I have twice now killed a GPIO input pin by forgetting that 5V is greater than 3V3. This is annoying, but at least it's not the whole Pi. I updated the schematic to include the simple potential divider that I keep forgetting (circled in red). The values aren't too critical, here I've selected them to keep the output (input into the Pi) below 3V even if the input gets to 5.1V.