incandescent light bulbs from an electrical perspective,
and as such, multiplexing them will simply act as a light
dimmer. A 1x6 (or even a 2x3) multiplexing arrangement
would leave them very dim, so is not a viable option.
Luckily — since their current requirement at about 23 mA
per segment is fairly low and it is a seven-segment device
operating at about five volts — we can drive them with a
standard BCD to seven-segment decoder designed for
seven-segment LED displays with a max driving output of
25 mA. The CD4511 also has a data latch built in, so it
was chosen for this design.
I opted for the SMD version since they neatly tucked
behind the Numitron tube, making for a nice clean
appearance (Figures 11a and 11b).
Since I am now rapidly running out of ports on the
PIC, I needed an efficient way to control the CD4511s
and their latches. This is done by the 74164 shift register
in a similar fashion as to how the CD4017 operates in the
LED section. The BCD data is presented
to all the CD4511s at the same time via
the data bus, but decoded and latched
only by the CD4511 selected by the
SN74HC164 shift register.
Synchronization of data and latch
selection is controlled in software.
Since the time only changes every
second, only one Numitron digit is
updated for each time the LED circle is
updated. This made the software easier,
and it is still much faster than actually
required. The CD4511 also has a “lamp
test” function which is used to flash all
segments during startup and for reset of
the clock, also providing an easy way to
30 September 2016
■ FIGURE 9. Schematic
of the LED driver
portion of the project.
■ FIGURE 10. Close-up of the top of the PCB showing the water-clear
LEDs in place.