Light Up Earrings

I made them out of clay

With the boards made, I moved to the housing.  My plan for these earrings was to assemble the case using typical methods for crafting jewelry.  Namely, Sculpey.

Sculpey or “polymer clay” is a synthetic clay product that can be cured in a conventional oven without the need of a special kiln.  I really liked it as an option for a number of reasons:

  • It doesn’t shrink that much when it’s cured meaning that I can size it very precisely while it’s still soft.
  • It can be easily cut and drilled after it’s cured without cracking or shattering.
  • It’s translucent.

This last bullet came as something of a surprise to me.  The original plan for the earrings was to make the case out of Sculpey and then provide some kind of semi-translucent window for the LEDs on the front.  It took me a while to come up with the phrase “flat glass gems” (in case you were wondering what these are called) which I thought were the perfect candidate for my window until I realized how heavy they are.

I decided to start playing with the Sculpey before heading to the craft store.  I made a bead of Sculpey and cut a cube out of the center using an X-acto knife:

The plan was to drop the circuit in like this:

But I immediately noticed how well the light traveled through the translucent Sculpey and decided to turn it around:

Oooooooo….

In addition to alleviating my window problem, flipping the board around added a new design element known as “dead fronting”.  This is a technique often used in old VCR clock displays and some more modern gadgets like the Nike FuelBand. The idea is that the display is there when you want it, but you can’t see any trace of it when it’s turned off.  It’s a cool way of hiding the technology away from the user.

This is especially cool for my design because people aren’t expecting a circuit inside a pair of earrings, so I wanted to make them as innocuous as possible when switched off.

Happy with the shape, I baked the clay for 20-30 minutes at 275F (the website recommends 15 minutes for every 1/4″ of clay).  To tweak the aesthetic and lighten the load a bit, I used an X-acto knife to cut around the sphere making it look a little more like a cube with rounded edges.

Next up was the charging ports.  I’ll go into details later, but I needed two electrical connections to the outside world to enable charging.  For one of these, I used a small loop of wire stopped off with a blob of solder and threaded through a small hole that I drilled in the top.  This doubled as a loop for the ear hook.

For the bottom contact, I cut the head off a screw with my Dremel tool and soldered a wire to it:

I then threaded the wire through a hole in the bottom of the earring and glued the screw head in place.  A small amount of countersinking around the hole helped the screw head sit evenly.

The advantage of the screw head is that it’s ferromagnetic, so a charging wire can connect to it with a magnet.  The top loop can be grabbed by an alligator clip.

The end result looked like this:

With that out of the way, I tucked in my little circuit and soldered in the appropriate connections to the back of the PCB.

I then made a matching door which was baked separately and finagled into place by cutting around the edges until it fit:

Despite my best efforts, it still wasn’t a perfect fit, so I held it in place with some super glue and sealed around the edges with extra Sculpey:

I then baked the Sculpey in place using my hot air gun set on 275F.  Although this hardened the Sculpey, it didn’t do a very good job of hiding the door.  The convection of the hot air gun heated the Sculpey too quickly and unevenly.  You can see what I mean in the picture below:

I would have loved to throw the whole thing in the oven and bake it all at once, but sadly 275F is just a little too toasty for the battery.

Curious about how much they weighed, I threw them on a scale:

I don’t have pierced ears, but a quick Google search indicated that 5 grams isn’t unbearably heavy for a pair of fancy earrings.

23 thoughts on “Light Up Earrings

  1. Really nice project! Now get these 3D printed and start selling them on Tindie 🙂
    I still shy away from using LiPos for the reasons you mentioned. Maybe I should give them a try as well.

    A few comments regarding your circuit:
    – when measuring the current draw from your circuit, did you account for the voltage divider (I calculated about 8.5 uA)?
    – you don’t actually need a voltage divider at all! Using the ADC mux connect VCC as reference and the 1.1 V bandgap as input. From that measurement you can calculate VCC.
    – almost did not see this: so you pull low PB0, right? What about the current flowing through R12?
    – when in power-down mode you can still wake up the device with the Watchdog interrupt (see http://www.avrfreaks.net/index.php?name=PNphpBB2&file=viewtopic&t=135288). That way you can configure your MCU to only wake up every 8 s.

    • Great comments!

      -The voltage divider certainly contributed to the 0.45mA draw. Also that measurement was taken before I added R12 (I added a note to make this clear).

      -That ADC mux trick is brilliant. I’ll definitely use that next time.

      -Someone else suggested the WDT as well. I’ve never used one before. I always assumed it ran off the same clock as the application processor (or at least that it drew more current than that). I’ll definitely try that out next time.

  2. Your break off programmer has inspired me to use that design in a wearable project idea that I have. Awesome and Thank You!

  3. Nice project 🙂

    One idea that could help storing the earrings in a charged state would be to use a small reed switch between the battery and the rest of the circuit (while keeping the connection to the charger).
    I think you could use it to simply cut the power mechanically instead of having a sensor+code to manage the case.
    For over-discharge protection when it’s in its box, you would only have the self-discharge effect to worry about, and that would not be a problem as long as the earrings are used at least once every few month.

  4. Yeah, using the 1.1vref for voltage detection works out all right – it’s not 100% accurate, but if you set your threshold high enough, you don’t risk getting near the bottom end of the discharge curve for the LiPo cell. I’ve been fiddling around with adding navigation lights to my RC helicopters now that the sun is setting so early in the evening, and having the lights warn me before the low voltage cutout in the model kicks in is a design goal, and it’s within a few dozen mV of my goal voltage.

    (viz: http://jeelabs.org/2012/05/12/improved-vcc-measurement/ although I’m using a different method.)

    And white Sculpey is translucent, huh? I’ll have to give my blinkenlights a shot with it. Right now I’m using some HDPE medicine bottles to diffuse the LEDs a bit, but it’s a bit weird looking. Pressing them into Sculpey and baking it up might look a _bit_ nicer…

    • It’s actually marked as “translucent” Sculpey. I think the white stuff is different.

      Neat project. I really like the coulomb graph.

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  9. do you have a schematic for the charging circuit? Been throwing around an idea that this battery would work well in.
    – K-tWizel

    • Page 4 has the schematic for the charger. You can also download the .zip file at the bottom of page 5 which contains the Eagle schematic files for the earrings and the charger.

  10. Have some master models of the parts 3D printed. Smooth up the surfaces as needed with some primer and sanding. Then make RTV silicone molds for casting in urethane resin.

    Smooth-On has a fairly quick setting white that when cast thin enough, light can shine through.

    You could make a multiple cavity mold with only one set of 3D printed parts by first making a small mold to cast one set, then cast several sets from that mold to make one or more multi-cavity molds.

    Shrinkage of the resin, especially with small sized items like these, is next thing to zero.

  11. This is an awesome project. I’m also doing really small circuits as well. I’m sure you get this a lot but I need help finding a component. I need something that will detect voltage on a battery to about 2.5 volts. Then after hitting 2.5 volts it will direct the power somewhere until the voltage drops back down to about 1.7 volts. Or something similar to that. I just need to find out what kind of component that would be so I can see what there is to offer. thank you

    • I don’t know of a specific part that can do that. There may be a number of battery management ICs that can be configured to do something like that though. What kind of battery are you dealing with? What’s your application?

      • Well I’m actually dealing with a capacitor but treating it like a battery, just for energy storage. The application is a small energy harvester that takes in small amounts of energy over a period of about a minute and then dumps that collected energy into a motor or an LED. I found a part that could do the job but I’m going to test it in a few days to see if it’ll work. The LM8364 seems like it’ll work. What do you think? http://www.ti.com/general/docs/lit/getliterature.tsp?literatureNumber=snvs232b&fileType=pdf

        • Well, that part will certainly detect when the voltage drops below a threshold, but I still don’t quite understand your application. Will this connect to your charging circuit?

          • Sorry for the long reply. Basically the circuit has a small solar panel* made up of photo diodes. I’m getting a little over 3 volts in normal room light from the solar panel. In parallel with the solar panel I have a 1200uF capacitor as a battery to store the electrical energy made from the solar panel. I want that stored energy dumped into an LED or a small motor once the voltage in the capacitor gets to around 2 volts. I just finished building the circuit just now and it doesn’t work with the LM8364. But the circuit did show that the small solar panel and the capacitor could power an LED for a few msec. Then, i could have also heated the LM8364 to high when I was soldering it. I’ll test the component now to see if it actually functions.

          • So after testing the LM8364 I figured out the problem. The component basically detects whether there is enough power to run a processor or not. And in return tells the processor to shut down or turn on. For every failure I learn more about what I’m doing.

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  13. Really good design work, unfortunately the Chinese have unlimited “research” funds
    and dirt cheap parts and labor. They are selling beautiful LED earrings on the web for 99 cents a pair. Including shipping!!! Sorry Guys!!!

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