# 19 LED Clock circuit

Posted on Oct 20, 2012

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...

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