# Help with LEDs

Discussion in 'LEDs and Optoelectronics' started by kingaxon, Dec 27, 2012.

1. ### kingaxon

3
0
Dec 27, 2012
I'm trying to build a small circuit that will send a pulse, so to say, down a string of LEDs. I've been looking around, and I can't find anything online explaining how to make what I need. All I can find is how to flash LEDs to music, make a flashing clock, things like that. Amy help would be greatly appreciated.

2. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,496
2,837
Jan 21, 2010
You need to explain very clearly what you want.

Something like "I want a string of 10 LEDs and make the first one light, then the second, third, and so on down to the last one. Then I want it to start again from the first LED. Each LED should stay on for about half a second."

3. ### kingaxon

3
0
Dec 27, 2012
Sorry, my bad. What I want to do is, say I have a string of 20 LEDs, I want 1, 2, and 3 to turn on. Then have 1 turn off and 4 turn on. Then have 2 turn off and 5 turn on. Then 3 turn off and 6 turn on. And so on down the line. So that it seems like the light is moving down the string of LEDs.

4. ### donkey

1,293
56
Feb 26, 2011
how many LEDs in total?

3
0
Dec 27, 2012
26 LEDs

6. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,496
2,837
Jan 21, 2010
For just 10 LEDs, a 4017 can do something dimilar to this (however it only has 1 LED on at a time.

However, there are ways of extending the outputs of the 4017 by cascading them (See this datasheet, page 11).

With steering diodes on the outputs you can have each output also turn on adjacent LEDs (this will require 3 diodes per output (so 60) and a transistor (20).

Also required will be an oscillator to clock the device (you could use a 555).

A more simple circuit would use a microcontroller. With charlieplexing you only need 5 outputs to control 20 LEDs. Whilst programming for this is complex, the circuit is a lot simpler.

A microcontroller solution will probably require more than an 8 pin device (whilst they have up to 6 I/O pins, some are input only). Something like the 14 pin PicAxe 14M2 would work nicely.

Charlieplexing only allows a single LED to be on at a time (well, not exactly, but that's the easiest way) so you would have to decide which three, then rotate through them quickly so that they all appear to be on, then after a suitable delay, change to another set of three, and so on...

7. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,496
2,837
Jan 21, 2010
OK, 26, not 20. With charlieplexing that means 6 pins are required for up to 30 LEDs