[Noisebridge-discuss] LED Calculations help

miloh froggytoad at gmail.com
Mon May 21 13:25:23 UTC 2012

Hi Phil,

I'm forwarding your question with responses threaded and a
question/challenge to sfanalogcircuits.


On Sun, May 20, 2012 at 10:16 PM, Phil Spitler <phil at bonfirelabs.com> wrote:
> Hi,
> For simple LED projects, I usually use an online LED calculator to get the
> values for the needed resistors.
> I have just ordered some LEDs online but the specifications don't list the
> forward voltage, just the current draw.

These aren't just leds. They have a little analog circuit in them
which switches the red, green or blue led to flash through a sequence.
 They aren't harder to work with because of this, they just are a bit
different than 'raw' leds.  Its probably worth understanding what
makes them different.

> http://www.adafruit.com/products/679

Like they wrote on the site you linked, you could use "3.4VDC" supply
for your lights (hookup in parallel) that would just have to source
enough current to turn them all on.

> How would I figure out how to power the LEDs and what resistors to use?
> I am thinking of using 10 LEDs and having them all powered, all the time.
> I would love to know how to calculate my needs.
> Any ideas?

Make some observations at Noisebridge with the constant current
feature on the Thandar power supply. Apparently, the optical
characteristics of led's (wavelength, brightness, light cone angle,
and packaging material, to name a few) vary enough in batches to make
this method very compelling over a resistance calculator or analytic

Phil,  you might be able to buy a spare Lumbrella board from Rolf (at
noisebridge for chm by arrangement) and wire them all in series, then
you won't need resistors and you might be able to run about 7 of the
leds in a simple series circuit. His board uses a constant current
driver that you can also find elsewhere.

So sfanalogcircuits, you're up: I know you've all seen these rgb leds.
Tell me what's that's the simplest circuit you would come up with for
cycling the three leds in a predictable order and brightness. Now, can
you tell us how that circuit get connected with the rgb leds into
packaging for a product?

Also, are there similar ic-on board smd versions of the flashing rgb
leds yet? I remember Meredith asked for this about a year or two ago
on sfmicrocontroller or another local list, it would be quite popular
I imagine.


anyone down for circuit hacking monday tonight?

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