I have a sketch that has been running fine on an Uno 24x7 for a few months now. The part of interest is an interrupt routine associated with a momentary contact switch [on pin 2]. When pressed, the interrupt routine handles debouncing and simply toggles a boolean variable. The main routine checks status, takes action, warbles a buzzer, and does a Serial.print of the event.
Today, I moved the unit to the other side of the room and plugged it into a different PC via USB. Whilst it was running, I happened to turn on a nearby fluorescent light [not connected to anything but a 120v outlet].
The Uno buzzer went off, and appropriate msgs were written to the serial connection. I flipped the light back on, and got the same kind of results. I repeated this numerous times and got the same. I could flip the light or push the button attached to the Uno, and the results were indistinguishable.
I unplugged the USB cable and used the power jack instead. I got the same results, sans the print msgs to the serial connection.
The board itself has only a DS3231, an 8x8 LED matrix, a button, two LEDs & resistors, and a buzzer.
What is happening? How can I resolve the problem? [If another person had posted this, I would be disinclined to believe it, but surely someone out there understands the situation.]
I can readily supply a minimal code set and a Fritzing if it would help, though I suspect it would not.