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By: Anonymous: dave () on Thursday, July 16 2009 @ 05:49 AM PDT  
Anonymous: dave

thanks. I can see a difference with the trim pots - added a few more leds to the board rather than just the one Smile and changing the value to 625 make it a little brighter... however I'm after much much more.

In a separate test I have one of the leds wired up to a 4.5v battery box with a test 57ohm resister (dangerous? it seems ok) that's the kind of burning brightness I'm aiming for. Can I just keep on decreasing the value of RA1 & RA2 until I get there? would it damage anything other than the LED?





       
   
By: MonsieurBon (offline) on Thursday, July 16 2009 @ 09:17 AM PDT  
MonsieurBon

Quote by: dave

thanks. I can see a difference with the trim pots - added a few more leds to the board rather than just the one Smile and changing the value to 625 make it a little brighter... however I'm after much much more.

In a separate test I have one of the leds wired up to a 4.5v battery box with a test 57ohm resister (dangerous? it seems ok) that's the kind of burning brightness I'm aiming for. Can I just keep on decreasing the value of RA1 & RA2 until I get there? would it damage anything other than the LED?



Dave, check out this thread - http://www.evilmadscientist.com/forum/viewtopic.php?showtopic=1730&topic=1730#1730 where Windell and I and others discussed how to get maximum brightness.

My solution to uncontrolled blinking of the array when programming was to solder male headers to two ZIF sockets, plug one into the main display controller's socket and the other in a Modern Devices BBB that is connected to nothing but a TTL cable. This way I can pop the AtMega168 in and out easily without frying any LEDs.

Also, I've upgraded to the STX790A for the transistors.


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By: Anonymous: dave () on Thursday, July 16 2009 @ 09:59 AM PDT  
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cheers for the link, some useful info there although I cant really see the answer I need.

If I reduce the value of RA1 & 2 to something low can this harm the driver? I need to make available the most current/voltage with what I have , and unfortunately I have no programming skills or electrical really Smile I know the leds can handle it but am scared of damaging the control circuitry.





       
   
By: MonsieurBon (offline) on Thursday, July 16 2009 @ 10:10 AM PDT  
MonsieurBon

Quote by: dave

cheers for the link, some useful info there although I cant really see the answer I need.

If I reduce the value of RA1 & 2 to something low can this harm the driver? I need to make available the most current/voltage with what I have , and unfortunately I have no programming skills or electrical really Smile I know the leds can handle it but am scared of damaging the control circuitry.



Following the driver spec sheet at http://www.st.com/stonline/books/pdf/docs/13093.pdf, I reduced the driver reference resistors to 150 Ohms. The driver can handle 100ma per driven pin, so you won't damage it if you drive it that high. I also replaced the demultiplexor transistors with ones suggested by Windell which can handle double the current of the ones in the plan.

But you do need to follow Windell's suggestion that you program off-board and not load any programs on the '168 that will turn on every LED without following the Peggy library's refresh rate.


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By: Windell (offline) on Thursday, July 16 2009 @ 10:17 AM PDT  
Windell

Can I just keep on decreasing the value of RA1 & RA2 until I get there? would it damage anything other than the LED?



You are welcome to push the limits of what Peggy 2 is capable of, but you should recognize that you are doing so and exercise appropriate care in your approach. You probably won't damage anything but your LEDs, although you should take care not to exceed the transistor current limit.

Peggy is a multiplexed display. Only one row of LEDs is ever turned on at a time. Doing this, we take advantage of the painfully bright LEDs that are available today, making an efficient display with reasonable refresh rate, that can even run on batteries. There are some enormous advantages to this, both in terms of cost and in terms of power consumption. (Hint: Peggy 2 costs $95, not $295, and consumes 0.5 - 3 W , not 20 W. It *can* run on batteries.) However, it does come at the cost of some absolute brightness. You *can* try to push it harder -- a few people (like MonsieurBon) have --but you will need to keep in mind that this really hacking it above and beyond the original design, and there is some subtlety.

As we discussed in that other thread, the display is uncontrolled at startup and during reprogramming. The *standard* brightness settings are designed to NOT blow up the LEDs, no matter what a regular user does. Make sure that you understand where that limit comes from before you step around it.


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By: Anonymous: dave () on Thursday, July 16 2009 @ 10:20 AM PDT  
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thanks, will give that a try hopefully the light will get through the diffusers now! I can unplug the grid of leds when I program as they are external using a couple 25 subs so no probs there.





       
   
By: Windell (offline) on Thursday, July 16 2009 @ 10:22 AM PDT  
Windell

>hopefully the light will get through the diffusers now!

Are you perhaps diffusing too aggressively? A simple piece of tracing paper, 1/2 inch away works pretty well... Also, consider sanded but transparent plastic-- it lets nearly all the light through.


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By: MonsieurBon (offline) on Thursday, July 16 2009 @ 10:38 AM PDT  
MonsieurBon

Quote by: dave

thanks, will give that a try hopefully the light will get through the diffusers now! I can unplug the grid of leds when I program as they are external using a couple 25 subs so no probs there.



Dave, what is your goal for this project? I think that might help everyone help you a little better. How big is your display? Are you wanting diffusing to blend neighboring LEDs together or to make each pixel appear larger?

We are doing an 8x8 foot display based on the Peggy, so we've done a lot of experimentation with diffusers that will keep each pixel isolated. Things we've tried have included plain paper with silver painted cubbyholes around each pixel, white plastic "party" drinking cups (these were pretty cool looking), and all manner of small white hollow balls.

Windell's suggestion of sanded acrylic is a good one, depending on how large your display is and how much you're willing to spend. But if you want pixel isolation, then you'd need to build a cubbyhole-esque matrix underneath it. As for tracing paper, you can buy 48" wide rolls of plasticized vellum (tracing paper) that should hold up nicely.

We'd love to see pictures!


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By: Anonymous: dave () on Thursday, July 16 2009 @ 11:20 AM PDT  
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Im making a 1.5m display (text and graphics) that is to be suspended rather high up. For diffusers I'm using 50mm expanded polystyrene spheres, with a 60mm spacing between the leds.

The spheres worked brilliantly in my concept stage with the leds brightly burning from a battery pack, and I kinda just thought I would get the same output from the peggy...silly I know....

Just dropped RA1 &2 down to 100 ohms, not really that much better to be honest. Will try and replace the transistors - problem is only have a few days left to get it sorted before I have to be onsite setting up! and there is soo much still to do!





       
   
By: Windell (offline) on Thursday, July 16 2009 @ 11:24 AM PDT  
Windell

What power supply are you using on the Peggy, and what type of LEDs are you using?

Are the 50mm expanded polystyrene spheres basically regular styrofoam?

Are you using the full 25 x 25 array, all the time?


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By: Anonymous: dave () on Thursday, July 16 2009 @ 11:30 AM PDT  
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I'm using a regulated 5V 1.5A supply.

Yep regular Styrofoam I think.

At the moment I'm just running the demo apps such as 16greysaclae animation, but will be running it as a video peggy.





       
   
By: Windell (offline) on Thursday, July 16 2009 @ 11:44 AM PDT  
Windell

>Yep regular Styrofoam I think.

OK-- that's *not* very transparent. You'll have to work *very hard* to make that visible.

Check out this comparison: http://tinyurl.com/pingpongballtests

The basic problem is that you're lighting up something that's not clear. What you really want is something that's clear but frosted, so that the light gets out, but in all directions. If it's up high, you may be able to get away with less than full spheres as well-- transparent plastic boxes (like from uline) or plastic cups or bowls may work.

There are also good old ping pong balls, even 55 mm (oversize) table tennis balls that you can probably still get in time as well.

Also, what LEDs are you using?


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By: MonsieurBon (offline) on Thursday, July 16 2009 @ 11:50 AM PDT  
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There are also good old ping pong balls, even 55 mm (oversize) table tennis balls that you can probably still get in time as well.
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We ended up choosing 40mm ping pong balls for our project because we could get 144 for $10.

I think the polystyrene balls will absorb more light than they let through.

We're also using 13,000 MCD green wide-angle LEDs because they look great in the ping pong balls and green has about the highest perceived brightness in human vision.


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By: Windell (offline) on Thursday, July 16 2009 @ 11:55 AM PDT  
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We're also using 13,000 MCD green wide-angle LEDs because they look great in the ping pong balls and green has about the highest perceived brightness in human vision.

mCd are funny units. They depend as much on lens shape and angle as the actual LED chip, but also account for the human eye response-- so a 1000 mCd green LED seems no brighter than a 1000 mCd red LED, in principle.


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By: Anonymous: dave () on Thursday, July 16 2009 @ 12:35 PM PDT  
Anonymous: dave

polystyrene balls look great with enough light in them, I originally choose them over ping pong balls because of the many large sizes available, I then had to settle on 50mm due to weight restrictions, cheap too 625 for $80.

Leds are: Ultra Bright White LED 3mm 9000mcd
Source Material: InGaN
Emitting Colour: White
Emitting Wavelength (nm): NA (Broad)
Size: 3mm
Intensity (mcd): 7000-9000
Viewing Angle (deg): 20
Forward Voltage (V): 3.0-3.4
Forward Current (mA): 20-30

Its just about acceptable now, so hopefully if I get some STX790ATransistors, It will be ok - is this just a simple swap job? Hopefully this will improve it enough. Unless there is anything else that be thought of hardware wise? would reducing the cable run have any noticeable effect or is that the same as reducing RA1?





       
   



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