The clock circuit above uses seven ICs and 19 LEDs to indicate binary coded
decimal time. The LEDs can be arranged (as shown in example above) so that
each horizontal group of 3 or 4 LEDs represents a decimal digit between 0 and
9 and each individual LED represents a single bit or (binary digit) of the
value. Binary digits have only two values (0 and 1) so a number written in
binary would be something like 1001 or 0011, which represents decimal numbers
9 and 3 respectively. From right to left, each binary (1) represents increasing
powers of 2, so that a 1 in the right hand place represents 2^0=1 and the
next place to the left is 2^1=2 and then 2^2=4, and so forth. This makes
binary counting fairly easy since each digit has a value of twice the one
before or 1,2,4,8,16,32,64,etc. Thus the decimal value can be
found by simply adding the values of each illuminated LED in the same row,
(the total is shown in the box to the right). For example, the binary number
1001 would have a decimal value of 8+0+0+1 = 9. But this is actually a
binary coded decimal 9 since only values from 0 to 9 are used 0000 to 1001.
A true binary clock indicating minutes of the hour would display values from
0 to 59, or 000000 to 111011. But this would be more difficult to
read since adding values 32 + 16 + 8 + 2 + 1 = 59 is not as easy as
8 + 0 + 0 + 1 = 9.
The circuit is powered by a small 12.6 VAC transformer which also provides
a low voltage 60 Hz signal for a very accurate time base. The transformer
is connected with the secondary center tap at ground which produces about 8
volts DC across the 3300uF filter capacitor. DC power for the circuit is
regulated at about 5.5 using a NPN transistor (2N3053) and 6.2 volt zener
diode. The 2N3053 gets a little warm when several LEDs are on, and may
require a little (top hat type) heat sink.
A one second clock pulse is obtained by counting 60 cycles of the AC line
signal. This is accomplished using a CMOS CD4040 12 stage binary counter
(shown in light blue). The 60th count is detected by the two NAND gates
connected to pins 2,3,5,and 6 of the counter. When all four of these
lines are high, the count will be 60 resulting in a high level at
pin 4 of the 74HC14 which resets the counter to zero and advances the
seconds counter (74HC390 shown in purple) when pin 4 returns to a low
state. The same process is used to detect 60 seconds and 60 minutes
to reset the counters and advance the minutes and hours counters respectively.
In both of these cases the 2 and 4 bit lines of the tens counter section
will be high (20+40=60). In all three cases (seconds, minutes and hours) a
combination 10K resistor and 0.1uF capacitor is used at the input to the
74HC14 inverter to extend the pulse width to about 300uS so the counters will
reliably reset. Without the RC parts, the reset pulse may not be long enough
to reset all stages of the counter since as soon as the first bit resets, the
inputs to the NAND gate will no longer all be high and the reset pulse
will end. Adding the RC parts eliminates that possibility. The reset
process for the hours is a little different since for a 12 hour clock we
need to reset the hours counter on the 13th count and then advance the counter
one count so the display will indicate one ("1"). The 74HC00 quad NAND gate
only has 4 sections with two inputs each so I used 3 diodes to detect the 13th
hour (10 +1 +2 =13) which drives an inverter and also a transistor inverter
(2N3904 or similar). The last 74HC14 inverter stage (pin 12 and 13) supplies
a falling edge to the hours counter which advances the hours to "1" a short
time after the reset pulse from the transistor inverter ends. The pulse
width from pin 12 of the inverter is a little shorter than from pin 10
which ensures that the hours clock line (pin 1 of yellow box) will move
high before the end of the reset pulse form pin 10. If it were the other
way around, the reset pulse may end before pin 12 of the inverter had a
chance to reach a high level which would prevent the counter from advancing
to "1". So it is important to use a shorter RC time at pin 13 than for the
other Schmitt Trigger inputs. I used a 10K resistor and a 0.01uF cap to obtain
the shorter time, but other values will work just as well. Only 2 sections
of the 4071 OR gate are used, so the remaining 4 inputs (pins 8,9,12,13)
should be terminated to ground if not used.
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Parts List:
3 - 74HC390 - Dual BCD counters
1 - CD4040 - 12 Stage Binary Counter
1 - 74HC14 - Hex Schmitt Trigger Inverter
1 - 74HC00 - Quad NAND gate
1 - CD4071 - Quad OR gate
1 - 2N3053 - NPN transistor (may need heat sink)
1 - 2N3904 - NPN transistor
3 - 1N914 - Signal diode (1N400X will also work)
2 - 1N400X - Rectifier diodes
1 - 6.2 volt - Zener diode
1 - 3300uF - Filter Capacitor - 16 volt
1 - Power Transformer - Radio Shack 273-1365A or similar
1 - 220K 1/4 or 1/8 watt resistor
1 - 150 ohm 1/4 watt resistor
19 - 220 ohm 1/4 or 1/8 watt resistors
11 - 10K 1/4 or 1/8 watt resistors
2 - 0.01uF capacitors
4 - 0.1uF capacitors
19 - Red LEDs (15 mA)
2 - Momentary push button switches (to set the time)
1 - Toggle switch (to start the clock at a precise time)
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