The controller is described as:
a still-flat Disc [with] a BROWNISH STRIPE around the ring, like a coffee stain on white linen.
The controller is one of two disc items created by the Maker.
Purpose / Behaviour
It is discovered that the Controller, when it is in the proximity of a funnel stack, causes the funnel stack to collapse tightly, then launch into the air. Carter is the first to discover this behaviour.
With much apprehension a hand slowly brings the Controller closer to the Stack which now stands on a small ALTAR of random lumber. When it is within a couple feet, the Stack starts to go compact and as it gets closer, the collapsing speeds up...
During scene 212, where the flowers become colour coded (green / red), and the flower limbs get rebuilt with alternating red & green flowers, the boys create new controllers that are also marked red & green.
Later on in the screenplay, we see that the boys wear thin leather necklaces with the Controller(s) attached.