The dark rooms with the invisible trapdoors make it a bit harder so I wrote down the right order on my map.

This one features another example of the Lightning Machines madness explained in my howto: you just need to use 2 switches to turn off all machines in the exit room. Thus you only have to collect 3 of the 4 keys to the exit room. If you don't wanna spoil the fun and try it for yourself don't move your mouse over the spoiler alert below: