I need help with project ideas that include the construction of an AD converter and....?
I think I saw this on an episode of Gilligan's Island.  You know, where the professor was given a couple of coconuts, a vine, "magical" rocks from a volcano, and a washed up old rocket booster and he had to build a washing machine for Ginger's delicates!

Humor aside, what you're asking for is difficult.  Normally I would just skip answering a question like this but these types of questions come up so much that it's worth answering.  So, let me try to put it in terms that people (not just you) can relate to.  

In your question you have stated what you and your classmates are able to do, which is a 555 based ADC converter.  Then you go on to say what you really want to do, mainly automation, robotics, or a CNC thing.  And you have 2-3 weeks to accomplish this.  This is like being able to make a really good paper airplane and then planning to make a supersonic fighter jet.  In 3 weeks.  Or graduating with a Bachelors degree today and expecting to get your Ph.D. next month.  It isn't going to happen.

I'm not saying this to squash your dreams!  I certainly had them when I was just starting out, as did many others.  I also don't want to make you stop learning about electronics.  What I am trying to do is give you some guidance so that you'll be successful.  Nothing will kill your aspirations faster than to bite off a huge project and never finish it.  

Making something with automation, robotics, or CNC-anything requires knowledge of microcontrollers and/or microprocessors, motor control and driving, real-time software programming, digital signal processing and control loops.  Doing the mechanical fabrication of these devices is another difficult skill.  And interfacing this to a PC will require another whole set of talents.  Any of these abilities could be a career by themselves, and to find a single person with this ability is, well, a gold mine.  By all means, pursue that!  But do something else for your 2-3 week project.  

Here are some projects that you could reasonable accomplish in 2-3 weeks that is staying in the spirit of the "Rudimentary A-D Converter", but requires some additional stuff:


A multi-digit ADC converter.  Don't stop at just one digit, do 3 or 4.  
Have the ADC measure something other than an input voltage.  Maybe ohms, or light levels, sound levels, weight, or temperature.  A multi-digit digital thermometer is non-trivial but achievable and practical too.
Add a bar-graph in addition to the digital readout.  This actually isn't that hard to do, but could be useful.


So, take it slower and you'll be more successful.  You'll have plenty of time to create really big and crazy things later!