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Topic: Building an addressable LED strand with WS2801 (custom pcb) (Read 9 times) previous topic - next topic

bograt

I found some of the Parallel ones: http://www.digikey.co.uk/product-search/en?x=19&y=9&lang=en&site=uk&KeyWords=+MLEAWT-H1-0000-0003E3
This company seem to sell all the cree modules so I guess it would be best to design it assuming I can get any part I need...
I would have liked and LED with CRI higher than 80 but it seems all of them have been wiped of the planet
Best

Jules



KirAsh4


I found some of the Parallel ones: http://www.digikey.co.uk/product-search/en?x=19&y=9&lang=en&site=uk&KeyWords=+MLEAWT-H1-0000-0003E3


No one has the parallel ones in stock.  I do know that Cree has recently switched to a larger wafer manufacturing, so maybe that's why they're behind.  It's also possible that they're focusing more on some of their newer technologies.  Or that there simply isn't a high demand for them.  The one thing with the parallel ones is that if one of the channels goes out, you will lose some intensity and might not realize it.  With the serial ones, you lose a channel and all three die, resulting in a dead pixel in an array.  Something's that a lot easier to spot.  I don't know, just speculations on my part.

The Avago Tech ones would do the job as well.  Depending on where you are geographically, you can get them in the US as well.  And they are low voltage so you can pair three of them up to one WS2801 and still drive the whole circuit with 5V.  Or you can create strings on each channel.  With 3 per channel, you can drive the circuit with 12V, or go up to 6 per channel and drive it with 24V.  That's a lot of light ...

I gotsa play with some of these.  Perhaps it's time I dig up my DIP version WS2801s and prototype some of this, see what happens.

bograt

I think I will buy some of the Avago ones to play around with, they have very good CRI as well. Also I am in the UK so it looks like I can source these easily.
How do I know which resistors, capacitors and transistors to use? I am trying to learn this stuff but its all very new to me...
I have been studying your circuit diagram and trying to figure out how it all works  :D I cannot figure out exactly what the capacitors are for...
If I have 12v coming in how can I make sure I the other components get the correct voltage?
I'm sorry, I don't mean to overload you with questions but I am desperate to make progress with this

KirAsh4

The caps are to stabilize the ripples in the circuit.  The Zener will protect the IC (and transistors.)  Anything over 6V, and you need the Zener to clamp the voltage.  This is specified in the datasheet as well.  That's really the only component you need to worry about when it comes to voltage.  The other ones are:

R1, R2, R3 are controlling the amount of current for each LED.  The formula is in the datasheet:  I = Vref/R (where Vref = 0.6V).  The Avago LEDs are 150mA, so you'll need those three resistors to be 4 Ohms.  0.6V/4 Ohms = 0.15A = 150mA.  In my case they're 33 Ohms because I'm driving 20mA LEDs with only 18mA: 0.6V/33 = 0.018A = 18mA

R4, R5, and R6 are there to provide sufficient bias to turn the transistors on/off.  22K, 27K, or in my case 33K works just as well.

R7 is there to control the total amount of current flowing to the IC and transistor bases, this is why it changes based on the voltage.  Again, this number is derived from the datasheet.  For 12V, they suggest a 2K resistor ... I picked 2.2K, so shoot me. :)

The transistors are BC848B ... simple NPN transistors, able to operate up to 100MHz.

KirAsh4

The one thing to keep in mind when creating multiple LED "pixels", is heat dissipation from the LEDs.  Normally I create boards that will have vias on the thermal pad going to the other side of the board where I can then vent the heat, or use a fan.

bograt

Amazing, Thanks for the info.
I'm slowly starting to understand what is going on here, I think I will benefit from a bit more study in this area though, might invest in a book.
guess the next thing to do is get some of these parts and see what I can make something happen :)
But let me get this right, using this circuit (with changes to the resistors) would 12v be suitable? (assuming R7 is 2k)
And I can't quite figure out how the zener does anything in that location, that's really boggling my mind! is the ground location important?
thanks

Jules

bograt

I am going to print some pcbs and reflow solder them so I will make use of the 'isolated heat pad' they mention, thanks for pointing that out.
They probably wont stay on for more than a few seconds at a time so hopefully I wont need a fan, I will certainly look into draining a bit of that heat away though, thanks

KirAsh4


guess the next thing to do is get some of these parts and see what I can make something happen :)


Best way to play.


But let me get this right, using this circuit (with changes to the resistors) would 12v be suitable? (assuming R7 is 2k)


Yup.  The only resistors you need to change are R1, R2, and R3.


And I can't quite figure out how the zener does anything in that location, that's really boggling my mind! is the ground location important?


Ground is ground, doesn't matter.  Just pour yourself a ground layer and tie them all together.

Here's a different view ...  Again, I'm using 20mA per channel LEDs.  So change the resistor values on R1, R2, and R3.  And the part listing is for 0603 parts.  The LEDs are 5050 RGB LEDs which I have several hundreds of so they're not listed.

KirAsh4

I can easily make this into a board, however, I've never done a DIY board ... I've always sent them off to a fabrication house.  Doing it by hand means you'll probably need thicker traces and what not.  This particular schematic is on a board that's around 30mm^2 ... I forgot the exact dimensions.

bograt

This information is absolutely priceless to me you know, I really owe you for this, I will have to give you some cash at some stage.
I was planning on getting the pcb's printed here in London along with a laser-cut reflow stencil, and then trying my luck at assembling them to cut costs. the quotes I got for assembly seemed a bit steep for the work...
I know things will be small, I have very steady hands a good eyes so I though I would try my luck, also I was hoping that using a stencil and oven would mean I could work smaller.
Out of interest did you get the schematic you uploaded made?

KirAsh4


fungus

#27
Nov 30, 2012, 09:42 am Last Edit: Nov 30, 2012, 01:14 pm by fungus Reason: 1
I just saw this on eBay - RGB LED with a built-in WS2811...! 

You should be able to pack those quite close together :)

Edit: I forgot the link: http://www.ebay.com/itm/160886742091
No, I don't answer questions sent in private messages (but I do accept thank-you notes...)

KirAsh4


I just saw this on eBay - RGB LED with a built-in WS2811...!

You should be able to pack those quite close together :)


Yes, those are single LED per IC.  We're talking multiple LEDs per single IC, which is also possible using the WS2801.  eBay doesn't sell those.

bograt

Thanks for pointing that out fungus, just looked them up and although not suitable for my current needs, it looks like an interesting component.

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