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DiscoverCircuits.com -- Hobby Corner
"Hobby Circuits for your
Hobby Projects"
Last Updated on:
06/19/2009 03:15:42 PM
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Reduced Power Sequential LED Flasher
designed by David A. Johnson, P.E. |
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December
1, 2008 |
| The
74HC4017 is a neat decade counter which has 10 decoded outputs. When supplied with
a low frequency clock and with a LED attached to each of the outputs, an interesting ten
step sequential flasher effect can be produced. The LEDs can be configured in a
wide variety of patterns of loops, lines or arrows. But, these circuits can often
be power hogs. To reduce the average current needed to produce the same effect,
you can strobe the individual LEDs with a short 10mS pulse. |
| The
circuit shown below uses a CMOS version of the 555 timer to produce a 20Hz clock, with
10 millisecond output pulses. The 20Hz frequency means the 74HC4017 IC completes a
ten LED sequence twice each second. Slower sequences are possible by increasing the
value of R1. |
| With the
parts shown, the LED peak current is about 10ma with an average current of about 2
milliamps. A set of 3 alkaline AA cells should power the circuit for about 500
hours. This is 5 times longer than without the LED strobing method. With a
solid 4.5 volts from the three AA cells, any LED color can be used. By restricting
the color of the LEDs to only red devices, the circuit can be operated at 3 volts, with
just two AA cells. The average current can be reduced even more by lowering the clock
frequency or by decreasing the peak LED current. Many high efficiency LEDs produce
very useable light with just 2ma of current. To program for 2ma of peak LED current,
change R4 from 68 ohms to 330 ohms. |
| If you
would like to have more than 10 LEDs in a flashing sequence, you can string multiple
74HC4017 ICs in series. I will post such a design later. |
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| Click on
Schematic below to view PDF version of this Circuit |
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