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Controlling 4 RGB Leds with minimal components


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i could still need a pointer to determine iref, because i don't know how to calculate Vref

 

That's actually easy. Look at the datasheet for the TLC5940(link below), page 12 has a chart in the top left that shows current vs reference resistor size.

 

http://www.ti.com/lit/ds/symlink/tlc5940-ep.pdf

 

now i feel really bad. i just pushed _some_ resistor in for it to work ^^. lucky me, i cought a 4.7k Ohm, but this just by accident, the one next to it would have been 500Ohm :shh:

 

when i get the rgb leds i will do the math beforehand. what i have to figure out now is what leds to get and how to configure the tlc in means of analog brightness correction. also i feel like the tlc just works when i constantly push the current status, but i recall something in the paper that might help with this :)

 

a great project btw: take an obsolete 10Mbit hub, connect all the nice indicator leds to the tlc5940 and have your own binary clock in a nice project box. but first some cursing because of the 5V standard and tons of resistors to replace, and yes,even back then it was smd

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As a note, plcc-6 5050 ""smd"" package rgb leds are neither common anode or cathode. Individual c/a leads for each color, in a small package. Those are the same size leds used in most of the 30 leds per meter rgb strips.

 

Ahh, that must be how that guy charlieplexed his RGB display...

 

now i feel really bad. i just pushed _some_ resistor in for it to work ^^. lucky me, i cought a 4.7k Ohm, but this just by accident, the one next to it would have been 500Ohm :shh:

 

Haha, breaking stuff is all a part of prototyping(I do it all the time because I like using LEDs as in-line visual current indicators) :P

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you just have to be alittle creative, but common cathode RGB LEDs can be charlieplexed.

 

Lol.. I like it.. so neat and simple

 

Dan

 

*edit*

Thinking about it.. you would have to be wary to sourcing/sinking too much current if more than one LED/colour was active at the same time.. But I still like the layout..

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you just have to be alittle creative, but common cathode RGB LEDs can be charlieplexed.

 

Lol.. I like it.. so neat and simple

 

Dan

 

*edit*

Thinking about it.. you would have to be wary to sourcing/sinking too much current if more than one LED/colour was active at the same time.. But I still like the layout..

 

Way I see it, you can only have four on at any given time. (2 pins high, 2 pins low). But the entire point of charliplexing is for outputs to be pulsed, pin by pin. No two should be on at the same time.

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I have set up the tlc5940 now, so i might go with this solution, though charlieplexing still have the advantage of less compoents, the tlc5940 seems a little overkill as well as not as easy to control as i thought.

 

one would have to find a rgb led that has the same forward voltage on all leds, or else one would have to add a lot of resistors, 3 for every led instead of 1 for every pin.

 

i still cannot completly figure out why so many leds light up, but it might be a combination of too much voltage as well as a few mistakes in wiring ^^

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  • 2 weeks later...

Ok getting slightly more creative you can isolate the colors per pin so you only have a single resister per pin based on the color that pin triggers. This requires a diode on each pin also to jump around that pins resistor when it is used as the common.

 

This is only quasi-Charlieplexing at this point because full charlieplexing would allow 30 LEDs (10 RGB sets) but since the cathodes are tied together, you are limited to a max of one RGB LED per pin, and isolating the pins by color limits you to sets of 6 pins at a time, but you still maintain 1 pin per RGB LED density.

 

I enjoy puzzles like this

post-12741-135135538941_thumb.jpg

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