Digital Clock

The complete circuit diagram is shown in Fig. 1. Heart of the system is lC2, National`s MM5402 clock chip. It is an MOS monolithic large scale integrated circuit. It`s pin configuration is shown in Fig. 2. The supply is derived from the mains through a low-voltage stepdown transformerXl. As shown in the diagram, 9-volt DC supply derived by diode
Digital Clock - schematic

D3 and capacitor Cl is fed to the counting circuit of the clock chip at pin 29 through a lk resistor. A bypass capacitor C5 is used at r input of clock chip for protection from high-frequency transients. The current requirement for the entire clock chip is around 4. 5 mA. . The clock input pin 35 is fed by 60Hz clocked pulses. These are derived by 3. 579 MHz quartz crystal oscillator formed with ICI, Nationals MM5369 chip, which is also ia frequency divider. Its output at pin I is 60Hz square wave For trimming the crystal frequency, a trimmer (C4) is also provided which could be used for improving the accuracy of time. For display, Fairchild`s FNDSOO seven-segment light emitting diodes (LEDs), D7 to D10, are used. These are common-cathode LEDs. Since the clock chip`s segment output is of the order of lO mA, no interfacing is required. The various LED segments are only to be connected to the respective clock outputs. In all, four LED displays are required, and these can be straightaway soldered on to a separate PCB or breadboard. Supply to the display LED is obtained from the 3V tap of transformer Xl. The pin configuration of FND500 is shown in Fig. 3. The blinking colon is also available in the FND500 display. It is provided at pin 5 of the display, and may be connected through a 220-ohm current limiting resistor to pin 39 of the clock chip. The a. m. and pm. displays are obtained by using two LED lamps, D5 and D6, and...

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