>> Perhaps you would have enjoyed a visit to Stoney Stratford telephone
>> exchange. It's a sprawling building, filled with rack after rack after
>> rack of cases, each case filled with dozens of relays. And they're not
>> in cases or anything, just "naked", so you can see (and if you desire,
>> move) the working components. There is also several tonnes of wire
>> overhead feeding this equipment. And all of it is as silent as death.
>> It's like a museum or something.
> 
> Some time ago I was reading a Wikipedia entry about some of the relays 
> in a central office. Someone described the sound as deafening back when 
> everything was electromechanical. Especially on heavy phone traffic days.

Looking at this cavernous, echoing building stuffed *litterally* to the 
roof with densly-packed racks of switches... Christ, it must have been 
like hell on Earth! Deafening would be an understatement!!

>> Myself, I visualised a computer made of pressurised water fed through a
>> series of simple mechanical valves. Sadly, I fear that for reliable
>> operation, you would need inside water pressure. And if you wanted a
>> clock speed of more than about 0.02 Hz, you would have to use steel
>> piping and pyrotechnics to sustain the necessary pressure! It would be a
>> very "kinetic" experience though.
> 
> Hmm. Water hammer. I can't imagine it working purely on water pressure 
> alone, though. I keep thinking solenoids to control the valves. Would 
> that be cheating?

Think about how a transistor works: You have one circuit that controls 
another. So how hard would it be to rig up a valve where pressure from 
one pipe moves the valve allowing (or blocking) water from flowing 
through a seperate circuit? In principle it ought to be pretty trivial. 
(Of course, making a valve that actually works well in practise probably 
requires far more equipment than I personally have...)

The problem is going to be that once you have more than a few of these 
linked together, effects like gravity and insertia become significant. 
These don't affect electronics, for some reason...

>> Also... when I tried to build my own machine out of 7400s, I quickly
>> discovered that the gates don't appear to function as their truth table
>> indicates that they should. (!) I also looked into playing with FPGAs,
>> but the cost is prohibitive. (And, knowing me, I'd just make something
>> that doesn't even work, and then spend months trying to find out why!)
>> Plus, Xilinix (?) have a free simulation tool available, and it's just
>> painful to use. I dred to think what actually synthesizing with it would
>> be like...
> 
> They do. If, however you have the gate connected to a switch, and the 
> switch opens the circuit, you'd better have a pull-down resistor, or the 
> gate will float. TTL is rather forgiving of this, the input will float 
> high (e.g. it will be interpreted as a 1) CMOS, on the other hand, can 
> potentially self-destruct if an input is left floating ;)

Maybe that's what I did wrong... I was expecting an open TTL input to 
float low. Anyway, I don't think I shall go down the FPGA route. (!)

>> In solomn truth, it's probably simpler and easier to code a small peice
>> of JavaScript that controls a little Flash animation on a computer
>> screen. But there's something impressive about being able to pick up a
>> physical object in your hands and see that there really are no tricks...
> 
> Yes, but, as you say ... there is something impressive about a physical 
> object. :)

Indeed.

Another thing I thought about was a lego-style kit where you have lumps 
of plastic in the shape of logic gates, with nice connectors for the 
inputs and outputs, and LEDs in each input and output to indicate which 
logic state it's at. The trick, of course, is power routing. ;-)

-- 
http://blog.orphi.me.uk/
http://www.zazzle.com/MathematicalOrchid*

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