I was searching for a good Arduino project for my end-year hollidays. But what to do? My little daughter was very surprised when I offered her this "electronical" necklace, and also very happy. I hope that the person to whom you will offer your achievement will also be very happy.

The jewel itself consists of a Micro Controller, and an RGB LED that has the same dimensions.
The necklace consists of a very thin brass wire, that can be soldered easily with small tin wire. The power feeding is a simple 3V coin cell lithium battery. I used a small sheet of adhesive paper, found in my own home pharmacy, to protect and isolate the battery pack.

Step 1: Tools & Materials

Tools

soldering iron, 0.5mm tin soldering wire
a magnifier, because wires to solder are so small
a computer with Arduino software installed
an ISP programmer, as explained here
a small wires cutter

Materials

a CR2032 battery with its battery basement (consisting of two parts, one for each pole)
very thin brass wire
one RGB LED in a 5050 package, with a WS2812B chip inside (this is important, because you can find 5050 LED without the WS2812B controller inside)
a small piece of medical adhesive paper
an Atmel Attiny85-20SU SMD Micro Controller Unit
a cheap simple necklace

Step 2: Electronic Schematic

The electronic schematic is very straightforward, because there is no passive components, like resistors, capacitors, or inductances, and because there is only 3 components, including the battery.

The power pack I have used is a 3V CR2032 lithium battery. Its voltage is lower than the one mentionned in the WS2812B datasheet, but after testing, the RGB LED did not suffer from this 2V drop.

The fact to be able to use a simple 3V coin cell battery was a very important condition for me to make this project alive. We cannot imagine a necklace with a big heavy battery pack as power source.

The Micro Controller Unit (MCU) is also working very well with this 3V voltage level.

I measured a mean current of 5.3 mA. Such CR2032 lithium battery has a typical capacity of 200 mAh. This means that, with a brand new battery, you could let the system on for 40 hours. But, even the half would be largely enough for a common use.

WS2812B.pdf
Download

Step 3: The Software

The Micro Controller Unit is an ATTINY85 (~$1) from Atmel. I programmed it with a cheap Arduino Nano (a clone found on ebay for about $5). But if you own a genuine Arduino board, you can use it for that as well.

The Arduino Nano has been programmed with the "Arduino as ISP" sketch.

The sketch to program into the ATTINY85 Micro Controller is given as attachment on this step: JeweLED.ino

Beware that you must burn the bootloader for the MCU to be fully programmed. This actually does not flash the Arduino bootloader on the MCU, but flashes some important configurations fuses. Without doing this, the sketch won't run at all.

The type of board to choose must be: "Attiny85 @ 8MHz (internal oscillator, BOD disabled).

BOD stands for Brown-Out Detect. This is a special feature that shuts down the MCU when the power goes under 4.3V. This is useful to avoid damaging rechargeable battery packs. But in our case, it has to be disabled, because we are going to power our MCU with only 3V, and even less.

JeweLED.ino
Download

Step 4: Assembling

The first step is to assemble the MCU with the LED.

Once programmed, only pin 4, 5 and 8 of the Atmel MCU must be kept. The other pins can be removed, because unnecessary.

Pin 4 of MCU must be soldered with pin 3 of 5050 package. This will be connected to negative pole of the battery.

Pin 8 of MCU must be soldered with pin 1 of 5050 package. This will be connected to positive pole of the battery.

Pin 5 of MCU must be soldered with pin 4 of 5050 package. Pin 5 correspond to PIN0 of Arduino for this type of MCU.

Use the medical adhesive paper to isolate the coin cell battery from the skin. This allows you to fix the negative part of the brass wire to the negative pole of the battery.

There is no power switch on this mounting. To shuts-off the LED, you must open the necklace, by pulling the negative wire out of the battery pack.

And that's all.

Step 5: Testing & Tuning

As you can see on the close-up picture, I have soldered two very small rings of brass wire on the GND and VDD pins. The purpose of this is to attach this "electronical" jewel to the necklace.

For first testing, I used only the brass wire as necklace. The brass wire is necessary to ensure electrical contacts, but is not enough. The brass wire is too light in weight, and the battery behind the neck is too heavy compared to the LED on the front. So I had to use a real necklace for the battery to stay in place.

You have to separate the necklace into two parts of even length, and close those two parts onto the jewel rings.

I twined the brass wire into each loop of the necklace. The wire is almost invisible, and ensure the electrical conduction as well as the rigidity of the whole construction.

Another way to make the electrical conduction would be to use stainless conductive thread, that you can find on Adafruit for some dollars.

On the video, you can see the JeweLED in action.

Enjoy!

See it in action