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Driving 740 LEDs with variable intensity


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Hi all,

My sweet wife wants a "dawn simulator" the ligthsource is to be LEDs, 740 of them in all.......
I've tried to discourage her, but she seems to believe i can do magic with electronics since i built her an amplifier/mixer for her computer stack.

I have been pondering how to drive those LEDs for some days now:
1: It seems,  to me, that PWM is the answer.
2: 24V is max voltage i'll allow out of the controller.

Any ideas and musings would be most welcome, as i'm still in the pondering phase.

Just to prove her belief in me, the sweet one bought 1000 white high brightness LEDs of Ebay. To get me committed as she says ::)
For revenge i have built a little teststand so she herself can pick out the 740 best of the LEDs , hehehe.

Any ideas?
Basil

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Yes, i think i'll make series of six LEDs with a resistor on the tail, seems the easiest.  A resistor for each diode seems to me to be a lot of work.

Now for a power-supply; i have been eyeing all those old computer power supplies i have. Stacking two of those would provide a nice stable 24V. Any ideas how those would take having a PWM circuit loading them?
Or should i rather build a conventional powersupply? I suppose a LM338 stabilised one would do just fine. I do have the LM338'ers in the junkbox.

Then there is the PWM circuit, i'm leaning towards making it voltage controlled, since that would make it easy to switch between manual and automatic dimming.
Any thoughts?
What about the switching frequency? A kHz or so?

I've disected an old clock-radio and have decided that it will do just fine for starting the "dawn". It turned out it was it's buttons that had died, digiswitches are easier to install in a panel anyway so that's no loss.

Another piece to the puzzle is making a ramp voltage that will simulate dawn.

Ideas, musings and downright "you are nuts'es" are most welcome. Please let me hear your thoughts on any of the above and (most important) what i have forgotten to think about

All the best,
Basil

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Hello,
You need to control all leds together or you must be able o change each led? The first is easy. Actually you have several possibilities, from custom DC/DC controller to using of standard LED controller you can see this type of controllers from TI Company.
You must know that the White led, actually all leds must be controlled via current. You must to have a several current sources for each group of LED's in series. In my application I am using either BD9778 - ROHM DC/DC controller, either TI controllers, this for application that must control all leds together. To build a current source generator you may use any DC/DC controller. Because the DC converter your input voltage can be in wide range. Because the number of leds is big, I can suggest you to use something like a matrix control.

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Are you using a microcontroller?
Ehhh, no.
Do you think i should?


I won't say should.  If you have experience with them you will already know the advantages.  If not, then this project alone would not justify the costs involved with starting microcontrollers.

I can't recommend using two computer supplies in series.  Some require a load on the 5V section.  Many have less than stellar regulation on the 12 part.
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You need to control all leds together or you must be able o change each led?

For a dawn simulation i need to control all LEDs together, fortunately. I have been considering using current control, but i think PWM is easier to build. I'll only need one output, which will make wiring up the LEDs easier. 
Are there advantages using current control instead of PWM?


I won't say should.  If you have experience with them you will already know the advantages.  If not, then this project alone would not justify the costs involved with starting microcontrollers.


I have some experience with the Parallax basic stamps, i've used them for making some tricky process controls at work.
I must admit i hadn't thought of using them for this project. But i think it's a splendid idea. That way i can get wifey to both pay the parts and program the controller  ;D  (She is the computer nerd in residense, after all) and i wont get any complaints about the clock being difficult or awkward to set.... Wonderful, hehehe.

All the best,
Henrik
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I fool around with BASIC Stamps too

hehehehe. Now, that's excactly how i'd describe my relationship with those  too ;)

Thanks a lot for bringing the PICAXEs to my attention.
It looks like they have all i need: A PWM out and a RTclock/LCD add on.

Seems, providing i stick with PWM, all i need to worry about is the powersupply and the driver section. 

All the best,
Basil

PS: Wifey is done testing and sorting the LEDs, result: 992 of 1000 LEDs are ok, which was a pleasant surprise, since they were a surplus lot of 1000 white LED (3mm 17,000mcd) for only $87+pp.... Sometimes you are lucky.
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Basil,

You might want to take a look at the stuff this guy sells related to PICAXE.  He has chips but, more importantly, he has prototyping boards that have a serial hookup and power supply on them so all you have to do is plug in the chip and program it.  I have bought several things from him and he seems to be a good guy.  Besides that, he is cheaper than the "official" US distributor for PICAXE.

Jim

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