Binary Coded Decimal (BCD) Clock
 | Tens of Hours | ||||
| |  | | | Hours | |
| | | | Tens of Minutes | ||
| | | | | Minutes | |
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| | | | | Seconds | |
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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)
