Martin,
Generally speaking, you will usually want to ask more specific questions than you have here. But I want to give this a shot anyway.
Since you seem to be stuck at the very beginning of things, I am going to focus on how to get started (not on how to program it):
First: define the goal,
Second: identify actions (outputs),
Third: identify things we watch (inputs),
Last: identify the dependencies (logic).
1) Define the Goal : Usually this means that someone (maybe you in this exercise) must define all the individual functions that make up the machine. So for now, lets assume this is your job. What are we building? You say "a carwash", so lets further assume you are talking about an automatic carwash. For my answer I am going to say that there are three cycles to this car wash: Wash, Rinse, and Dry.
2) Identify Actions (Outputs): For the Wash Cycle you obviously have supply water so lets open a valve. We also want to scrub a little so lets rotate two vertical brushes and oscillate one overhead brush. Because of various car sizes, these brushes will need to move in/out and up/down, respectively, we will turn on reversible motors for each of these actions. We want to pull the car through so lets start a conveyor motor, too. You need to do the same for the other two cycles as well.
3) Identify Things We Watch (Inputs): We need to be able to start and stop this thing, so we have a "Start" button and a "Stop" button. We want to accommodate different height cars so we need a "height" sensor. Similarly, since different cars are different widths, we need a "width" sensor. Once again, you can do the same for the other cycles.
4) Dependencies: Now that you have defined the inputs and outputs, all you have to do is exercise you gray matter and decide which inputs need to be used to make which outputs do their thing and then write your program.
Now keep in mind that GOOD programming and system design require a lot more consideration than I have talked about here, but I am just trying to help get the creative juices flowing.
Steve
