BARRY  FONE  MUSIC
A few of my cool projects of little interest to anyone else
The DIGIAC Logic Trainer - from 1966!
Since I've worked on analog and audio equipment almost all my life, and now work in Avionics which uses
the latest in digital and microprocessor circuitry, I figured it was time to learn something about digital and get
a Logic Trainer - and boy did I find a dandy!  Thanks to an eBay seller who was willing to accept a money
order since PayPal stole all my money (or at least tried) and I refuse to ever use them again, I managed to
snag this super-cool Digiac 3013KD Logic Trainer that was once used at Miami State University - in 1966.  

To many, this thing looks like a very early computer - and in fact, it can indeed be wired to perform a variety
of crude computer functions.  But the layout and where it came from strongly suggests that it is actually an
ancient (and surprisingly well-equipped) digital logic training console.  Like newer trainers, it has its own
internal power supply, and all gates are pre-wired with their needed supply voltages to minimize the number
of jumpers needed to construct various experimental circuits.  As a brief crash course in electronic design,
engineers "breadboard" their designs to work out the bugs before committing to a circuit board layout.

A typical breadboard is simply a block with thousands of tiny receptacles, into which you can fit transistors,
integrated circuits (or "chips"), and small wires to connect the parts together.  But the really cool thing about
a trainer is that all pins of the internal components are wired to very convenient jacks on the outer panel,
with the jacks surrounding the SCHEMATIC symbol of the part.  This means that you can spend all your
time wiring the circuits without constantly referring to data sheets to find out which pins connect to which
part of the chips.  Basically, you go straight from the schematic to the wiring operation with fewer hassles.

Here is an example of how much easier this method is - a basic NAND gate:







Incidentally, a NAND gate stands for NOT-AND.  With an AND gate, if input one AND input two are both
high (say, +5 volts), then the output will also be high.  If only one input is high, then the output will be
low (usually ground, or 0 volts).  With a NAND gate, the output is INVERTED, meaning that a high output
out of the AND gate is NOT-ted (inverted) and will be low instead of high.  The tiny circle at the right side
of the gate means NOT, and the signal is inverted.  Without the little circle at the output, the symbol is
a simple AND gate.  Okay, let's get back to this cool project that I'm so obsessing over!

Upon testing all gates, flip flops, indicator lamps and displays, I was very happy to see that EVERYTHING
still works - which is great since the chips in this unit became obsolete decades ago!  But no matter, since
literally ALL digital equipment, even brand new cutting edge stuff, still uses these exact same basic gates -
just thousands of them in some cases to produce more complex function chips.  In fact, almost all the chips
inside this unit - from the full adder to the D/A converter, and even the display drivers and keyboard BCD
encoder - are all just combinations of NAND gates inside.  And yes, even the website you're looking at right
now, as well as ALL computer functions, are only millions of combinations of NAND gates - or even more
simply, nothing but combinations of 0's and 1's - everything, literally everything that happens inside any
computer - is because something is either on or off.  Your computer is nothing but a bunch of switches!
If you could hook up millions of toggle switches to a 9V battery and flip them fast enough and in the right
sequence, you could do everything you're doing right now on your home computer.  AMAZING !

You've seen 7-segment LED displays in all sorts of places - calculators, digital clocks and watches,
gas station pumps, DVD players, etc.  Well, feast your eyes on one of the very first types of 7-segment
displays - basically, 7 light bulbs inside a glass tube!






















Okay, so I've tested everything and found that the whole thing still works.  Now, the fun part:  Where can
I find the many JUMPER WIRES needed to connect things and make circuits?  Well, the tiny plugs are no
longer available to make jumpers from.  Sure, I could twist wires and stick them in the jacks, but this is a
pretty cheesy and unreliable way to do it.  Then I thought about it awhile, and got this COOL idea:


















Pretty nice, almost professional looking probe tip, hmmmm?  Here's how I make them:

Use drill bits to find the exact size of the hole you want to plug into.  In this case, it was 5/64, or .080".
This size bit fit nice and snug - with no play, and not requiring excessive force to pull back out.  Then, I
used that bit to drill a hole in the wooden top of my bench.  Next, I stripped and tinned about 5/8" on each
end of several wires, which I obtained by purchasing a cheap 16 gauge extension cord.  !8 gauge would
have also worked, but would have used up too much solder to form that nice probe tip.

Once the wire ends were tinned (just a very thin coat of solder so it will fit into the 5/64 hole in my bench),
I then inserted the wire end vertically into the hole, leaving just enough tinned wire to be able to touch with
the tip of my soldering iron.  By the way, the reason I tinned the wires beforehand is because the solder
coating conducts heat much faster than the bare wire.  Once the wire was in the hole, I then heated
the little bit I left exposed while feeding a nice amount of solder to the exposed wire.  The solder melts,
flows down into the hole and forms a nice rounded tip as gravity pulls it down.  Remove the heat, let the
wire sit for about 10 seconds, then (again using the small exposed portion) pull out the wire using a pair
of needle nose pliers.  It's nice to drill your hole about an inch from the edge of the bench, so you can
use the bench corner as a lever - the wire is pretty hard to pull out.  But once the task is completed, you
have the nice rounded, snugly-fitting tip you see in the picture above.  And these things work GREAT!

The last thing I did was convert several jacks to perform a different function.  There is a row of 16 jacks
that connect to a receptacle called "Interface Plug" and nothing else.  Since the interface plug was probably
used to connect a classful of trainers to the instructor's station, the plug is useless for my needs.  So, that
frees up 16 jacks to use any way I want.  Well, sometimes I might want to connect the output of one gate to
the inputs of several others at once.  The unit currently has only 2 such "tie points", 2 rows of 4 jacks.  So,
I created 4 more tie points by connecting 1-2-3-4 | 5-6-7-8 | 9-10-11-12 and 13-14-15-16 together, to give
me a total of 6 tie points instead of the original 2.  Then I used a silver paint pen to make ALL tie points
the same color, original and self-created:








Now, all I need to do is mark a line between each set of 4, since each quad set is isolated from the others.
Usually, even a very basic NAND gate chip contains 4 or more
actual gates, plus a supply voltage pin and ground pin.  This results
in a black rectangle wth 14 or more pins, with no external clue as
to which pins connect to which input, output, etc.  This picture of
ONE SINGLE gate makes cooking up circuits MUCH easier!
The DIGIAC uses something called a
NUMITRON tube - to be exact, it has 3
RCA DR-2000 Numitron tubes.  This tube
has 7 filaments that, with voltage applied,
causes the filaments to heat and produce
light - exactly like how light bulbs work.

Of course, which segments light depends
on which pins you apply voltage to.  The
DIGIAC display requires 4 inputs (all of them
either low or high) in binary - coded - decimal
format (BCD).  Then, chips composed of
NAND gates convert the 4 BCD inputs to the
seven numeric segments you see here.

Since computers only work with two voltage
states, binary is used (0 or 1).  BCD is a
more efficient way to represent binary.
Now I'm ready for some serious experimenting!  I started working on a cure
for cancer last night, but the wife made me take out the trash instead.  Women!
Special thanks to George Flohn at IDEAL SURPLUS, where I purchased this trainer.