the counting circuit. Figure 3 shows this circuit and the
display circuit that receives its outputs. Counting uses four
ICs: a CD4518 dual decade counter; a CD4510
presettable decade counter; a CD4027 dual J-K flip-flop;
and a CD4081 quad AND gate. The counters and one of
the flip-flops generate signals that indicate the minutes and
hours digits. Let’s follow the 1/minute signal from the time
base through these ICs.
The 1/minute signal enters one decade counter of the
4518 which counts the minutes and generates a binary-coded decimal (BCD) representation of the minute’s 1s
digit at its four outputs. The 8 output of this counter is
connected to the input of the second counter in the 4518;
when the minute’s 1s count rolls over from 9 to 0, this
output goes low and the second counter increments the
minute’s 10s count.
In the minute’s 10s count, 0 should follow 5 as 59
minutes rolls over to 00. To accomplish this, the 2 and 4
outputs of the minute’s 10s counter connect to an AND
gate — one of four in the CD4081. When the minute’s 10s
count reaches 6 at the end of an hour and the beginning
of the next, its 2 and 4 outputs are high so the AND’s
output goes high. This resets the minute’s 10s count to
zero and sends a pulse to the next counter — the CD4510
— which counts an hour.
As mentioned, this is a 12 hour clock, so the hour’s
10s digit is always either blank or 1, and we don’t need a
full counter to keep track of it; one of the flip-flops in the
CD4027 will do the job. When the hour’s 1s count in the
CD4510 rolls over from 9 to 0, the 4510 sends a signal to
that flip-flop whose Q output goes high to indicate a 1 in
the hour’s 10s position.
At this point, things get a little tricky for two reasons.
First, the hours count must roll over after 12; that is, the
hours count must be reset when it reaches (very briefly)
13. Second, that count must roll over to 1 — not 0 — since
1:00 follows 12: 59. Two more AND gates in the 4081
handle the first issue. Together, they notice when the
hour’s 10s digit is 1 (the Q output of that flip-flop is high)
and the 1 and 2 outputs of the 4510 are high; that is, the
hours count reaches 13. The output of the second AND
gate goes high which resets the flip-flop — its Q output
goes low — and raises the LOAD input of the 4510, whose
preset inputs specify a value of 1. (It is possible to
accomplish this transition with a non-presettable counter,
but it requires more logic. That implementation is left as
The three decade counters and one flip-flop are now
correctly counting the minutes and hours indicated by the
1/minute signal from the time base. All that remains is to
light up the AM and PM indicators correctly.
It makes sense to assign this task to the second flip-flop in the 4027. It has complementary Q and ~Q
outputs, and exactly one of the indicators will be lit at any
time. However, morning becomes afternoon and evening
becomes morning again at 12:00, not 1:00, so we cannot
use the hours-reset signal to toggle this flip-flop.
Fortunately, we already have (as Figure 3 shows) an
18 April 2017
■ FIGURE 3. The