just use radius

If I'm understanding correctly you're talking about how you used to be able to use /testfor @p[r=5] in a repeating command block and a comparator would output a signal if someone came within that radius. If that's the case you can still do the exact same thing only the command and syntax has changed to /execute if entity @p[distance=..5] This command will execute from the position of a command block, to execute from a different location (like your door) you'd specify that in the targeting selector like so: /execute if entity @p[x=100,y=64,z=100,distance=..5] This will turn the comparator on if a player comes within 5 blocks of 100 64 100 and will deactivate when they are not in that area.

Hope that helps. There are a lot of things /execute can do and I don't think I can be much more helpful without more details of what, exactly you want to have happen. Remember that for these to work the way you want they should be in a Repeating Commandblock that is Always Active and should be in an area that's loaded when the player is at the target area (meaning nearby on in the spawn chunks).