Here is an interesting application for random light display. With this circuit as many as 72 LEDs of different colors can be used. LEDs can be fixed at random to suggest lighting them at random. They can be fixed one after the other suggesting running lights. They can be fixed in rows giving an effect of layers. It is an ideal choice for Christmas Tree Lighting and other festival lighting such as Divali. You can also fix them around statues, arches or banners. Imagination is unlimited.
CD 4020 is also a 14-stage divider IC, as it divides clock pulses by 16384 times at the final output. Similar to CD 4060, but this IC has no built in oscillator; the clock pulses from an external source are to be fed into the input of this IC. Basic pulses are also available at Qo at pin 9. All but Q Q2 divisions are available as outputs. For instance Q3 pulse division i.e., after dividing 8 times is available at pin 7. It is similar to CD4060, but CD4060 has built in oscillator whereas for CD4020, external clock should be supplied. Pin out detail of CD4020 is given in Figure 25.
CD4020 is selected because an external clock can be given from the mains frequency as this circuit may always work off mains. Hence separate timing components are not required.
50Hz clock input is taken from mains supply. So 6.25 Hz is available at Pin 3, 3.125 Hz at Q4 and so on. Each of these outputs drives a series of 6 LEDs at 10mA. LEDs light up in the order of Q0, Q3, Q4.. ..Q14 and so on but not at random. 12 outputs are shown with the exception of Ql and Q2. LEDs at the lower end flicker fast and those at the higher end stay on for long and go off for long.
Now the trick is to place these LEDs at different places to get a look of random distribution. Even LEDs within the same series are positioned at different places. Unlike other circuits, this circuit is shown along with a half wave rectifier and transformer, as reference frequency of 50 Hz is given through a 10k resistance (Rl) from the secondary of transformer to CD 4020. Reset Pin 11 is pulled down by R210k resistance. The counter stops if it is pulled up. The circuit is shown in Schematic 17. Switch is not shown in the schematic.
Again this is a CMOS IC and you are warned. You may have to hard wire 72 LEDs around. Hence please take care not to make shorts. Outputs can handle only 10mA of current and so do not compromise on LED series resistance (R3 —R14) for more brightness. Do not use different colored LEDs in the same series, as they will not light up properly.
Other ICs like CD4060,4040,4541 also can be used for this application. Make suitable changes following the appropriate pin-out and application notes.
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