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Power Budgets Suck

A project log for Trinket Timer

A small pocket timer for anything. Reminders, countdowns, etc. Great for kids when you don't want to put a $200 iPod in their hands.

KorishevKorishev 12/04/2014 at 05:0311 Comments

So I originally wanted to go heavy on some RGB LEDs on the timer instead of some kind of LCD display (character or graphical). I wanted it to be functional, but have some kind of colorful fun factor. I was worried about the power requirements of a bunch of LEDs, and for good reason.

I've been looking into using some 8mm diffuse NeoPixels from Adafruit, but the power rating on them maxes out at 60mA at full brightness. The 15 I wanted to use adds up to 900mA if all are on a full brightness. That's almost twice what I can draw from the LiPo Boost converter I'm using!

Now, I'm probably not going to run all of the LEDs at full brightness at the same time, but I'm probably closer than I'd like to be.

So I have to do something. But I have two options: reduce power drain, or increase power supply. Decisions, decisions.

Discussions

zakqwy wrote 12/04/2014 at 13:01 point
15 NeoPixels would be difficult to look at, they're crazy bright! Maybe just plan to run them at 10% brightness or something?

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Korishev wrote 12/04/2014 at 13:03 point
Yeah, these are the individual 8mm "diffuse" ones, so they aren't as bright at the "regular" NeoPixels. I also want them to be "daylight" bright, but that may be too much too. I don't have them in hand yet, so I don't really know the difference in brightness levels.
It may be that I can run them at 10%, and be totally fine. More to follow :)

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Stryker295 wrote 12/04/2014 at 16:50 point
If you're using them in daylight, even 25-50% brightness on the diffuse ones are more than enough! I have the opportunity to play with both—though I own neither—and they're simply blinding.

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Korishev wrote 12/04/2014 at 17:17 point
Thanks for the info! I should get them in my hot little hands today, so I'll see if I can toss up a video with a breadboard soon. This certainly sounds like good news!

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davedarko wrote 12/04/2014 at 08:41 point
you still want to look at them, I guess? You'll probably don't want do have them shine that bright anyway... and making them work as a display will probably give you a "one-at-a-time" scanning so your numbers will reduce accordingly :) Red is always good for the last 5%

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davedarko wrote 12/04/2014 at 08:41 point
and I totally forgot you use neopixels... nevermind the scanning thing.

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Korishev wrote 12/04/2014 at 13:09 point
yeah, the trade off is one data pin vs. 15 pins. Multi/Charlie-plexing would reduce the power draw, but with NeoPixels they kind of run themselves. If I went with single color (red or blue or whatever), I would need to have some kind of multiplexer added in to reduce the number of pins needed from the trinket. Always a trade-off, huh?

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davedarko wrote 12/04/2014 at 15:18 point
45 pins with 15 rgb leds ... would be quite a challenge to charlie-plex them ;) 8pins.. enough on a trinket, but a pain to wire.

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Korishev wrote 12/04/2014 at 15:30 point
yeah, if I had to drop the NeoPixels (still an option), I'd probably skip the RGB and default to a single color. That would reduce the pin count and the power draw considerably. In my head, lighting an rgb led roughly equals lighting three leds and blending the colors, so dropping from rgb to single color leds would potentially cut up to 2/3 of the power requirement in that step alone. I'd still need to address 15 leds instead of one daisy chained strip, but it would be easier to wire than the equivalent of 45 leds..

Of course, I really don't *know* what I'm doing, so this is all conjecture :)

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PointyOintment wrote 12/04/2014 at 05:44 point
15 RGB LEDs? How would you use those to display timer info?

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Korishev wrote 12/04/2014 at 13:05 point
ahh, that's the secret! Just kidding, I've got a bit of an sketch I'll post to the log ASAP. That might help visualize what I was thinking about.

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