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I'm building some LED juggling balls. I'm planning to power an Arduino with two AA batteries, and power the LEDs with a 9V battery.

I want to drive three of these 3W RGB LEDS in parallel, with PWM. I'll probably need 3 to get an even glow around the sphere. I'm planning to use a 9V battery (perhaps something like the Energizer LA522) to power the LEDs. They say this battery can provide up to 1A continuous discharge, so at max brightness there will be about 100mA through each LED. That would give a power of about 0.65W for reds and 0.56W for greens & blues.

I found the NCP5623T LED Driver with PWM, which has 3 outputs for RGB, and I2C support. I'll be needing about 300mA for each color (3 LEDs in parallel), and the IC can only provide 90mA, so I was thinking to use darlington pairs for each of the colors.

Questions:

  • Would it be better to use these 1W LEDs at 100mA? I chose the 3W ones because they were $2 cheaper. Or is it likely that I will get the same brightness at 0.6W?
  • Would you recommend using a different LED driver IC with PWM? (Non-SMT would be easier to breadboard)
  • Should I use MOSFETs instead of darlingtons? (Maybe NTD80N02-001?)

Please correct me if I've made any wrong calculations / assumptions, and please let me know if you have any suggestions. Thanks for your time!

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I hope you’re aware that 9V batteries have a fairly low capacity. The one you cite may have less than 700mAh at the high discharge rate you’re planning for, so one battery would be good for about 40 minutes. AAs have considerably higher capacity. –  microtherion Sep 3 '13 at 9:04
    
Frankly, 3 rgb leds in parallel is 9 leds in parallel, each with about 2.2~3.2v forward drop. You can power everything with three AA batteries in series in stead. –  Passerby Sep 3 '13 at 9:40
    
Er, which arduino are you using? –  Passerby Sep 3 '13 at 9:48
    
Regarding "... power an Arduino with two AA batteries", = 3 Volts nominal: That would be iffy with most older Arduino boards. I can get an Arduino Pro Mini to work by feeding in 3 Volts directly to the 5 Volt pin, but the timer goes badly off. My Arduino Uno doesn't even get past the bootloader with just 3 Volts. On the other hand, a 3.3 Volt Nano clone I have, works perfectly with even 2.5 Volts. So, have you verified whether your board works with the 2 AA cells? –  Anindo Ghosh Sep 3 '13 at 10:11
    
Those 3 Watt LEDs run pretty hot. If you are placing 3 of them inside a juggling ball, you might have to juggle pretty fast to avoid cooking your hands :-) –  Anindo Ghosh Sep 3 '13 at 10:14

1 Answer 1

up vote 1 down vote accepted

If you power off 9V then consider using some of the 3W LEDs in series: -

enter image description here

Typically red leds need about 2.5V across them so wiring three in series will need at least 7.5V then there should be some current limiting circuit that might take 0.5V.

Typically for blue or green leds they need 3.4V across them so maybe only series together two LEDs.

You might just about get enough brightness with the NCP5623 chip this way but if not you'll need a current limit circuit and mosfets. The NTD80N02 looks OK but you need to limit current into the LEDs.

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Hi, thanks for your answer! Could you please explain why it is better to wire some LEDs in series, instead of parallel? Is this more efficient? –  nathan.f77 Sep 5 '13 at 3:17
    
I think I've figured out why it is better to wire some LEDs in series. (Sorry, my physics/electronics is very rusty.) So in parallel, the LEDs will be wasting voltage, which could have been divided over multiple LEDs if they were wired in series. So series will use less current, and the full voltage. In that case, would it be a better idea to use this 11.1v RC battery? Then I could wire all 3 colors in series, and 1300MAH would be plenty. –  nathan.f77 Sep 5 '13 at 3:24
    
@nathan.f77 quite correct dude –  Andy aka Sep 5 '13 at 8:46

protected by Kortuk Sep 3 '13 at 13:34

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