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It seems like I should be able to ~abuse~ use the input pullups on the Pi to drive an LED, and that means the input is open so I can also sense a switch for occasional use. I can't talk myself into or out of it, though.

If I break it into two problems, I think I can drive an LED by connecting it to +3.3v through a resistor and connecting the cathode to the GPIO. If I use a pullup it will remain off, if I use a pulldown it'll turn on. Right?

Given that, while in pullup mode, a NO switch connected from the GPIO to ground will trigger. In pulldown mode it'll do nothing.

That 'just' leaves the other parts of the problem.

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Unfortunately No!

Pullups are ~50kΩ so would only supply 6µA which would be invisible - ALSO they only work on INPUTs.

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