Well, I've never tried modelling a signal. But I have played with motors and microcomputers. If I had to design a mechanisim...
First thing is, I have no clue how to model everything above the thick green line. But, I might be able to help below. My basic scheme would be to have a servo motor spin a wooden disk with a hole in it. A stiff wire would thread through that and move the signal, passing though the ground. I would reccomend a servo motor specifically, because you can move it precicely instead of just spinning it. Only catch being, you proabaly need a tiny computer to operate it.
If you're good with computers and programming, you're clear. I've used Parallax's BASIC stamps (~ 30 dollars each, for the BASIC Stamp 2) to great success in various projects, but I hear you could use PIC computers to run it too. These can cost mere quarters each. Then again, just one Stamp could run a few servo motors by itself.
If you're a mechanical type, maybe you could heavily gear down a can motor to run the wooden disk. Then the computer isn't necessary, but you have to pay for more parts. I dunno how much that will cost. Or maybe you could put the motor on a heavy resistor, to make it run very slowly (the mechanism will be dirt cheap if you do it this way).
Have fun with the building!
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