This stage can be a little tricky if you do not know what you are looking for.
1. Your first task is to solve the puzzle on the floor similar to chapter 1-9. You will have to light up the floor in a certain order to obtain the bucket at A. To light up all the squares, press the buttons in the following order: 6-3-4-2-5-1
The notes have some symbols that give you clue for the final answer. Place the lever into B and you will see some light bulbs starting to shine. Use 5 as your center starting point. Count the direction of the arrows to obtain the code: 7239
You can instantly pass this stage by inputting 7239 at the passcode panel.