Sure you can use all three. I was just suggesting the path of least resistance to get to a partial goal. You're a good planner (diagram is pretty good) so maybe your first stop is your final goal!So get the sensors, play with them a bit. Type up some code and post back here for critique!The parallax ping is a very popular sensor. It is not like a potentiometer. You need to talk to it to start the measurement, then listen on the same pin you talked to it for a pulse. The width of the pulse tells you distance (round trip).Here, take a look at my codes for this car reverse obstacle sensor project:http://liudr.wordpress.com/2011/02/12/arduino-parking-sensor/It's got an intro to the sensor.When you download my code, do the nutshell first. It's easy to read through and understand what I do. The full version has interactive features you don't need for your project (at least at this stage).Let me know if you have questions!