|An Exercise in Jurassic Silicon.
|the long hard bttf coc.. um, i mean clock
|Findsomethingandburnit.com 2004-2020. Doc J.Vicious & Murph-O-Minion enterprises. Godfrey, ON.
Content may be used on the condition that the original author be clearly credited.
|Last updated Nov 20, 2019.
|Why would you ever do this?
It may seem hard to believe now, but those
frightening little now ubiquitous talking boxes and
pucks infiltrating your home that allow Big Brother
and his corporate minions to catalogue everything
from your dietary preferences to your personal
porn predilections weren't always a thing.
Yes millennial fuck, people actually managed
without iphones and Google in the days of the
proud full bush, when integrating a few hundred
transistors was relatively impressive. We even
used to smoke on the bus.
Statistically speaking, the 1980's are as long gone
to anyone reading this paragraph in 2019 as the
1950's are to me. Some other weird time where
old people who didn't have Sony Walkmans did
weird things. Bryl Cream? A little dab'll do ya?
Soda Jerks? Record players in cars? Say what?
This in itself not really of any concern to me. I
accept my obsolescence. The unrelenting arrow
of time. The new guard. So be it.
Vape on grasshopper.
But imagine a time if you will, The time predating
movie CGI. Pew-pew lasers were drawn by hand
on film negatives (Rotoscope), and exploding
space ships were in fact little model space ships
with fire crackers rammed up their wee arseholes.
The era silicon still so anemic and expensive
relative to todays standard as to be mostly
disregarded even by the cutting edge Hollywood
special effects legends of the day.
The now technically trivial 40 second green
monochrome wire frame rendering of Snake
Plissken's approach to the World Trade Center
displayed on the instrument panel of his then
futuristic 1997 glider? in actuality video footage of
a scale model - a fucking hand built model! - of
Manhattan, painted matte and outlined in
reflective tape, then filmed under black light.
Likewise, Doc Emmet Brown's DeLorean BTTF
clock simply never existed. I believe for the
production, nothing more than back lit litho-film
mask. Or so I recall reading.
The old boys certainly knew their smoke and
In reality though, The BTTF clock might have
existed. To be certain, available 1980's medium
scale integration would have made construction
readily possible, albeit tedious and relatively
A week's pay worth of Archerpack 7400 logic, and
a fortnight of evenings with the hot stick would
have made such a clock a 1980's reality.
Due to the nature of a clock display, it seemed more
natural to work right to left.
The specific part numbers only chosen at the time due
to familiarity with the parts. Other suitable replacements
naturally would work just as well.
The first four decades display minutes and seconds
from 00:00 to 59:59 and are comprised of two identical
sections utilizing a 2 input AND gate which on the sixty
first clock pulse excludes displaying a nonsensical "60"
by resetting the decade counters. The low to high
transition of the reset pulse also carries ahead and
increments the "next" decade of the clock.
Military time rolls over to 00:00 from 23:59, as opposed
to 1:00 from 12:59 for the civvies.
One half each of a 7420 4 Input NAND excluding "13"
and "24" o'clock. The mode select when high, both
enables the 12 hour reset and preloads "1" into the
least significant hours decade.
Although this satisfies the logic for the six decades so
far presented, the plot thickens. We are now presented
with the problem of incrementing the day of week
display, as the AM/PM time format will send an advance
pulse at twice the frequency - once every 12 hours - as
the one per day generated by the 24H output.
The last decade of course, with the availability of
ultra cheap programmable silicon certainly has
seen the realization of many functional "what if"
clones available for sale on line.
So what would have been required of 1984
contemporary electronics to create an actual leap
year through minutes clock display?
The answer is: It depends.
For a static dummy display simply and forever
illuminating one particular moment in time, a lot of
resistors - assuming the use of actual off the shelf
For a working clock, a quarter pound or so of 70's
date coded TTL and a week's worth of evenings.
More still to realize direct keypad entry, although
possible with glue logic and 16 pin DIPs, one
would likely at this point have utilized an MPU,
especially if the clock were to be actually
interfaced with a time machine.
This discussion of course is likely academic.
I have no plan at the moment to build this clock in
this particular manner. To do so in an era of
$0.30 one chip solutions would be .. well, crazy.
One possible exceptions being the possibility of
an art only build. For several years I have wanted
to build a "dead bug" circuit on a clear substrate,
using adhesive copper tape as interconnects.
The schematic presented is, I believe, fully
functional, even accounting for leap year
correction - a reproduction of a pen to paper
exercise long ago performed, cup at hand, for the
sheer hell of doing it, in fact, everything beyond
the tens of hours decade an addition to an actual
6 decade clock designed and assembled as a
high school science project for my then girlfriend's
At the time I recall expecting the full MSI long clock
(Years through seconds) circuit schematic would
quickly grow into an unwieldy monster that would
be offhandedly abandoned at the last sip of my
beverage, crunched into a ball, and forever
purged from memory.
As it turned out, the ripple nature of the
sequential logic and simple incremental rules of
displaying time made for surprisingly quick and
easy, if not somewhat ugly, solutions for each
The resulting circuit however was never intended
to be elegant. Indeed almost the opposite; It's
creation and presentation entirely for
entertainment, although still a valid representative
journey back through the wispy smoke of rosin
core real tin-lead solder, for a look at how things
were sometimes done when your grandma was
still wearing leather tassled boots and rocking out
to Iron Maiden.
No one would have built this in this particular
manner - most likely having implemented a Z-80
instead. Still, they may have.
Notes and errata:
This clock as drawn is only capable of counting ..
or being set - forward. Perhaps one day the
whiskey cup will compel me to work out a
bidirectional clock, and perhaps even overflow the
year counter to display BC/AD.
Maybe even keypad entry, but I wouldn't hold my
breath: Time, ironically is the one thing that I'm
perpetually short of.
The circuit as presented is in logical order here ..
That is, displayed (L-R) as a 4 digit year,
abbreviated month (ie, JAN, FEB, etc..), date, day
of week ( MON, TUE), Am/Pm/H, and then
HH:MM:SS .. Well - any builder insane enough
may want to physically reorient the actual displays
for a more real world intuitive readout - For
example, Mon Aug 03 2004 11:59:59.
As well, this is more a logical representation than
an electronic tutorial. Although the circuit should
certainly be functional, specific pins not used for
active logical functions, for example, the preload
pins on the year display counters, were ignored or
omitted in the schematic.
With that said, a gawk back in time before this was
possible with an Arduino and a few shift registers.
And so the ugliness begins. A 4017 is used to select "A"
or "P" and divide the 12 hour carry by two. With the 24H
mode selected, "H" is displayed.
The fast eye will note that the ripple pulse transition
between decades is all over the place. This of course
would be unacceptable for any type of numeric control,
but the ugliness will pass too fast for the human eye to
notice in an output only display like this.
The current limiting resistors on display segments a and
c might require a slight value tweak to compensate for
dimming of the segments due the forward voltage drop
of the steering diodes, although this appeared to be far
less noticeable on eighties deeper red LEDs.
Holy diode shit Batman! Yes, steering diode ROM was
actually a real thing once upon a time.
Note that LSD segments "f" and "l" do not require
decoding as they follow the state of "e" and "i"
respectively. As well, segment "e" of the center display is
always illuminated, and therefore can simply be tied to
Although this may appear archaic, there really weren't a
lot of early 1980's solutions available for solving this type
of logic. Steering ROM could be built relatively compact
and had no obvious disadvantage over any other
method for an array approaching this size.
However, with our total IC count already now at 18 (not
including the time base prescaler, the contemporary
nerd may have at this point began to consider a full
But that's not what this silliness is about, is it?
Satisfying the four possible month rollover conditions to
display the date as 1 to either 28, 29, 30, or 31.
The date is incremented by the tens of hours carry
reset, but the end of month reset is determined by
looking ahead to the month display decoder section.
Leap years are determined by dividing the year (which
will be 0000 upon power up) least significant digit clock
pulse by 4, either inhibiting or allowing a 29 day count.
Note that the logic will allow invalid dates after a power
failure. This deemed unnecessary to correct based on
the assumption that the clock would likely be readjusted.
Are we having fun yet with those silly cunt rectum fires?
Decoding the month display is again straightforward, if
not tedious. Note the switch to common cathode
displays and PNP drive transistors. I don't recall why I
chose a 74154 to increment the months (next panel),
this chip requiring a counter anyway - I might as well
have simply cascaded another pair of 4017's.
Many ways to skin a cat depending on what's in the junk
The month least significant digit must be able to display
the letter B, requiring either a 16 segment display, or
accepting a lower case letter. The diagram as drawn,
with the g segment full display width, would produce in
my opinion a somewhat ugly character.
All logic to properly display the day of month end count,
and February 29th on leap year, has now been
satisfied. The only part of the clock remaining is to
increment the month display (there wasn't enough room
in this panel), and overflow to increment the year.
The actual month counter is straightforward. A counter
increments a 74154 data selector, the overflow (month
13 disallowed) of which resets the month counter to
January, and increments both the year counter and leap
year divider by one.
The four decade year counter is likewise
straightforward, simply displaying 0000-9999 with no
Although we've satisfied our Back To The Future clock's
actual display and decoding logic with .. Lol - 36 MSI
chips and a hundred or so 1N914 diodes, it's still going
to require a timebase to actually make it a functional
clock, as well as a means to set the time and date.
As we're in 1984 .. and our DeLorean is devoid of a
60Hz AC line signal to divide, Doc likely would have
hacked a cheap digital watch for the former, and using
steering diodes and pushbuttons, injected the raw
timebase into the clock inputs of the various stages for