The doorway machines are a type of device introduced in The Room Two. They can be used to open Null portals to specific locations. Null users such as The Craftsman use doorway machines in order to circumvent the disorienting effects of the Null and avoid becoming lost in their travels. The machines are powered by soul energy.
All doorway machines have two main components: a projected beam of light and a receiver, mounted on the wall above where the doorway will appear. The light is beamed into the receiver from some form of mechanical device, the construction of which may vary.
In The Room Two
The first occurrence of a doorway machine is in The Room Two - The Lab. Prior to this chapter, the player traveled between levels by finding Null samples, which would open the next doorway for them. This changes at the end of The Lab, where with A.S.'s guidance the player must activate a door machine, which opens a portal to the Cog Room. From there, the player is able to solve the final puzzle that sends them home to A.S.'s Manor.
This machine is powered by the souls of A.S. and/or Lucy. The final step to activating the machine is restoring power to Lucy's heart. Upon doing so, a beam of light will be projected and the doorway opened.
In the original (mobile) version, the door machine is silver in color, and appears to have a number of buttons and dials. The beam that activates it is a red laser, projected out of the box at the center of the room. The design of the receiver has been changed and given a less high-tech look since the game was remastered for PC, but the red laser remains the same.
In The Room Three
The Craftsman employs a number of doorway machines to navigate Grey Holm without becoming lost in the maze created by the Null. Before each chapter, the player must activate the machine that leads to the corresponding next location. They are found in the study (which leads to the Pyre), the library (the clock tower), the greenhouse (the forge), and the elevator room (the observatory). The penultimate doorway, in the altar room, is opened not by a machine, but by The Craftsman's Key.
There are many components to the Grey Holm doorway machines. Each machine has a main breaker, a switchboard, a lever, and a frequency tracker, all of which the player must activate to open the doorway. The pieces are connected by a long length of electrical cord that runs through them, and a string of lights along the length of it indicate when a part has been turned on. When the machine is fully on, a beam of white light is projected from the switchboard. The receiver of the beam of light is more antique in appearance than the original Room Two doorway machine, with a bronze color and simpler, more vertical design.
The internal power structure of these machines is unknown, though at least part of their design is purely electrical. Presumably, like many of The Craftsman's machines, these doorway machines require the power of souls to operate. In one of his hidden notes, The Craftsman laments that the crystals he uses to power his doorway machines are running low on energy, and that he will soon no longer be able to bypass the Null's effects, making his need to capture the protagonist's soul all the more pressing.