The recognition of the different kind of obstacles is given by the different output signals that can increase or decrease in intensity respectively at the increasing or decreasing of the obstacle dimension. We think that the simplicity of the output signals that the user have to learn to interpret is the goal of this device, for this reason we take the information strictly necessary on the surroundings of the user and give to her/him only two kind of signals for each foot, depending on whether the obstacle is positive or negative. The introduction of the ultrasonic sensor, as after explained, is due only to functional reasons and to make more reliable the system.