RCA's Numitron Readout
1970 Popular Electronics Article
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
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.
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.
See all articles from
By Vincent Wood
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
Simplified Seven-Segment Display in One Tube
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 display plane.
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 desired result.
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.
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 power.
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.
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
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
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
*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 January 29,