Automatically lock my computer when I walk away from it

I would like to lock my Ubuntu after I walk away from it. Ideally, it should be a small Bluetooth tag / card that I carry in my pocket with a proximity sensor.

Is there some software and hardware that I can use for Linux for this purpose?

Basically, I'm looking for something like GateKeeper (but I don't know if this works for Linux)

The proximity feature will automatically lock and unlock your PC or Mac when you walk away from it. That will keep your machine from sitting unprotected.

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