the inside of the assembled unit.
Self-tapping sheet metal screws hold the top to
brackets at the rear of the base. The metal parts of the
enclosure were cut from a salvaged panel painted with
lightly textured gray paint. The paint cracked at the bends,
but another light coat of gray fixed that and preserved the
texture. The minimal labels were made with a Brother
label maker. Figure 7 shows the finished clock.
Adjustment
The trimmer capacitor in the crystal oscillator allows
slight adjustment of the oscillator’s frequency. Set the
clock, determine over a day or two if it is running fast or
slow, and modify the capacitor’s setting accordingly. It
helps to sketch pictures of the capacitor’s setting, and the
process may take a few weeks. Be patient.
A Second
Clock
In designing and prototyping
the clock I’ve just described, I
built a second power supply and
time base board. This one
requires an input voltage of about
8 VAC, and therefore includes a
full-wave bridge and an
electrolytic filter capacitor. Also —
in place of the CD4029 — it uses
a 74C193, also a presettable
binary counter which has the
advantage of not requiring an inverter between its carry-out output and its load input. Since this board was already
assembled and tested, I decided to build another clock
using it.
Again, there were three boards: the power supply and
time base; a counter implementing the same circuit as
before; and a driver and display board. The physical
arrangement, however, was different. This time, the power
supply/time base and counter boards formed a horizontal
sandwich.
The display board — a section of a Datak 12-600B —
was mounted to the counter board with long right-angle
headers and two small Keystone right-angle brackets,
which are threaded for 4-40 screws. (Jameco stocks these
brackets as part number 1581530.)
The set switches and a small 8 VAC transformer were
mounted on the unit’s back panel, along with a three-wire
computer-style line connector. As before, the sub-assemblies connect via headers and connectors, including
(in this case) a five-wire connection from the counter
■ FIGURE 9. The two 12 hour clocks and a 24 hour clock.
■ FIGURE 8. The inside of the second clock.
■ FIGURE 7. The completed clock.
April 2017 21
