.
Now, the following oscilloscope trace shows voltage at TP (CH1) and VCC (CH2, AC coupled) at the time, when CCFL lights are switched off in the room.
I do understand that two conductors can become inductively coupled, and that current change in one conductor induces voltage in other conductor. By looking at the oscilloscope trace, I see that signal voltage drops- my assumption was, that only more positive voltage can be induced.
Is my assumption wrong, and negative voltage (i.e. signal voltage drop) can be induced in a conductor which is close to other conductor?
Or this oscilloscope trace does not show inductive coupling, but something else I'm not aware of?
After I added an external pullup of 3.3kOhm, flicking light switch does not result in LOW level input signal seen by microcontroller anymore- I just wanted to understand what exactly caused it.