Here as mentioned 5 IC7490s are used as counter ICs and as you can see the 5 displays are connected with seven segment displays with their respective decoder. IC4511s are used as BCD to Seven Segment decoder. All seven segment displays used are common cathode. That means all lines are grounded.
Also, the circuit shown counts only upto 8hrs. So it requires only 5 displays otherwise needs 6 displays. The circuit given is very well tested and in working condition.
The Heart of the circuit is the "Second's pulse". It was very tough to derive the 1Hz signal. After searching through the web found the idea to build the circuit using AC power lines.
In some countries the power grid operates on 60Hz i.e 60 cycles per second; and in other countries power grid operates on 50Hz.
There are some problems involved with is:
1. It is necessary that this Frequency is maintained accurately. The frequency might vary as many different electrical generators are connected with the grid and all pour power into grid.
2. Different generators must match with each other in frequency and they must also be phase locked to each other as well. Otherwise generators might overload each other.
3. Also their voltage outputs must be matched so that no generator will draw current from each others.
One can use this power line along with some CMOS ICs & can derive the 1s signal. But considering the above circumstances it requires to maintain the frequency. Which we found not so easy job.
Other than that I found a circuit which uses the Crystal circuit to get 1 second signal.
As shown the cct is built around the crystal along with 3 resistors and 2 capacitors.
Component
U1 CD4060 FREQUENCY DIVIDER
U2 CD4027 DUAL FLIP-FLOP
R1 330K Ohm Resistor
R2 3 6.8 Meg Ohm Resistor
C1 2 33pF Capacitor
Y1 32.768 KHz Crystal
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