Grimrock

Hi,

i'm a room with 9 traps, one of which is closed.
I see the key, but no way to reach it.
what's the catch?

throwing stones in the teleporter, i managed to close one, but eh.

another trap closed, but i totally dunno what i did... perhaps it's timed?

edit : indeed, it's timed... but far too long
or some mobs are randomly going over a plate somewhere.

I'm stuck at the same spot. I walking around in the level, and each time i come back the following plate is up. I managed to get the key, but it's so long, must have another way.
Now i'm walking to put the right plate up...

i suspect it's like a clock, try to rest until the hands are in the right direction?