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Topic: 100 x 100 possible? (Read 2052 times) previous topic - next topic

Galardo

Hey i been wanting to do a LED display but on a half dome surface...just one color white...i will use around 960 LEDs but in section...is this possible?  by the way it's a school project to, i'm wanting to go all out on this :D

example: http://www.indieshuffle.com/wp-content/files_mf/1293135498deadmau5_led_head.jpg

madworm

Have a look at Peggy2 by Evilmadscientist. You can adapt it to your needs.
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Galardo

so from the Peggy 2, i would just add more to it?

johnwasser


Hey i been wanting to do a LED display but on a half dome surface...just one color white...i will use around 960 LEDs but in section...is this possible?  by the way it's a school project to, i'm wanting to go all out on this :D


100x100 is 10,000  not even close to 960

Peggy2 uses two 74HC154 chips to control 25 rows and two STP16DP05 led driver chips to control the 25 columns.  Those chips each have 16 outputs so it should not be hard to extend the design to 32x32 (1024 LED's).
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scswift

A 32x32 array will give you 1024 leds.  You can build that using four TPIC6b595 shift registers on the cathodes, and four ULN2803A darlington arrays for the anodes.

Multiplexing an array that large though would result in the leds being on for 1/32 of the time though.  That means 1/32 as bright.  They make pretty bright white leds though, so you might be okay.  If you supply 40mA to each super bright led, that would mean you'd need around 1.2amps to light each row, which is not unreasonable.

Note that with this setup, you wouldn't have greyscale control over the leds.

Galardo

thats right...got my math wrong lol...thank you correcting me and for the info :D

Galardo

do you guys know a good website where i can buy a bunch of LEDs for a good price?

tomm

eBay if you're willing to buy from HK. Otherwise there are loads of other online shops, depends on your country.

scswift

An addendum to my post above...

As I said, with a 32x32 array, the LEDs will be on for 1/32 of the time, so you'll need bright LEDs in order to counteract this dimming.  I would reccomend whatever LED you plan to use you try running on a 1/32 duty cycle first at however much current you decide you will put through the LED, and see if that is bright enough for you.

Also, there is the issue of how much current you can put through those chips I listed.  For the TPIC, each drain can handle a total of 150mA, or 500mA total for the whole chip.  So if you have one drain on at at a time, it can drain 150mA.  If you have 8 on at once, 500mA/8 sinks = 62.5mA each.

For the ULN2803A, the datasheet seems to indicate each pin can source 500mA, and that the total that can be sourced at once from all pins is 2.5A.
(http://focus.ti.com/lit/ds/symlink/uln2803a.pdf)

This is important, because you're going to be lighting 32 LEDs at a time, either in a single row, or a single column.

If we take the 150mA for a single drain on the TPIC, and divide that by 32, we get 4.6mA per LED if we drain 32 leds at once.  That's not much, so even with a super bright LED that won't work.
If on the other hand we take the 500mA total for the chip, and drain 8 leds at a time, one per drain, we can drain 62.5mA per LED.  This would work fine... IF the ULN2803A can supply adeqaute current.

So how much can a single pin on a single ULN to supply all of those at once?  Well, as statted previously, the datasheet says each output can handle 500mA.  So we divide 500mA by 32 LEDs, and we get 15.6mA per LED.  Oops!  That's not good.  Not terrible.  But not good.  Not when each LED will be off 96% of the time.

What you'll need to do then is one of several things:

1) Find the brightest LEDs you can, and run then at 15mA on a 1/32 duty cycle, and see if that is bright enough for you. 
2) If it's not, then instead of a ULN2803A, you could use individual transistors which could supply 1A or more each.  Then you could get that 15mA per LED up to 30mA.
3) Go with one of the other suggestions here which may be better than what I'm suggesting.  I'm just a newb when it comes to this stuff.

Galardo

So i would need 32 transistors for each column or row? or both?

and i found these LEDs
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=320674396304&fromMakeTrack=true&ssPageName=VIP:watchlink:top:en

these LEDs have a viewing angle of 160-180 degrees and a mcD of 16,000, FV= 3-3.2, Reverse Current of 10uA
they look like what i would exactly need

And i found this project as a reference...it's a 32 x 32 coffee table turned into a LED pong table :D
And this project looks like it has the circuit board and chips i need too
http://www.instructables.com/id/Super-Pong-Coffee-Table/step11/Tidy-Up/

And this guy is using transistors...i'm assuming it's the same concept your telling me to do :D
http://www.instructables.com/id/810-LED-Matrix-with-4017/


scswift

Yes.  Though of which type, I'm not sure.  I think PNP though. 

I can't comment on the LEDs.  I checked Digikey for you, but 1000 white LEDs would cost several hundred dollars there.  And they only carry one tpye with a similarly wide viewing angle.  I'm usually wary of what you get from ebay sellers, especially the mcd rating, as they don't provide the datasheets.  But you can't beat the price.



Galardo

ok i'll probably watch the item for now lol

ok...I have the dome and i drew out on it where all the LEDs will go but they are kinda scattered...i mean the markings are nicely put on the dome, equally spaced...but they are not in parallel :(

So how bout if i just controlled the LEDs in a series.  There will be 30 rows but with different amount of LEDs in each row...how would that work?  In my head i'm thinking it's going to be easier...Just the programming is gonna be hard on account i would have to synchronize each indiviadual LED on it's own :D lol

scswift

Well you can't wire that many LEDs in series, because each LED will drop some of the voltage.  Assuming you have a 5v supply, you probably can't even wire two leds in series if you use white leds, as their forward voltage will be much higher than 2.5v.

You could on the other hand wire the leds in each string in parallel.  You'll need one resistor per led though.  It also won't matter that there are different numbers of LEDs per string if they're wired in parallel.

There's still the issue of being able to supply or sink enough current to consider.  But with this setup, you only need to worry about one.  Which one depends on how you want to switch the LEDs.

If you use transistors, you can stick them on the anodes or cathodes, but you'll need 15 of the right type of transistor depending on which side of the led you connect them to.

If you use two ULN2803As, a transistor array on a chip, you would connect that to the anodes, and connect the cathodes to ground.  You would need to use 15 pins, one per row, on the Arduino with this method however.

And if you use two TPIC6B595 shift registers, you would connect that to the cathodes, and connect the anodes to +5v.  You would only need three pins to control this setup, since you send serial data to them and can chain them.  The +5v you connect to however would need to be able to handle the current.  I don't know what the Arduino's onboard voltage regulator can handle.  But you could use a seperate voltage regulator to supply the +5v needed.  You just need to make sure to connect its ground to the Arduino's ground.  The right regulator could handle somehwere between 1-1.5A.

The question is, how much current would you need to supply? 

It looks like you have something like 68 LEDs in that diagram.  Assuming you power each LED with 15mA, That's 1020mA total.  Just over 1A.  That's not unreasonable.  However, the TPIC6B595 can only handle 500mA max going through it.  Any more and you will definitely burn it out, and you should use less than that.  Some people tell me 50% below absolute maximum ratings.  The datasheet seems to indicate 90mA continous per drain is okay.  90/7 leds on a single sink = 12mA.  But you may as well play it safe and go for the 50%, which is 75mA per sink or roughly 10mA per led.  Which is not at all unusual to right a bright LED at.  I am running some bright LEDs in one of my projects at only 4-5mA because any brighter and they would be too bright to be comfortable to look at.  And if you're gonna have 68 of the things... well, 10mA per LED is a safe bet.

So, now we consider the total current.  10mA per led, with 34 leds on each TPIC = 340mA per TPIC.  That's a lot more than the 50% of absolute maximum I'm told is reccomended.  That's more like 70%.  So it would be safer to drop the mA per LED to 7mA, to get down to 238mA total for the chip.

To summarize...

1) Get two TPIC6B595 shift registers.
2) Get a T-220 package voltage regulator capable of handling at least 1A and a couple capacitors for it.
3) Get one resistor per LED, connect it to the anode.
4) Each LED can use no more than 7mA.  So Google LED wizard, and put in the smaller of the two forward voltages listed for the led, 7mA, 5v source, and only 1 led in series, and it will tell you the smallest resistor you can use.  If the LEDs seem too bright, figure out how much they need to be reduced by, and pick a new value for mA.  4.5mA will be roughly half as bright, and 2mA will be half as bright as that.
5) Wire the cathodes for each row together, and then to the appropriate pin on the shift register.
6) Wire the resistors attached to the anodes for each row together and then to the voltage regulator +5v source.
7) Wire the ground of voltage regulator to the ground of the Arduino and the shift registers.
8) Connect the appropriate pins on the shift registers to the Arduino, to eachother, to ground, and to +5v. 
9) Send serial data to the shift registers to tell them which rows to light.

I think that about covers it. 

It may be easier to just use those premade boards the others suggested. :-)

Daneel

Because it hasn't been mentioned yet, you can get large numbers of LEDs from here at a not unreasonable price:
http://evilmadscience.com/productsmenu/partsmenu/89-led

About halfway down the page are 5mm single-colour LEDs - you can get 640 for $83.50, which I'd consider an insanely good price :D

best of luck.

Galardo

Alrighty...bookmarked this page and Thanks everyone for the info, seriously, i greatly greatly appreciate it.
The only difficult part is ee for myself is placing the LEDs on the dome in a form that it will work with the chips n' stuff

I will post up pics when i get the LEDs placed out on the dome, cause I would really appreciate your feedback :D
THANKS!!!

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