Interference on interrupt pin


Just put this up as it might help someone

I was using a straightforward hardware interrupt on pin 2 of a Uno to catch a gas meter pulse.
Every time I switched a table lamp on or off, the interrupt would fire. Tried FALLING, CHANGE, RISING, nothing.

Tried Schmitts, RC filtering, software delays but nothing worked. The lamp had an LED GU10 fitting. Changing to resistive halogen lamp was better, but not perfect. Nothing showed on the scope. Tried different powers supplies to the Uno - still no good. The answer was, of course, a snubber on the lamp.

A 120R + 0.1uF X2 snubber across the switch terminals fixed everything, rock solid interrupts, no false counts. All to do now is get the lamp back together. I still don't know how the RFI/EMI was getting as far as the pin, but sometimes the unexpected can derail a project