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Abuse my LEDs?

Discussion in 'LEDs and Optoelectronics' started by galantida, Nov 1, 2012.

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  1. galantida


    Oct 27, 2012
    Hello All,

    I have a 4x4 grid of LEDs I want to light. I am quickly flickering through them one by one and they all look like they are on. When I expand the grid to 8x8 they get very dim. I have a 3D cube so eventually I want to light 512 LEDs but these will apear to be barely on. Would it be OK to use a smaller resistor and take them beyond their spec since they are only on for a fraction of a second or will this still damage them?

    Note: They are wired in parralel so its not power loss that is dimming them but the low duty cycle (I guess you would call it).

    Thanks in advance.
  2. CocaCola


    Apr 7, 2012
    You need to increase the frequency you are rotating them on/off or redesign the circuit so that more than one is on at a time so as to reduce the loop size... There is simply limits to what you are doing...
  3. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    In addition to increasing the frequency you need to increase the current.

    An LED with 20mA going through it constantly will have a certain brightness.

    The same LED with 20mA going through it for 10% of the time will only appear as bright as one that has 2mA passing through it continuously.

    If you only light 1 LED at a time, you can see that the peak current needs to be quite high to have it appear at a reasonable average brightness.

    The best answer is to arrange your drivers so that you light more than 1 LED at a time. It should be quite east to light a whole row at a time. This means that 512 LEDs (8x8x8) will illuminate 1/64 of them at any time. To maintain the same brightness as a constant 20mA, you need a current pulse of around 1.2A. Check the specs for your LED, they may allow a 1A peak current.

    Another solution is to treat the LED array as a 64 x 8 array and set the value of 64 LEDs at a time (32 x 16 may be more feasible). In this case, the LED current only needs to be 160mA

    You may not want your LEDs to be that bright, but the inescapable fact is that the shorter time they are on, the dimmer they will appear.
  4. galantida


    Oct 27, 2012

    I actually have made some progress. They were originally running at about 25% their full brightness.

    Hardware Change
    The first thing I did was modify the 8x8x8 cube to that each column had its own feed from a decoder chip. This allowed me to select any single column using 6 output bits on the Arduino (3 for x location and 3 for y location). I then took eight outputs from the Arduino and connected one to each layer of the cube. Since these output are independent of each other this configuration allows me to light any combination of LEDs in the column in one pass. (10%) brighter

    The second thing I did was put a capacitor in to keep the LED on during the downtime between changes. Each of the 8 feeds from the Arduino that power a layer of the cube have a limiting resistor. I put the capacitor parallel with the resistor and they got a little (5%) brighter. Not sure why this worked?

    The third thing I did, which seemed to help a lot was modified my software. I have animations that run at about 2 FPS. I had the entire cube render nearly every processing cycle. Not sure how fast this is but I am guessing about 30 FPS which is why they are dim or flicker.

    What I found was a lot of time was taken to change the status of the outputs on the Arduino for reach new LED. This is dark time since I have to turn the cube off or you would see the LED move it x and y coordinates one at a time.

    Now knowing this is processor intensive, I rewrote it to render only one column per cycle rather than the entire cube. I also modified the loop to only change the outputs it needs rather than reconfigure the entire x,y and z output for the cube every time. Now the cycle when an LED is on is longer than the time cycle when one is off. This gave a huge brightness increase. I would say they are running at near 60%-70% now.

    “If you only light 1 LED at a time, you can see that the peak current needs to be quite high to have it appear at a reasonable average brightness.”

    Steve, your quote above is exactly what I was asking. I haven’t taken that step yet. I am not quite confident yet that the cube won’t get stuck on one LED too long and burn it out on me. If I do boost the power I’ll have to think of a way to protect my LEDs in case I get stuck on one for some reason. This is why I was trying the capacitor route. I figure it would give them a little boast of power that would never be sustained.

    Well my RGB LEDS just came in, so now I have to redo my entire bread board to support 3 colors. This should keep me busy for a couple of weeks. At least they seem brighter. :)
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