Instructables

Best way to light small bank of LEDs?

I was trying to help someone put together a small lighted prop for a Halloween costume, and quickly realized I knew enough just to be dangerous. 

The result should be simple, for someone who better knows the intricacies:

9 superbright 3V LEDs, arranged in a circular pattern about 4in dia., powered by a small pack of batteries which CAN be located elsewhere on the costume, with a small wire run.

Initially the thought was "oh, well, 2 AA's gets you the 3V, you're good..." Then we thought, "to make hooking up the LEDs simpler, attach them end to end, and then each end of the loop of LEDs goes to the two batteries." 

Of course, totally wrong.  After realizing the glaring problem, it occured to us that 3 9V batteries would instead get us the requried voltage to light them up.

Then, realizing again this should work but that there might be some finer points to know, I started looking up some information online for similar projects.  My big fears, having not tried this out yet, are that:

1) The string of LEDs or batteries may get hot. (Seen a lot of posts like this.)
2) There is some glaring inefficiency in this setup that could be more simply redone.

For example, is it better or worse in any way to run three strings of 3?  Would batteries last longer if a different configuration of batteries or wiring were used? 

dcenteno (author) 2 years ago
Thank you all for your input, it definately set me on the right path. The led array calc made things so much simpler! No guesswork as to the est way to arrange them that way.

My only thought now is why the 9V battery seems to universally be the least liked way to power LEDs.

Without getting too technical, is there an easy way to explain why say a bank of 4 AA would be better? Or maybe is there a web link that explains it simply? I've seen some allusions to how the power drain determines battery life etc, but not a concise explanation of what's really involved in this math. Any thoughts?
bwrussell2 years ago
Use a parallel/series array. Put the specs into this calculator and it will show you how to hook everything up. With one 9V you can put three parallel series (of three) LEDs with a 1 Ohm resistor on each series.
LED series parallel array 3x3.png
Yes, but they'll never be full brightness
Not sure if the 20mV loss on the resistors will be that noticeable. Any other arrangements will be quite wasteful.
Well, yeah, but the "3V" is likely to really be between 3 and 3.3 volts, unless you keep the LEDs chilled, so you can't develop the rated current in them from that supply.

I think you're really forced to a 2 + 2 + 2 + 2 +1 system, inelegant I know.
mrmerino2 years ago
You might try connecting them in parallel, or else arraying them. Maybe have three-by-three, so you have three times three volts equals nine, and you can connect a nine volt battery or two to provide the current.
you'll need to use resistors to run the leds. Do you know what their rated current is?
When working with LEDs you always need a current limiting resistor. Use this LED calc to help you figure out the best way to wire up the LEDS and what resistors you will need.

But you need to know the forward voltage and current of the LEDs. The more current the LEDs draw the faster you'll use up the batteries. And a 9V battery is not the best solution. They definitely won't last as long as a pack of 6 AA or AAA batteries would.