March 1970 Popular Electronics
Wax nostalgic about and learn from the history of early electronics. See articles
published October 1954 - April 1985. All copyrights are hereby acknowledged.
RCA's Numitron was
their answer to the Nixie tube (manufactured by Burroughs Corporation). It was
a simpler 7-segment incandescent display (DR2010) that, with
all lines energized, formed the number 8. It worked off of +3.5 to +5 volts,
with each element requiring 24 mA of current. The number 8 drew 192 mA
of current and dissipated 0.672 W at 3.5 volts and a whopping 0.96 W
at 5 volts! RCA marketed a BCD*-to-7-segment display driver (the
Numitron was pitched as a sensible alternative to the 7-segment LED display,
but with an element size of 0.35" wide by 0.6" high, there was no real advantage
over the LEDs, which were just entering the electronics market in 1970. Numitrons
do have a certain nostalgic 'cool' factor, though. It is interesting to note
that the author's last name, Wood, is the same as that of
Frank Wood, who was issued the first U.S. patent for a 7-segment
numerical display in 1910.
By Vincent Wood
Simplified Seven-Segment Display in One Tube
There are many ways of displaying digital information. As readers of Popular
Electronics are aware, digital readout can be achieved with ten small incandescent
lamps; with a Nixie® glow tube, or with an incandescent-lamp, seven-segment
display.* Each of these readouts is unique and has its own merits - and their
use has made it possible for the electronics experimenter to build relatively
low-cost digital systems.
Now RCA has developed a new type of seven-segment readout that is so simple
that your editors wonder why no one thought of it before. Basically, a seven-segment
readout consists of seven narrow illuminated bars arranged to form a rectangular
figure 8. As the associated logic circuits determine the numerical value to
be displayed, the appropriate bars are illuminated to form the numeral on the
The RCA approach to a seven-segment readout (called a Numitron) uses seven
short filament wires to form the numerals, The filaments are suitably supported
against a dark background and encased in a conventional 9-pin miniature tube
glass envelope. As power is applied to the various filaments, they glow to form
the appropriate numerals, Operating at voltages between 3.5 and 5 volts and
requiring 24 milliamperes per segment, the Numitron has an expected operating
life in excess of 100,000 hours. Since the segments are voltage operated, segment
voltage may be varied to control the display brightness. Also, since the segments
glow white, any color filter can be used in front of the display to get the
Fig. 1 - The seven-segment decade counter reflects
the latest in integrated circuits and display devices. Unlike other display
tubes, the brightness of the Numitron can be determined by applied segment
Fig. 3 - Component installation is simple as long as
you observe the notch code on the IC's. The connector is optional.
Fig. 4 - Power and signal connections for each decoder.
Segments should be tested before each use.
To accompany the Numitron, RCA has also developed a decoder-driver integrated
circuit. Although the Numitron can be driven with the seven-segment board described
in our February 1969 issue ("Third-Generation DCU", p. 43) , a much smaller
and simpler PC board can be made using the new RCA CD 2501E IC. To further simplify
the board and reduce the size, a Texas Instruments IC (SN7490) can be used as
the decade counter to decode the incoming pulses. The circuit for the decade
counter and display is shown in Fig. 1.
Construction. To take full advantage of the small-size IC's and to avoid
any possible wiring error, the readout should be constructed on a printed circuit
board. A foil pattern for a board is shown in Fig. 2; you can make your
own or purchase one already etched and drilled. Install the components as shown
in Fig. 3. A conventional 9-pin printed circuit tube socket is used for
the Numitron, V1. Be sure to observe the correct terminal placement on the IC's.
Fig. 2. Actual size printed circuit foil pattern
for the readout.
Connections to the board may be made by soldering directly to the foil
terminations or through a conventional 10-pin Amphenol PC board edge connector.
Testing and Operation. Once this board is complete, make external connections
for testing as shown in Fig. 4. The 5-volt d.c. line, which can be used
for both logic and segment power, should be capable of delivering about five
hundred milliamperes. With the two 5-volt supplies and the ground terminal connected,
connect the "ground to test segments." All seven segments should light. This
test insures that all segments are operating and should be performed each time
the readout is used to make sure of correct display. For example, if the center
segment is not operating, all 8's appear to be zero's. To test the decimal point,
connect the "decimal point" terminal to the 5-volt source. Pushbuttons may be
used to perform these two functions.
The CD2501E integrated circuit also has an optional blanking circuit. The
board terminal that connects to pin 5 of IC2 is grounded in the readout associated
with the most significant digit of a multi-readout display. The connection to
IC2 pin 4 is then connected to the preceding board at IC2 pin 5, and so on down
the line. These connections permit the IC to blank the associated display if
the input to that readout is a zero. Thus, if six readouts are used in a circuit,
and only the first three digits are required, the unused displays would be dark
rather than displaying a series of three zeros which may add confusion.
With the reset lead grounded, apply an input signal of about 2 volts to the
board terminal marked input in Fig. 4. The signal from an audio generator
may be used. As the low-frequency audio signal is applied, the display will
start to indicate. To reset to zero, lift the "reset" terminal from ground and
apply +5 volts d.c.
*See the following Popular Electronics articles:
"Build a Low-Cost Counting Unit," p 27, February 1968; "All-Purpose Nixie®
Readout," p 67, November 1968; "Third-Generation DCU." p 43, February 1969.
Posted March 3, 2021(original January 29, 2014)