help w/ led string for umbrella!

[illegalsmile]

Hello and I hope this is being posted in the right place! My friend is doing a long rafting trip and we were talking about making a led umbrella. Probably somewhere in the neighborhood of 32ish (4 per segment) white lights, or blue, or white and blue, i don't know yet. My biggest problem is I don't know the best way to do this with batteries.

There are off the shelf options such as the battery powered strings but those usually come with a limited amount of lights (~10). So I want everything to run off of one battery pack using AAs. Let me run what I figured out. If i use 10 AA (1.5v/ea) batteries, i can have a 4 (led) x 8 (series) array assuming a led forward voltage of 3.3 and 30mA continuous voltage. Is this right? How long would something like this last?

Ideally I'd like the most amount of leds and the least amount of batteries.

Also, is there any circuit board preprogrammed to easily do things like chase, scroll, dim in and out that I could add on?

Any help is appreciated, THANKS!!!

P.S. I'm sorry I posted this in the flashlight - led section. Mods could you please move this post to the right spot or refer me to another forum to post this message?

 

[illegalsmile]

I was just looking around to find how many leds some headlamps use and how many batteries they take. This headlight takes 3 AAAs and powers 19 5mm white leds for ~20 hours. I was reading somewhere about how some flashlight/headlamp manufacturers do not use any resistors at all because they have leds that are all the same quality with little variance. Could I do this with 32 of them? Thanks again.

 

[riffraff]

What do you wish to use as a battery? Once you know that, we can go from there.

 

[illegalsmile]

here.

So I want everything to run off of one battery pack using AAs.

 

[riffraff]

Ah, right. I missed that.

Okay, so we're looking at around 15 volts, nominal (10 AAs in series). As you know, the number of LEDs on a series string is limited by the forward voltage. White and blue LEDs have a higher Vf than green and red. Check out this page:
http://www.superbrightleds.com/leds.htm
There's a datasheet linked to each part number in the left column.

So, you can drive four white or seven red LEDs off of 15VDC, per string. You could have twenty strings of four white LEDs, for instance. That would use 20 100 [size=-1]Ω[/size] resistors, each dissipating 40mW each, for a total power draw of 6080 mW (6.08 Watts) or a total current draw of 400 mA. Assuming 2850 mA of capacity (Duracell MN1500), alkaline AAs should run an 80-LED array for about 7 hours, give or take (assuming Vf=3.3VDC, If=20mA for each LED).

Your example of a 4 X 8 array at 30mA would draw half that (180mA) and so theoretically should last about 15 hours.

Plug in different values here http://led.linear1.org/led.wiz in order to play with other configurations.

Scrolling, chasing, etc. could be done with a small microprocessor, or even a counter (latch) chip. I don't know of a programmable off-the-shelf solution, per se.

Anyway, let us know what you end up with. We will want pictures, of course!


BTW, welcome to CPF!