In the RoboCupJunior, the wall of the play ground is smoothly painted black.
I've used buttons(touch sensors) for the detection of the wall, but it is not the most suitable sensor for the detection to the play ground's wall, cos everytime when the button was triggered, the robot has already "crash" to the wall.

I've tried Ultrasonic sensors, but the circuit is too compricate (the sensors that have already tested are too expensive to me).

Can anyone give me some suggestions about the sensors to detect the play ground wall? and also, any ultrasonic sensors data?