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16  Forum 2005-2010 (read only) / Development / Re: Big LED Ambient System on: October 16, 2009, 03:45:11 pm
Nope, that's all you need.  

Whether or not your PC PSU can handle it depends on how many LEDs you're using and the PSU itself.  Figure at least 20mA per junction so at least 60mA per RGB LED--although you may be able to drive them at higher current; check the datasheet--times however many LEDs you plan to use.

You definitely want to use a 5V supply; a 12V supply will have you dumping an additional 7W straight to heat via the current limiting resistors for every amp your LEDs consume.  So if you have 100 RGB LEDs at 20mA per color, you'll be drawing 6A.  At a 5V supply that's 30W, with say (3.4+3.4+2)V * 20mA * 100 LEDs = 17.6W consumed by the LEDs, and 12.4W by the current limiting resistors.  At a 12V supply, LED current is the same (6A), so you'll be consuming 72W.  The LEDs are still only using 17.6W of that, so the current limiting resistors eat up the remaining 54.4W.  

Check the specs of the PSU you want to use.  Newer models have shifted more of their capacity to the 12V line since newer motherboards and video cards have shifted to deriving their low voltage/high current power from the 12V bus via switching converters rather than the 5V or 3.3V lines.  If you're hoping to piggy back onto your computer's PSU, you'll also have to figure how much of its capacity you're currently using.  The quality of the PSU also becomes important, since lower quality units will get flaky when loaded near their ratings.  

I'd recommend getting a separate 5V supply for any sizable array, though.  You can find switching supplies in a variety of form factors and capacities at decent prices at most electronic surplus sites.  This also gives you the added ability of easily divorcing the lighting system from the PC if you choose to do so at any point.
17  Forum 2005-2010 (read only) / Development / Re: Big LED Ambient System on: October 15, 2009, 11:24:14 am
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Regarding the other option, using a logic-level N-channel MOSFET: I would wire it to the PWM port of the Arduino and the all of the cathodes of the LEDS (one color), but what is connected to the third leg?
The negative supply (ground).  Basically, the MOSFET is a switch, with a voltage at the gate, current can flow between the source and the drain.  In your particular case, during the high portion of the PWM signal current will be allowed to flow between the negative supply and the cathodes of your LEDs.  

Since MOSFETs are voltage controlled devices, you will want to put a pulldown resistor at each gate, otherwise when the Arduino is off or executing the bootloader the gate can float high enough to turn on (since all of the IO pins are in high-impedance state at that point).
18  Forum 2005-2010 (read only) / Development / Re: Big LED Ambient System on: October 12, 2009, 09:09:01 pm
1: Two RGB groups fit perfectly within the Duemilanove's 6 hardware PWM pins, so you're good to go there.
2: Depending on how much you want to spend versus how much work you want to do in wiring/mounting, you can find pre-build RGB LED units at places like dealextreme, allelectronics, etc.  I've got some of these on my desk right now that work alright.  Or you can build up your own out of discrete R, G, & B LEDs (though you will need to choose exact wavelengths carefully if you want an optimal gamut) or RGB LEDs.  The disadvantage with single-device RGB LEDs is that you can't wire them in series, so you need to use a lower supply voltage and many more current limiting resistors which makes the whole assembly drastically less efficient.

3: If you use discrete R, G, and B, you will want to wire them in series strings to match your supply voltage.  Higher voltages make for more efficient arrays (more LEDs in wach series string means fewer current limiting resistors), and means less current for you to control.  If you buy premade RGB assemblies, go by whatever voltage they're spec'ed for.

Common anode assemblies are preferable because you can tie the anode to a higher voltage supply and use a logic-level N-channel MOSFET (like an IRF510) with the gate wired directly to an Arduino PWM pin to control the cathode.  This is much easier than trying to use a MOSFET at the anode.

You can control as many LEDs as you want!  Of course you will need a supply that is suitably sized, and suitable switching devices.  12V supplies are readily available, look for one that is rated for at least as much current as your entire LED array will draw when all colors are at full blast.
19  Forum 2005-2010 (read only) / Development / Re: Switchmode Power supply. on: November 08, 2009, 01:52:24 pm
I think you're going to need to post a schematic before anyone can help you.
20  Forum 2005-2010 (read only) / Development / Re: LED help on: November 03, 2009, 10:08:38 pm
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it would "work" but each bank of 3 will be dimmer
It won't just "work", it will work (no quotes) just fine.  smiley-wink   120R is just right to drop ~2.5V at ~20mA which is about what will be left of the 12V supply with three LEDs in series.  
21  Forum 2005-2010 (read only) / Development / Re: homemade servo-bus for arduino shield on: October 12, 2009, 08:40:26 pm
Not a bad idea!

You can make it much smaller, though:





 smiley
22  Forum 2005-2010 (read only) / Development / Re: Arduino as a programming bridge to another Arduino on: August 23, 2009, 11:27:24 pm
If you pop the ATMega out of the arduino board, you can then simply carry the appropriate connections to the chip on your breadboard (TX, RX, RST, GND at least), no modifications necessary.  Alternatively, you can get one of the various FTDI boards like this one to program breadboard arduinos.  
23  Forum 2005-2010 (read only) / Development / Re: 200 LED project. on: August 16, 2009, 01:50:30 pm
You still don't have your power supplies properly bonded.  Start by wiring the (-) of G1 DIRECTLY to the (-) of G2 and then leave it that way forever.  

The reason they all come on is because by connecting all of the MOSFET sources to the arduino ground you've put all of the LED banks in parallel and all if the MOSFETs in parallel.  MOSFETs are like switches, if you place a bunch of switches in parallel, it doesn't matter which one you flip, the load will always turn on.  So eliminate the connections between the sources and ground.
24  Forum 2005-2010 (read only) / Development / Re: 200 LED project. on: July 25, 2009, 12:52:26 pm
You've got your transistors hooked up wrong. . .check out this tutorial.

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Transistors will work fine for you... But you won't get fading from the transistor so you might as well use a 555 timer chip to control it instead of an arduino...
Fading as in brightness control?  That's what PWM is for.  The transistor is just there to amplify current.

The only thing you really need to worry about when it comes to the lifespan of the LEDs is the operating current.  In any lot of LEDs, especially the cheap bags of 'em you get on eBay, expect some to die early.  Your best bet is to burn them in for a while to weed out any weaklings before you button everything up.  It'll also help if you stay a bit below the rated maximum current.

One disadvantage to wiring LEDs in series is that when one fails open, every LED in series with it will go out as well; if any fails short circuit the other LEDs will stay on, but will be getting much more current since the current limiting resistor is dropping a much higher voltage.  So watch out for one LED goes out and the one next to it gets a whole lot brighter and replace any that have failed short ASAP.
25  Forum 2005-2010 (read only) / Development / Re: 200 LED project. on: July 23, 2009, 09:42:58 am
If you are using two separate power supplies for the Arduino and the LEDs as your schematics indicate you will need to join the negatives together so that there is a common reference when you supply power to the transistors.  Otherwise it won't work.

Why would you not wire the LEDs in series pairs?  If you wire them entirely in parallel, that's 200 * 20mA = 4000mA consumption at full bore.  If you run them from 9V, that's 36W total, of which 22W is going straight to heat through the current limiting resistors.

On the other hand, wiring them as series pairs, they'll only consume 100 * 20mA = 2000mA, totaling 18W, of which only 4W will be dissipated by the current limiting resistors.  Efficiency will be massively improved and your battery life will be doubled.
26  Forum 2005-2010 (read only) / Development / Re: 200 LED project. on: July 20, 2009, 11:15:00 pm
Arduino pins can supply a maximum of about 40mA, LEDs typically draw 15-20mA, so more than two LEDs in parallel will require a transistor.  If you use a 9V power supply, you can wire at least two LEDs in series which will halve the number of current limiting resistors you'll need and making the whole arrangement vastly more efficient.  The number of parallel LEDs will determine the total current that each transistor will need to supply.  What color are the LEDs you'll be using?

For the board itself, do you mean 20x30 inches?  For that size and as simple as your design is, you could use a sheet of masonite (more durable) or foamcore (lighter) or gatorboard (like industrial foamcore, in between the two for weight/durability), drill holes for your LEDs to poke through and wire it all up on the back.  Glue some spacer strips around the edges and a few blocks in the middle, and then you can attach another piece of board on the back to protect the wiring.  A couple coats of Polycrylic or similar all over will help to weatherproof it.
27  Forum 2005-2010 (read only) / Development / Re: Charlieplexing on 1-sided PCB 90LEDs 10 Pins on: August 07, 2009, 02:38:05 pm
You're not going to be able to do a single sided layout without lots of wire links added.  With only ten pins wired straight to the display you will only be able to light one LED at a time which will allow a duty cycle of 1/90--very low if you're trying to display an image with any more than a handful of LEDs on at a given time.  The common method is to use shift registers to drive columns and a demultiplexer or individual pins to drive rows (via darlington or FET or somesuch to provide sufficient current).  That way you can feed an arbitrary amount of column data into the shift registers with 2-3 pins and display it all at once when you activate a row.  You can control your matrix with as few as five pins with this method.  
28  Forum 2005-2010 (read only) / Development / Re: Editing nets in Eagle on: August 03, 2009, 12:25:41 pm
In my experience, as long as there was still something assigned to each net somewhere in the schematic eagle was remembering that those nets were supposed to be connected (apparently).  Hence why I had to delete all of those segments to make it forget the association; deleting just the misassigned segment was not enough.  The problem originated with a stub wire that was assigned by name to the wrong net, not with an errant wire between two components.  And since both nets were already wired to multiple components, simply renaming one into the other was not a solution.
29  Forum 2005-2010 (read only) / Development / Re: Editing nets in Eagle on: August 03, 2009, 09:48:16 am
It seems the solution was to delete every segment attached to the relevant nets and rewire the affected pins.  It seems that Eagle forgets a net name once all its segments have been deleted.  Still wish there were a better way, but if there is I can't for the life of me find it.
30  Forum 2005-2010 (read only) / Development / Re: Editing nets in Eagle on: July 28, 2009, 11:11:14 am
I was rearranging the functions of some header pins in the schematic and went to reassign the VIN pin to RESET,  Eagle asked me if  I wanted to connect the RESET net with the VIN net and because I was tired and not paying enough attention I clicked Yes.  Now as I work on the board layout it keeps drawing a ratline between RESET and VIN.  So the problem is in Eagle's concept of what should be connected to what rather than in anything I've drawn which is why I can't figure out how to fix it.  It seems to me like it'd be a serious shortcoming to not have any facility to view & edit the net list.
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