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Thread: Model K

  1. #1

    Default Model K

    I've been looking for a while about info on George Stibitz's Model K.
    All I can find is a poor quality image of the machine, scattered over teh interwebs. I want to know how this thing works, and maybe even a schematic.

    Can anyone help?

  2. #2


    Whatever it was it was probably just as impressive as....

    maybe sounded like....
    (check out the video)

    And was more expensive than...

  3. #3


    Its actually quite different from those. I have a picture, prepare for a great anti-climax!

    The first link you sent nig, by very creepy coincidence, i was actually viewing in another window after googling paperclip computer. Thats far too large.

    The Model K is from around 1937, made of torch bulbs, a few relays and a tobacco tin.

    here's a pic that I found, the only one that I know of in multiple locations on ter webs.

    All that I can find on it is that it is a 'Simple Boolean logic based binary adder'. You could input 2 binary numbers and it would display the sum on two torch bulbs. I assume by multiple flashing (fnarr fnarr) or something.

    Im curious because I want to build a simple computer. This isn't really a true computer, but you have to start simple and work up, and this is the simplest soundiong one I have found, thus I am looking for schematics. If I knew a little more about what I was dping, I could probably deduct it from the picture, but I'm not, so there
    Last edited by bbcmicro; November 26th, 2006 at 09:03 AM.

  4. #4


    what I was looking up seemed to indicate that it had 2 then 3 teleprinters attatched to it, although that could have been his later "complex number calculator"

    I suspect that the model "k" was just a simple relay half-adder, used to demonstrate the principles of mechanical maths. The relays being switched on by single pole momentary switches made of beer cans, the two light bulbs being "sum" and "carry", and the two cans on the left being batteries.

    Possible schematic attatched???
    Attached Files Attached Files

  5. #5


    It certainly looks simple enough. his Complex Number Calculator developed from this, and he had a teletype conected ~200 miles away to feed it sums, I think it was the first example of remote computing. Anyone one to talk me through it/how it works?

  6. #6


    The circuit is just a half-adder (it has 2 inputs A & B but no Carry bit in, and 2 outputs, the SUM of A & B, and a Carry bit out)

    If you aren't familiar with circuit diagrams etc... the relays are in each of the red boxes. the magnet is the square, and the double-throw (relay) switches are shown in the "normal" or magnet-switched-off position. when the magnet is energised, the ganged (shown by the dotted line) switches (arrows) move together to the other contact (circle)

    You can construct a truth table for each of the 4 states that the inputs can be, ie 00, 01, 10, 11

    then you get an answer for the SUM output that says "when only one input is on, the output is on" i.e. an exclusive-or function.

    you get an answer for the Carry which is when bits A and B are on, an AND function.

    The rest is just bulbs and batteries stuff, The AND function for the carry is two switches in series which must both be on for the light to light,

    and the EXclusive OR is rather like 2 way light switches, both switches have to be in opposite states for the light to come on.

    when you start to think further down the line, the input switches can get replaced by relay contacts from a previous logic gate, and the lightbulbs can be relay coils for a later logic gate, which is how the system joins together.

    relay logic is dead easy on one level, because an AND just has contacts of each relay in series, an inclusive OR has them in parallel, an inverter has contacts that are closed when the relay in't energised, and open when it is.

    The fun really starts when you add delays to relays with resistors & capacitors, and use bistable and current sensing ones. I was lucky, I had loads of them to play with when I was a kid, because my dad was a railway signalling engineer.

  7. #7


    I'm still a little unsure....I can input 2 states, either on or off by the two switches at the top left, thats 00,10,11,01. If i input 00, will it add 0 and 0, 10 would be the sum of 1 and 0, 11 would be the sum of 1 and 1, therefore only being able to display 1 or 0 on its two lightbulbs, but it wouldnt matter because the most you can get with this machine is 2?

    If this is correct (unlikely) then the highest possible sum would be of 11, (1+1) to get 2, which when displayed on the two bulbs would be on and off, or 10

    So the relays are arranged as two logic gates? Not sure which ones though.

    Sorry to appear dense, but the theory is like spaghetti in my head right now, and I'm only guessing the binary part

  8. #8


    print it out 4 times, and tipp-ex out the relay contacts. Draw them in for each state, then follow the circuit.

  9. #9


    I built a "2 bit adder" out of just 4 switches (double throw, triple pole), wire, battery, and 3 light bulbs.

    I wrote out all the logic table, turned it into boolean logic statements, then used boolean algebra to reduce the statements to a min.

    From those, I did the wiring.

    It worked, even did 1 1 + 1 1 = 110 correctly.

    What's really funny is, now at 57 years old, I don't think I could pull the same trick off again.

  10. #10


    helo !

    I also have been looking for a while about info on George Stibitz's Model K. but I have not find schematic. This "Microsoft Word - relay adder.pdf" attached it isn't exactly Model K because it work like 00 + 01 = 01, 01 + 00 = 01 that his good but A and B both 01 + 01 = 11!!! this is different should be 10 = decimal =2


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