If you are seriously interested in learning how to build your own digital clock, the following circuit and its control description is what you’ve got to grasp first:
The circuit is primarily based on the main National Semiconductor MM5402 IC, which is a monolithic large MOS Integrated circuit capable of producing all the discussed functions single handedly, with a clock signal being the only external trigger required for all these. The figure below shows a detailed view of the pin configuration of this IC.
The circuit functioning can be understood by referring to the figure alongside and with the following points:
The power supply section is a standard step down transformer/diode/capacitor circuit capable of supplying 9 volts pure DC @ 500mA maximum current output.
The 9 volt DC is fed directly to pin #28 which is the Vdd input of the IC through via a 1K resistor.
Capacitor C5 provides protection to the IC from high frequency input transients.
Another chip included in the circuit which also performs an important task is the MM5369 counter/divider/oscillator IC again from National Semiconductor.
The IC generates accurate 60 Hz pulses with the help of a 3.579 MHz crystal and a few other associated parts. The frequency becomes available at pin #1 of the IC and is applied to pin #35 of IC2, which forms the base for all the timings.
The trimmer C4 can be used for adjusting the 60 Hz output frequency for making the clock extremely accurate with the timings.
The display used by me was Fairchild’s FND 500 seven segment LED displays, which are basically common cathode displays, equivalent types may be tried and should work satisfactorily.
Since the output from IC2 is current controlled, it never exceeds 10 mA and therefore can be directly applied to the relevant pins of the IC without incorporating limiting resistors. In my prototype though, I used 1K resistors to avoid the displays glowing too brightly. The figure below shows the pin arrangement of FND 500.
The various display control facilities are achieved by simply adding push-to-on switches to the relevant pin-outs of the IC which makes the whole control system pretty straight forward.
Pin 25 of the IC provides the alarm output. Since the pulse from this point is too feeble, an extra current amplifier stage is incorporated for amplifying these signals, which ultimately drives a 555 oscillator and speaker stage.
The “sleep" output from pin #27 may be used for triggering external electrical appliances at a particularly fixed predetermined time via a relay, for example a radio, TV, oven, geyser or other similar gadgets.
The circuit is also equipped with a battery back-up facility which keeps the clock “ticking" during even at power failures. Though during back-up the display remains shut off, it immediately springs into action with correct time the moment power is restored. The facility can also be used during night time if one feels the light from the display is disturbing.