Another great video for wall detectio using the ultrasonic sensor.

great video for wall detection and how the robot will make its way through obstacles.

When Roomba knocks into something, its bumper retracts, activating mechanical object sensors that tell Roomba it has encountered an obstacle. It then performs (and repeats) the sequential actions of backing up, rotating and moving forward until it finds a clear path.

This is a good video that has to do with cliff detection
.

This is a good video to do with cliff detection. it appears to be a light sensor or an ultrasonic sensor that is doing the work

if a light source is detected on the left of
the robot, the left motor will speed and the robot will veer away from
the light source.increahttp://gicl.cs.drexel.edu/wiki-data/images/7/72/LearningRoomba-SensorsActuators-StudentsGuide.pdfse