February 1970 Popular Electronics
Table of Contents
People old and young enjoy waxing nostalgic about and learning some of the history of early electronics. Popular
Electronics was published from October 1954 through April 1985. All copyrights are hereby acknowledged. See all articles from
The mere sight of a
Nixie tube evokes passion and nostalgia in the hearts and
minds of vintage electronics aficionados. For the uninitiated,
Nixie tubes were one of the most successful early numeric display
formats. They had wire filaments shaped in the form of numerals
0 through 9, stacked front-to-back inside a vacuum tube enclosure.
Rather than the filament (wire) doing the glowing, the neon
gas (plus traces of others) fluoresces (glows) in the vicinity
of the wire. 7-segment LED displays had not yet hit the commercial
market when this story was published in 1970, so even though
the numeric display uses vacuum tubes (Nixie) the power supply,
counter, and display driver circuits use semiconductors rather
than vacuum tubes.
Build Numeric Glow Tube DCU
Nixie Readout at $15 per Decade
By Don Lancaster
Now it is possible to build a high-speed, decimal counter
module (complete with logic and Nixie® tube readout) at a cost
of $14.90 per decade. This counter, with speeds from d.c. to
either 8 or 12 MHz (depending on the type of logic used), can
be built with 2 1/2 decades (0-199), 3 1/2 decades (0-1999),
or 4 1/2 decades (0-19999) using a single printed circuit board.
No mounting or front brackets are needed and there is a minimum
of interconnections to be made.
The design provides an overflow indicator and latch which
operate when full scale is exceeded. This function is useful
for overrange indication or as a "turn-around" command on dual-slope
DVM designs. Display blanking, in which the readout can be turned
off or on by an external 0-2-volt d.c. control signal is also
available. This feature eliminates display bobble or blur and
back-and-forth numeral motion during rapid counting.
There is also a self-contained "gate" input that permits
turning the counters on and off and is useful for period or
frequency measurements. This feature eliminates quite a bit
of external circuitry.
You have a choice of the type of logic you use in building
the DCU. If RTL is used, the unit is fully compatible with previous
POPULAR ELECTRONICS projects. Or you can use Utilogic® (Signetics
Corp.), a faster type of logic with a higher voltage swing that
is compatible with industrial TTL and DTL circuits. Both types
of logic cost the same.
The IC counters are "weighted" in the industrial 1-2-4-8
manner to provide electrical as well as visual outputs if de-sired.
A simple modification and an external adapter can be used to
convert the RTL version of the DCU into an "add-subtract" counter
which operates in either direction. The units are useful in
computers, calculators, and positional controls.
When RTL is used in this new DCU, the unit can be used in
POPULAR ELECTRONICS projects such as the "Digital Voltohmmeter,"
the "Universal Frequency Counter," the "Sports Timer," and the
"Electronic Stopwatch." In fact, with a few mechanical changes,
the new 2 1/2-digit assembly can be dropped into the "Digital
Voltohmmeter" without adding any new parts. This makes a DVM
that looks like the industrial models that cost many times as
Because of space limitations, construction details
are given here for the RTL counter only. Complete information,
including PC layouts, for the Utilogic version is available
without cost from the source given in the box.
whether you want to use RTL or Utilogic in your DCU, consult
The circuit for one decade of the DCU is shown in Fig. 1 and
that of the overflow counter is shown in Fig. 2. Although these
are shown as separate circuits, in practice, one overflow counter
and as many decades as are necessary are mounted on one PC board.
Interconnections for the units are shown in Fig. 3. Note that
the Gate connections of all decades except the first are grounded.
In this way, if the input (units) decade is turned on or inhibited,
the counter operates or not accordingly.
Fig.1. The schematic for one decade counter.
As many decades as desired can be built using this same schematic.
The readout is a conventional Nixie tube with a glowing numerical-shaped
Fig. 2. The overflow counter is coupled to
the last decade used. When an excess count is received, power
is applied to the external overrange (neon) indicator.
Underside view of the PC board showing how
some jumpers are connected. These below-board jumpers must all
Decimal counting units can be built in a number of configurations:
1 1/2 (counting to 19), 2 1/2 (to 199) , 3 1/2 (to 1999), 4
1/2 (to 19999), etc. In each case the 1/2 stands for the "1"
of the overflow counter, while the whole number stands for the
number of decade counters (each counting to 9).
Construction details are given here for the popular 2 1/2-digit
assembly. Because of the complexity of the circuit, a printed
board is mandatory. A board is shown actual-size in Fig. 4.
A commercially made board is available (see Parts List for Fig.
1). If you prefer to make your own, it is recommended that you
use the better-grade, G-10 fiberglass.
Fig. 4. Actual-size pattern for the 2 1/2-decade
board, with associated overflow counter. By judicious re-arrangement
of the foil pattern the number of decades used can be extended.
Boards for multi-decade readout can also be purchased.
IC1,IC2-MRTL dual JK
flip-flop (Motorola MC791P)
IC3-MRTL quad two-input gate
Q1-Q10-2N3877 transistor (Allied Electronics
49D30 2N3877 SPR, no substitute)
R1,R4,R5-470-ohm; 1/4-watt resistor
R2,R3-330-ohm, 1/4-watt resistor
V1-Nixie tube (Burroughs B5750)
jumpers, insulated sleeving, solder, spacers, mounting hardware,
Note-The following are available from Southwest Technical
Products, Box 16297, San Antonio, Texas 78216: Etched and drilled
PC boards-2 1/2-digit, $4.00; 3 1/2-digit, $5.75; 4 1/2-digit,
$7.50. Complete kit of all parts- 2 1/2-digit, $43.50; 3 1/2-digit,
$59.50; 4 1/2-digit, $75.00. Write for a complete list of related
circuits, kits, and instruments. All prices post-paid in U.S.A.
Besides drilling details, Fig. 5 shows the location of the
32 jumpers located on the component side of the board. In addition,
there are four jumpers that are "sewn" through the board, so
that they alternate from one side to the other and pick up five
connections each. Details of this are also shown in Fig. 5.
The long bare jumper is soldered at one end and then threaded
through the holes in the board. Use insulated sleeving over
the exposed parts to prevent shorts to the transistor leads.
Fig. 5. Board drilling and jumper installation.
Some jumpers are "sewn" through the board as illustrated above.
Start at one end, and pass the wire through the respective holes,
inserting the insulation at the required places.
Once the various jumpers have been installed, the components
are inserted in accordance with the layout shown in Fig. 6.
Use a low-power (40-watt) soldering iron and thin solder to
make all connections. The IC's are identified by a notch and
dot code for positioning. To insert the 20 driver transistors,
hold them with the flat facing away from the readout tubes.
Then bend the center lead back toward the tubes and insert as
Fig. 6. Component installation of 2 1/2-decade
board. Other than placement of R6, both decades are similar.
This illustration shows external connections needed.
The 2 1/2·decade board. Each Nixie indicates
up to 9, and at the 100th count, both Nixies indicate zero while
the special"1" neon lamp comes on. The combination indicates
to 199. At 200th count, a special over-range neon lamp (not
shown) glows indicating that counter has progressed beyond its
General view of a portion of a 2 1/2-decade
board. This view shows the correct way to install the ten switching
transistors for the Nixie drive.
In inserting the Nixie tubes, put the leads in two at a time.
Before soldering, make sure that all leads are tight, none are
doubled over or shorted to each other and the viewing face of
the tube is aimed in the correct direction. Also be certain
the tube is vertical.
Overall view of the 2 1/2·decade board. When
mounted in enclosure, only the readouts will be visible.
Mount the neon lamp (for numeral 1) so that the metal rods
within the tall narrow bulb are at the same height as the numerals
in the Nixie tubes.
Use. The 2 1/2-digit module can be used in anyone of a variety
of chassis styles - as long as it has a rectangular front-panel
cutout for the two Nixie readout tubes and the neon light. A
special polarized optical filter is available (see Parts List
for Fig. 1) to improve readout visibility. This filter should
be oriented to produce the blackest instrument interior when
viewed and illuminated through the filter. Once the correct
orientation has been found, glue the filter in place behind
the front-panel cutout.
General view of the second decade of the
counter. Even though the three portions extend across the board,
the three readouts are very closely spaced.
External connections to the module are shown in Fig. 6. The
2 1/2-digit module requires +175 volts at 5 mA for the readouts,
and + 3.6 volts at 340 mA for the remainder of the circuit.
A power supply (such as the one shown in Fig. 7) is required.
It has low ripple with high-frequency bypassing - an essential.
Fig. 7. Low-ripple power supply for the 2
1/2-decade board. By changing D4, the supply can be used for
either RTL or Utilogic circuits.
Ground leads should be short and of heavy gauge wire (at
least #16). The "Out" terminal on the board is used only in
some special DVM circuits and is normally left unconnected.
The terminals along the rear of the board are for use in the
future with an add-subtract adapter and are also left unconnected
for routine applications.
The "Gate" input, if used, goes to an RTL-derived signal
that is positive when the counter is to be inhibited and ground
when the counter is to count. If you are not going to gate the
assembly, the Gate terminal should be connected to the ground
terminal FOR UTI LOGIC DCU DETAILS
Complete construction information, including full-size PC
layout replicas and all other details, is available free upon
Alvin R. Smith, Section Head Digital Design
Southwest Technical Products, Inc. Box 16297
Antonio, Texas 78216
Please limit free requests to single
To provide a blanking feature, connect the "Unblank"
terminal to an RTL-derived signal that is positive when you
want the display to light and ground when you want it off. Remember
that the Unblank input does not stop the counter from working
- it just determines whether or not the display can be seen.
If you do not want to turn the display off, connect the Unblank
terminal to the +3.6-volt source.
The two terminals
marked "X" are connected to a neon overrange indicator (usually
mounted in a red holder). If you don't want the overrange indication,
leave these two terminals unconnected.
are activated by connecting the selected decimal point terminal
beside each Nixie tube to the "DP" terminal on the overflow
counter through an external switch. Decimal point operation
is independent of display blanking.
The "Reset" terminal
is normally connected to ground through an external switch.
Raising the buss to +3.6 volts momentarily resets the assembly
The Reset button need not be bounceless. If you
use an electronic reset, a 2-microsecond pu1se with a fanout
of 30 is required. Input.
input must be a waveform that changes abruptly from +3.6 volts
to ground each time a count must be registered. For the counter
to operate properly, the input must be both noiseless and bounceless
and have a fall time less than 0.2 microseconds. Thus it is
absolutely mandatory that the input be properly conditioned.
Four possible signal conditioners are shown in Fig. 8. Circuits
(A) and (B) are used for mechanical-contact inputs, while (C)
and (D) are for electronic inputs. Circuit C (C) is used for
input levels of about 2 volts r.m.s. If the input frequency
is below 1500 Hz, the capacitor must be included. For higher
frequencies, omit the capacitor. Circuit (D) is a Schmitt squaring
Any of the circuits used in previous POPULAR
ELECTRONICS DCU projects have the proper conditioning circuits
built in. Thus, if you have built or are considering building
the Digital Volt-ohmmeter (December 1968), besides making the
mechanical modifications that are necessary to use this new
counter module, connect the "Unblank" input to the existing
"Gate" terminal on the V/F module in the Voltmeter. Should the
brightness of the display be inadequate, the original DVM transformer
should be replaced with the one called for in Fig. 7.
A recommended power
supply with sufficient regulation is shown in Fig. 7. This supply
is wired point-to-point after all parts have been mounted in
a suitable chassis. PARTS LIST
C1-100-µF, 250-volt electrolytic capacitor
C2-6000-µF, 10-volt electrolytic capacitor
6-volt electrolytic capacitor
C4-0.1-µF, 10-volt disc
D1,D2-1-ampere, 600-volt silicon diode
(1N4005 or similar)
D3-1-ampere, 50-volt silicon diode (1N4001
D4-4.2-volt (RTL) or 5.6-volt (Utilogic) 1-watt
F1-0.5-ampere fuse and fuse holder
transistor and suitable heatsink
S1-Power switch (usually
a part of other instrument or circuit switching)
transformer; secondary 135-0-135 V at 50 mA, 6.3 VCT at 1 A
(Southwest Technical #TR-DVM or similar)*
spacers, hardware, wire, solder, terminals, line cord and strain
*Available at $6.50 plus 4 lb postage from Southwest
Technical Products, Box 16297, Sail Antonio, Texas 78216.
HOW IT WORKS
One decade counter can be divided into four sections: the
actual counter, the decoder, the readout driver, and the readout.
The counting portion (at bottom of diagram) consists of four
JK flip-flops arranged to count to 9 before reverting back to
zero and simultaneously delivering a "Carry" output to the next
decade. To force the counter to count only to 9, an inverter
in a feedback loop is used. The voltage levels, which are unique
for each count, are taken from the Q and Q outputs of each flip-flop
for use in the decoder. The flip-flop outputs are in the common
1-2-4-8 code. If more than one module is to be used in an instrument,
the "Gate" input terminal of the counter is connected to ground
in all but the first counter. When the gate is grounded, the
counter operates normally. When it is made positive, the counter
is inhibited. In this way, an externally generated signal can
be used to determine when the counter is to operate.
In the decoder, consisting of four gates and two discrete
transistors, the 1-2-4-8 output of the counter is converted
into a biquinary (divide by 2, then by 5) code. It has seven
outputs: even, odd, 0 and 1, 2 and 3, 4 and 5, 6 and 7, 8 and
9. These form the input to the readout drivers.
readout (Nixie tube) is a gas filled tube with one common anode
and 10 discrete metal cathodes, each formed into the shape of
a number (from 0 to 9). When B+ is applied to the common anode
and any of the cathodes is grounded, the gas around that particular
piece of shaped metal glows causing a number to appear in the
The readout drive consists of 10 high
voltage transistors, driven in pairs by the decoder outputs.
The transistor collectors are connected to the 10 cathodes of
the Nixie tube. The emitters of all of the odd-numbered transistors
are connected together and to the "odd" buss, while the even-numbered
transistors have their common emitters connected to the "even"
buss. The even and odd busses are driven by the two transistors
in the decoder.
The system can be considered to operate
like a switching network. When, for example, the even transistor
in the decoder is saturated (with its emitter grounded), the
even buss is essentially at ground. Then, if a signal is applied
to the bases of one pair of driver transistors, only the one
whose emitter is connected to the even buss saturates and acts
as a switch to close the circuit to the appropriate cathode
on the readout. Suppose, for instance, that the count is 7.
Since 7 is an odd number, the odd decoder transistor is saturated
and the odd buss is grounded. Simultaneously, the 6 and 7 output
of the decoder applies signals to the 6 and 7 driver transistors.
Because only the 7 transistor is connected to the grounded odd
buss, only the 7 transistor saturates, causing the number 7
to glow in the readout.
Note that we said previously
that the odd or even buss must be grounded for the decoder transistors
to work. The grounding is made external to the counter through
a connection to the "Blanking Input" terminal. A circuit in
the overflow counter determines when this terminal is grounded
for display viewing. In this way, rather than have a blur of
numbers while the counter is counting, the blanking input keeps
the display off until the counting is complete. Then a steady
display is shown.
HOW IT WORKS
The overflow counter consists of a counting section, a display
driver, and a display.
PARTS LIST OVERFLOW COUNTER
C1-0.1-µF, 10-volt disc ceramic capacitor
dual JK flip-flop (Motorola MC 791P)
Q16-Transistor (National Semiconductor 2N5129)
The counter contains two JK flip-flops the first of which
is a divide-by-two and the second a latch. The latch flips positive
and stays positive when there is an overflow. Resetting the
counter resets the latch. The outputs of the flip-flops drive
high-voltage transistors which act as switches in series with
special neon lamps. The first flip-flop and its transistor energize
the neon lamp that displays a 1 which is similar to the 1 displayed
by the Nixie tube. The lamp driven by the second flip-flop and
its transistor is a standard neon lamp on the front panel and
it indicates "Overrange." Resistors in the B+ circuit of the
neon lamps provide for differences in breakdown voltages.
The emitters of both driver transistors are connected
together and to the "Unblank Input" through a switching transistor.
A positive input to this terminal saturates the switching transistor
and causes the display to turn on. The switched signal is supplied
to the decimal counters through the "Blanking Output" terminal.
Remember that counting continues whether or not the
display is lit. The blanking merely controls whether or not
the display is on.
The overflow counter also contains
a bypass capacitor for the supply, resistive loading for the
reset buss, and a decimal point resistor. These elements are
connected to their respective circuits through the instrument
(A) Set-Reset Pushbutton Conditioning
(B) Monostable Contact Conditioning
(C) Hex Inverter Input Squaring Circuit
(D) Schmitt Trigger Input Squaring Circuit
Fig. 8. Four approaches to "bounceless" signal
Either A or B can be used for mechanical switching, while
either C or D can be used if the input signal comes from a conventional
Posted June 5, 2013