LED Fireflies

LED Fireflies that pulse at dusk

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A couple of years ago I made a set of LED firefly's for my girlfriend's valentines present. They were little free-form circuits built around an attiny and a couple of small, flexible solar panels. These worked great for a few weeks, but then the batteries ran down and I discovered that the panels weren't able to charge them up high enough to light the LED (That's what you get for building things at the last minute and not testing them!).

I've finally got around to redesigning these (hopefully working this time). I've gone for surface mount components on a small PCB which gives them a much tidier aesthetic. I've also added a boost converter so that I can eek a bit more usable juice out of the battery before it drops below the LED's forward voltage.

I'm still not convinced that the battery will charge, I've blindly thrown it in because it looks nice and is about the right size. I get the impression that NiMHs don't really do trickle charging so my naive approach of sticking the output of the solar cells directly onto it's terminals may not work. I'll probably experiment with swapping them out for a super capacitor if I can find one that can store enough charge and is small enough.

  • 1 × TI REG710NA-5/250G4 DC-DC Converter able to boost voltages as low as 1.8V up to 5V
  • 2 × PowerFilm SP3-12 Small flexible solar cells
  • 1 × ATTINY45-20SU The brains of the operation... Chosen because I have loads of them!
  • 1 × CREE C503B-BCN-CV0Z0461 Bright, blue LED
  • 1 × VARTA 55602303015 Small rechargable NiMH cell. I'm not sure I've made the right choice with this but it's tricky finding rechargeable cells this size

View all 12 components

  • Oops...

    David12/03/2015 at 23:28 0 comments

    It's time to admit that I made a rather embarrassing mistake. I was just about to order my revised PCBs from OSH Park, when I spotted some text that made the blood rush to my face:

    Detected 2 layer board of 1.54x0.71 inches (39x18mm). $5.45 for three.

    It wasn't a hot flush from the excitement of spotting a deal, it was a blush from the realisation that my board doesn't actually fit in one square inch when on the diagonal...

    As a result I'm sadly going to have to withdraw from the square inch project. Whilst the prizes looked great, none of them can compete with the prize I stand to win when I complete this project: my girlfriend's heart (or at least an end to the annoyed looks she gives me when I ignore her because I'm working on the valentines present I promised her 3 years ago!).

    Anyway... Lesson learned: Actually measure stuff rather than just assuming it fits because it's small!!!

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alpha_ninja wrote 12/07/2015 at 01:05 point

[verified: no design files missing]

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alpha_ninja wrote 12/02/2015 at 00:44 point
This is your one-week reminder to upload design documents:

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Klima wrote 11/11/2015 at 20:24 point

What about energy harvesting ICs like BQ25504 or similar?

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David wrote 11/12/2015 at 11:20 point

I've had a look at them for other projects before. They would be absolutely perfect for this if it weren't for the fact that I'm intimidated by the size of the package and the number of external components required! I probably won't switch to using one in this iteration of the project because I'm not confident I could actually solder it but I might have a go on a future revision once I've got a reflow set up going.

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Klima wrote 11/12/2015 at 20:55 point

Do not be afraid. If you have a hotair station, that is all you need. The parts are, as I believe, quite forgiving.

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DENIS Guillaume wrote 11/04/2015 at 17:24 point

Brilliant project, I do love the idea.

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Steven Gann wrote 11/04/2015 at 14:24 point

This looks brilliant. Simple, elegant project.

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deʃhipu wrote 11/04/2015 at 13:58 point

Have you considered to use a capacitor instead? That may be smaller, but more robust?

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David wrote 11/04/2015 at 15:29 point

Yes :)   I think it may be the best option as long as I can find something that can store enough charge and is small enough to not ruin the way it looks. 

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davedarko wrote 11/04/2015 at 09:27 point

Awesome design! Your link is leading to a login form :( Do you have a prototype ready yet? 

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David wrote 11/04/2015 at 12:27 point

Yeah, I'm sorry about that... I decided to use Altium CircuitMaker for this project just to see what it was like. Unfortunately it means that everything is in their cloud and you need to register to be able to look at projects made in it. I'll put a PDF of the schematic and the Gerbers on here at some point though. I do have a prototype built and I'll stick some photos up when I have time. I'm planning to write a few project logs over the next couple of weeks detailing the design process which will have more information.

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davedarko wrote 11/04/2015 at 12:49 point

awesome :)

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