## Home-Built Turing Machine 123

stronghawk writes

*"The creator of the Nickel-O-Matic is back at it and has now built a Turing Machine from a Parallax Propeller chip-based controller, motors, a dry-erase marker and a non-infinite supply of shiny 35mm leader film. From his FAQ: 'While thinking about Turing machines I found that no one had ever actually built one, at least not one that looked like Turing's original concept (if someone does know of one, please let me know). There have been a few other physical Turing machines like the Logo of Doom, but none were immediately recognizable as Turing machines. As I am always looking for a new challenge, I set out to build what you see here.'"*
## Re:You're Doing It Wrong (Score:5, Informative)

RTFA. I know that's a sin, but seriously, do. You'll discover you are wrong.

The microcontroller loads the program as written in ascii on an SD card. It also can write the initial data onto the tape. After that, the computation is, indeed, performed by the "machine". Hence the optical reader for the characters on the tape.

## Re:You're Doing It Wrong (Score:3, Informative)

Actually, if you'd bother reading the article, you'd find that the micro-controller is being used to do drive the electric motors, image process and maintain the turing machine's "state" [wikipedia.org]. that's it.

## Re:You're Doing It Wrong (Score:5, Informative)

Now, let's look at how this works. It reads a symbol from the tape using the camera. Then it checks its internal state, and sees what it should do with that symbol (should it change the symbol, change the state, and/or how should it move). Then it does that action and moves on to the next symbol as instructed by the last "rule". Considering that the only thing that the machine keeps track of from position to position is the state, it is indeed a Turing machine. The microprocessor's job (as he states) is to act as the read/write head for the machine. Turing never described HOW the head worked, just WHAT it did. And this head performs EXACTLY what Turing described. And that's why this

isa Turing machine. If you wrote a program on your computer that did this, it too would be a Turing machine. The delineation is in how it handles and stores states, not the method in which it "processes" data... And Turing's original work described a Turing machine as using a person to perform the actions (but strictly following the ruleset). So I fail to see how this could possibly NOT be a Turing machine...## Re:hmmm (Score:1, Informative)

A "Turing machine" is what you see in the video—or at least it would be, if it were an ideal abstraction with an infinite supply of tape and the ability to remember an unlimited number of states (although the number of states would be finite in any given program). A "Turing-complete" machine/language is one that is capable of simulating a Turing machine—again, adjusting for finite memory within the bounds of common sense.

The significance is that Turing proved that a Turing machine can execute anything at all that we would intuitively call an "algorithm". So, if you have a new programming language, all you have to do is prove that it can simulate a Turing machine like the one in the video—a very easy task in most cases, doable even with a perversely simple language like Whitespace or Brainfuck—and bang, you've proven that you have a language capable of executing any algorithm at all, a "Turing-complete" language.

## Re:Cool...I think. (Score:3, Informative)

Strictly speaking, limiting a Turing machine to a finite tape makes it something other than a Turing machine (e.g. a linear bounded automaton).