Electronics > Microcontrollers

Multiplexing problem - White LED ghosting

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ColinB:

--- Quote from: andersendr on August 13, 2012, 08:04:50 pm ---When I look at that video, it looks like the light fron the LED that is being lit, is reflecting off the non lit LED's.  To me the tell tale sign was that you had to hold the board at the correct angle to get the problem to show up in the camera.  One thing to try would be to put some paper or tape around the LED and see if the problem still exists.  Another way to verify this would be to hook up a scope or a volt meter to the LED that lights up that should not.  This would tell you if there was trully any voltage across that LED.

--- End quote ---

I considered that it might be reflected illumination from the adjacent LED as well, but it seems to be a greater effect at higher multiplexing frequencies, so I don't think it's the problem.  The reason you need to get the angle exactly right is because the LED lens focuses fairly tightly and if you really want to get a direct shot at the LED die, you need to look directly down on it -- when it's so faintly illuminated, any angle offset may obscure the output.

hun_yeti:

--- Quote from: ColinB on August 14, 2012, 03:52:57 pm ---
--- Quote from: andersendr on August 13, 2012, 08:04:50 pm ---When I look at that video, it looks like the light fron the LED that is being lit, is reflecting off the non lit LED's.  To me the tell tale sign was that you had to hold the board at the correct angle to get the problem to show up in the camera.  One thing to try would be to put some paper or tape around the LED and see if the problem still exists.  Another way to verify this would be to hook up a scope or a volt meter to the LED that lights up that should not.  This would tell you if there was trully any voltage across that LED.

--- End quote ---

I considered that it might be reflected illumination from the adjacent LED as well, but it seems to be a greater effect at higher multiplexing frequencies, so I don't think it's the problem.  The reason you need to get the angle exactly right is because the LED lens focuses fairly tightly and if you really want to get a direct shot at the LED die, you need to look directly down on it -- when it's so faintly illuminated, any angle offset may obscure the output.

--- End quote ---

Reflection was my first guess as well, but i ruled that out.
And yes, i really have to hold the LEDs straight to to camera because these are very focused LEDs, with a viewing angle of about 15-20°.
I wish i had an oscilloscope, that would instantly reveal the problems i think.

kripton2035:
can you try to place a small capacitor like 100nF in parallel with each row of leds causing that problem ...
just my 2 cents...

Colin55:
First of all we don't know which way you are scanning.
Turn on one LED in a high-speed scan and let us know if the mirror is before the LED or after the LED.
The fault is definitely due to not turning off the LED fully after illuminating.


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