|Cold War Era Glow Discharge Redundant Rev Counter for the Bus.
|a universal nixie tachometer
|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 July 28, 2019.
There are far simpler ways to skin this kitty with
cheap and readily available modern
programmable logic, and arguably, point to point
hand rolling a potentially less functional fancy end
result using a greater number of parts seems
somewhat .. archaic.
But this is in fact exactly what we're going to do
No microcontrollers, no IDE's, no code downloads.
Like the Heath offerings of an era passed: Wire it
up correctly, adjust a trim pot or two, and it works.
Admittedly I have no "good" reason for such an
approach, Engineering, in theory, should employ
the simplest and most elegant solution to any
"But I remember a time of chaos, ruined dreams,
this wasted land..."
There was a time when nerds of my vintage would
rush religiously to the news stand, anticipating the
arrival of the latest Gernsback periodical.
Only to discover that the featured project required
a pre-programmed PROM, a Don Lancaster
schematic too complex to publish in it's entirety, or
some other obscure part that required sending
Although to be certain the DIP parts selected for
this project are decades obsolete, they are
(surprisingly) still relatively trivial to source.
This Nixie tach build, which will take place in the
evenings, after hours on my Mississauga work
bench will as well be filmed in it's entirety as a
video experiment, without editing from concept to
At the time of this writing, I have yet to breadboard
a fully functional circuit (Although I already have a
general block layout in mind and know which
components will be utilized), or generate a
schematic diagram. The experiment will be to do
Edit 2021: The build is finally underway, and
footage - such as it is .. Is now out there.
The heart of the project is an ICL-7135 4-1/2 digit
analog-to-digital converter. This old school LSI
chip configured as a precision voltmeter with
multiplexed BCD outputs which will utilize the four
least significant digits to display .0000 - .9999
representing revolutions per minute. Like I own
anything that doesn't peg at 3000.
An LM-2907 frequency-to-voltage converter IC will
convert the engine's derived tachometer signal
into the appropriate proportional voltage to drive
the A/D converter to displaying useful information.
Several support chips will be required as well, and
various options will be explored to make this tach
project truly universal .. such as implementing an
amplified audio coupled "front end" input to detect
the injector line pulse spikes on a fully
mechanically injected diesel engine.
The high voltage supply, cathode drive, and
anode switching should be relatively
straightforward, but of course will be covered as
I have placed some thought into final alignment,
and I believe that the microphone will in fact be my
During the meat of the build I will be using an
adjustable frequency 555 as a visual indicator
that everything simply "works" .. but for final
alignment, an audio driven front end and a
calibrated audio generator - in the form of a free
phone app - no test kit required, and with a bit of
simple "four banger" math .. I should be able to do
Then finally some fanciness. Mucking about with
glee and abandon .. The addition of an IN-9
Russian neon bargraph display, a 6E5 eye-tube,
of which the eye will open proportionally from idle
to redline, and perhaps a Decatron "spinner" that
will also appear to do it's thing somewhat visually -
if not instrumentally accurate - proportional to
granny up and full on Burt .. May god rest his
marvellous and handsome mustachioed soul -
The money shot. Four new old stock planar Russian
"Nixie" tubes and sockets. These, what I happen to have
in sufficient quantity with large enough digits for easy
viewing, but virtually any tube in any style will work with
little or no adjustment to the circuit.
The meat and potatoes with 5V buses in place. An
LM2917 will do scaling duty, an ICL-7135 and SN-7441
will decode and display the results.
One adjustable frequency 555 will be used for the time
being to drive to 2917, the other fixed to supply the
120KHz clock signal to the 7135, selected do to their
relative ease of use and stability - in effect, so as not to
add "wild cards" to the circuit during the initial
breadboard thrash phase.
As well, arguably a bit of a tease, the Nixie's themselves
will not be placed in circuit until the analog section is up
and working, instead for now a 4 decade common
cathode LED display.
The "Working" schematic: Really nothing more than the
example circuits found in the respective chip's literature
pasted together as if silicon Lego.
Initially, I had several problems getting the circuit to
work, most involving a lack of familiarity with the two
'main' chips, but all eventually resolved by looking closer
at the datasheet and a bit of frigging around.
The "tach (-)" input (pin 11) of the 2907 required a
voltage divider, and the 7135 references were arsing me
- in fact the 7135 will work single ended, negating the
need for the 7660, but this is the version that I made
work first, and so left it for now.
Note the lack of component values. The app note values
were used to get it going, but some will likely be
changed during the alignment phase of the build.
With the LED circuit functional, it's Nixie time!
The high voltage supply is a throw together 555
adjustable duty cycle driven open loop boost converter
fashioned from totally random parts at hand - the output
range adjustable from 70 to nearly 600V (diagram below
A 7441 was paralleled to the 7448 inputs (I decided to
retain the LED readout during the build), and the anode
drive is an NPN opto-coupler Darlington coupled to a
2SC945 general purpose NPN transistor, collector on
the HV through a 33K resistor, emitter "pulling up" the
tube anodes. HV is set at 250 Volts.
Whoa you say .. A general purpose tranny doing HV
switching? Well actually there's only 45 Volts collector to
emitter in this configuration.