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 Edit

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...

Coloured Controllers Edit

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.

Necklace Edit

Later on in the screenplay, we see that the boys wear thin leather necklaces with the Controller(s) attached.

Alternate Method Of Control Edit

Later, in scene 152 ,Euclid discovers that his stack of Funnels also response to Voice Command.