Thanks for all the responses on this. We solved the problem (for now) by putting the 100 nF capacitor between GPIO pin and ground. I had originally thought that this wouldn't work because the RC time constant of 3k and 100 nF would be too small to deal with low frequency EMI.
Seemingly it does work ok although the ultimate test will be a nearby lightning strike which could produce substantive low frequency components. The impedance of the 100 nF capacitor is about 50 ohms at 30 kHz.
https://www.desmos.com/calculator/av27hinyoi
Seemingly it does work ok although the ultimate test will be a nearby lightning strike which could produce substantive low frequency components. The impedance of the 100 nF capacitor is about 50 ohms at 30 kHz.
https://www.desmos.com/calculator/av27hinyoi
Statistics: Posted by neilparker62 — Fri Jul 25, 2025 8:00 pm