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February 1970 Popular ElectronicsTable 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 Popular Electronics.
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.
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.
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.
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.
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.
IC1,IC2-MRTL dual JK flip-flop (Motorola MC791P)
IC3-MRTL quad two-input gate (Motorola MC724P)
Q1-Q10-2N3877 transistor (Allied Electronics 49D30 2N3877 SPR, no substitute)
Q11-Q13-Transistor (National Semiconductor 2N5129)
R1,R4,R5-470-ohm; 1/4-watt resistor
R2,R3-330-ohm, 1/4-watt resistor
R6-15,000-ohm, 1/4-watt resistor
V1-Nixie tube (Burroughs B5750)
Misc.-#24 wire jumpers, insulated sleeving, solder, spacers, mounting hardware, etc.
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.
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 shown.
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.
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.
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.
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.
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.
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
1C4-MRTL dual JK flip-flop (Motorola MC 791P)
Q14,Q15-2N3877 transistor (no substitute)
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 wiring.
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 audio generator.
Posted June 5, 2013