Friday, March 30, 2012

Digital Clock Circuit Diagram

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 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 connected to pins I and 40 of clock chip through 220-ohm current limiting resistances. All the U’Eplay functions are obtained by simply connecting push-button switches at various control inputs, and grounding the respective input. In the prototype a self- assembled seven-reed assembly of switches was used. However,though costlier, the miniature calculator keyboard switches made by O/E/N India Ltd are the best for this purpose. Alarm output is available at pin 25 of the clock chip. Since this output is quite low, a current driver is used. This driver is formed by the transistor Tl and a few supporting components, as shown. Fora gentle alarm sound, tone is generated by lC3, which is an Indian timer IC 555. The tone can be varied by changing the value of capacitor C7. An 8—ohm, 0.5- watt speaker of 6.3 cm dia should produce sufficient sound to be heard across the room. _ The sleep output at pin 27 of the clock chip is similarly amplified and used to drive a relay which can control any audio or electrical equipment. The function of sleep timer is to switch off a radio set, tape recorder, TV etc after a preset time, within the maximum duration of 59 minutes. Since the clock chip as well as the crystal oscillator-cum- divider IC operate on 9V supply (actual operating range : 7 to ll volt), a battery back—up system can be used to keep the clock running during mains failure. There will be no display of time while the clock is running on this back-up system, but on the resumption of power through AC mains the clock will automatically display the correct time. lf due to any reason supply to the clock chip is cut off, all the digits will flash at lHz to indicate that time displayed is incorrect. 


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