This is a more sophisticated Edge Detector robot. It's like the Basic Edge Detector, but you can "tune" it using the Knob Cubelet. You'll need a Knob and a Minimum Cubelet, which aren't in the KT06 kit.
Battery Block
Knob Block
Distance Block
Drive Block
Passive Block
Minimum Block
Start with the Basic Edge Detector robot: a Battery, Drive, and Distance Cubelet in a line.
Take the Knob Cubelet and attach it to the Minimum Cubelet, knob facing up.
Attach the bottom of the Minimum Cubelet to the top of the Drive Cubelet, making a little tower in the middle of the robot.
Now relocate the Distance Cubelet one cube space up, attaching to the side of the Minimum Cubelet.
Next add the Passive Cubelet on top of the Battery, to balance the robot so it doesn't tip over.
Finally, you'll need to tune the robot by adjusting the Knob. The robot should move slowly when it's on the surface, and stop when the Distance Cubelet is over the edge.