Thanks! I'll certainly swap the Pi to test this theory. A first glance doesn't show anything suspicious. The solder joints of the headphone connector look fine. Ground is actually soldered twice, and both of them look fine.
If this turns out to be a hardware problem (even though I was able to fix it with software) then I agree it would make sense to move it into its own thread.
Edit: I have put in a brand-new, out-of-the box Raspberry Pi 3, and see the exact same behavior. When I unplug the connector from the Raspberry audio-out, then the board behaves as expected (except for the bouncing).
I also tried a Raspberry Pi 2, and here the results are even weirder:
- Headphone connected: Stable "1" state (i.e. "0" on the input, which is inverted by your code).
- No headphone: Very rapid back and forth between 1 and 0
Note that the Raspberry Pi 2 was only connected to the Picade board with the audio cable, not also via USB.
One last edit: My version of the code does not work with a Raspberry Pi 2 properly connected. The pull-up always wins. When I connect or disconnect the headphone, input 5 goes down for a moment, and then up again.
