I've been thinking about entering the LED contest but couldn't think of an entry. I have a major problem when it comes to the LED contest... My electronics skills are fairly rudimentary. I know how to use a soldering iron and I can identify the basic components but don't ask me to design a circuit or, for the most part, understand how to modify circuits. I was looking over the open contests and when I was checking out the Jewelry contest, the idea for making a pendant with LEDs popped into my mind. Instead of just putting LEDs into a design on the pendant, I thought it might be more interesting to make the circuit itself part of the design. Thus was born this instructable for a flashing LED pendant.

A couple notes before beginning:
1. The knowledge and items in this instructable are fairly basic. This is a limit of my current skills and tools/equipment on hand. On the upside, this means that anyone with basic skills and equipment can make the pendant.
2. I don't work on jewelry either by trade or hobby so my skills with the decorative wirework is poor at best. Hopefully, anyone making use of this tutorial will have better skills than me in this regard.

Step 1: Parts and Equipment

I've broken down the items needed into a few sections: the circuit, the pendant, and decorative touches.

Here's what you'll need for the circuit:
2x - 1K Resistor (1/4W)
1x - 1µF Electrolytic Capacitor (63V)
1x - 470K Resistor (1/4W)
4x - LED (Red)
1x - NE555 Bipolar Timer
Power source (not shown)
Switch (not shown)
Soldering iron
Solder
Wire
Hacksaw (not shown)

Note that a 1µF 50V capacitor also works. However, at the electronics store I purchased parts from, the 50V capacitor was black and the 63V one was blue. The resistors were also blue so I went with the 63V capacitor for aesthetic reasons. The colour of the LEDs should also be chosen according to your aesthetic tastes. I didn't list a specific power source or switch here. These parts can be worked into the design or hidden so the exact parts used can be varied according to your needs. I've tested the circuit and confirmed that it works with a 6V battery (one designed for headlamps), a standard 9V battery, and a 12V combo (four 3V batteries).

For creating the pendant, you'll need the following:

1x - Casting mold(s)
1x - Resin (polyester resin is shown but epoxy resin is probably a better choice)
1x - Catalyst
1x - Mixing cup
- Resin colouring (Optional)
1x - Mixing stick (not shown - I used a disposable wooden chopstick)
Craft foam
Glue gun

For decorative touches, I used the following:

18 gauge aluminum jewelry wire
Bent nose pliers
Side cutter
Scotch tape (not shown)
Heat shrink tubing, 16mm diameter (not shown)
Heat gun (not shown)

Step 2: Prepare the Circuit Board

Using the mold as a guide, mark off the desired size and use the hacksaw to cut to size. The size shown marked off is 13x10 (in terms of available holes). This was cut down to 12x9. I had two problems with the 13x10 circuit board. The first problem was the board needed to be 13x9 to fit within the mold. The second problem was purely aesthetic - I wanted the 555 timer to be dead center and a width of 13 was slightly off-center so I cut the width down to 12.

After cutting the board, sand the edges and corners to remove anything that might result in injury.

Step 3: Add the Circuit Components

The circuit is based on the one at http://www.555-timer-circuits.com/flashing-led.html. The only significant change I've made is to add LEDs (in parallel). Add the circuit components to the board and solder in place. Add the smallest (in terms of height from the board) components first. With the components I had, that meant the 1K resistors first, then the 555 timer, then the capacitor, and finally the LEDs. The long prong of the LEDs goes on the outside track.

Important note: Do not trim (or leave a fair bit after trimming) after soldering the components in place - we'll need to attach wires to the protruding metal.
Also note the layout in the first image is the back of the board (i.e. what you'll be seeing when soldering).

Step 4: Add the Wiring

This will by far be the most annoying and delicate part of the operation. It'll take a lot of patience, delicate work, and time. We need to create and connect the wires for the circuit. The first image shows a rough diagram of the wiring I planned to use. I started by creating the correct length wires to attach the LEDs. The ends of the wires are bent like hooks to snag onto the ends of the circuit components (which is why they should not be trimmed yet). The other wires I created as I went. Once the wires are all soldered in place, test the circuit and make sure it works. If it works, trim the protruding wires.

The main annoyances at this stage were: 1. The time it took to create wires of the correct size and 2. when soldering the wires, the other end often heated up enough to melt the solder and loosen up. There are that might have helped draw the heat before it traveled further down the wire but I didn't have any on hand.

Step 5: Embed in Resin

Use the measuring cup to mix up 1 ounce of resin, following the instructions for the chosen resin. I added 2 ounces of resin to the cup (which was overkill - for a single pendant, it'll require less than 1 ounce). Then I added in the transparent dye (8 drops). The colour shown is actually darker than than the final result - the depth of the resin in the cup (almost 1") was a lot higher than the depth of the mold (3/8") so keep this in mind when checking the colour. I made sure the colour was mixed in (using slow strokes to avoid creating too many air bubbles).

Then I added the catalyst and mixed it in. Pour the mixture into the mold and add the circuit. When hardened, the resin will shrink so make sure the mold is as full as it can be. You may want to use a syringe (the ones you find at art shops, not medical syringes) to top off the mixture. If you do, make sure it's disposable, as it'll probably be clogged with resin afterwards. You'll probably need to add a weight on top to keep the circuit in place (the wires hanging off the side tended to drag that end down, lifting the other end. A 9V battery works well for this.

Put the mold in a well ventilated place and let it cure. The curing time is dependent on the resin you use, ambient temperature, the thickness of the cast, and the amount of catalyst that was added. Typically, you should let it sit for a few days before removing it from the mold.

IMPORTANT: Polyester resin produces harmful fumes. If possible, let it cure outdoors so you are not exposed to the fumes. Failing that, place it in a room with a ventilation fan that pumps the air outside. Epoxy resin is supposed to be much less toxic in terms of fumes so it is probably a much better choice. The only reason I used polyester resin was I had some on hand. If you don't have either, look into epoxy resin.

Step 6: Add the Backing

Cut out small section of the craft foam. Trim the corners if desired. Position the wires going out to the power supply as desired. Using a glue gun, drop copious amounts of hot glue onto the back of the circuit. Press the craft foam into the glue and let dry. I suggest using a lot of glue because that will help hold the wires in place and reduce the risk of wires snapping off at the solder joints. Once the glue sets, remove the pendant from the mold.

Step 7: Add a Wire Frame

Adding a wire frame serves two purposes: 1. Additional decorative touches and 2. creates a way to attach the pendant to chains, backings, etc. I decided on a simple wire frame that would include a couple loops to allow attaching a chain. Using the jewelry wire, create a loop. I held the wire near one end with the bent nose pliers, twisted the short end around the end of the pliers, then wrapped the short end around the wire. Then I twisted the wire around the pendant and created another loop. As mentioned before, my jewelry skills are poor at best so with skill or time and patience, you'll be able to do a much better job than what you see here.

I then attached the chain and looped the power wires through the chain (every 5th link). This configuration assumes that the power source will be attached at the back, where it's out of sight. To attach the chain, I first detached a couple links. To do this, I closed the bent nose pliers and inserted the end into the final link in the chain. I pressed my fingertip to the end of the pliers, essentially pressing the link down onto the pliers. Then, I slowly opened the pliers. This pushed the chain link open. Repeat and you'll have two open links. Attach the chain to the loops on the pendant with the open links and use pliers to close the links.

Step 8: Add a Power Source

I decided to add the power source to the pendant rather than have it hidden around the back. To accomplish this, I used 4 small 3V batteries similar to watch batteries. First I prepared the wires by stripping the ends and forming loops with the exposed wire. Next, I stacked the batteries (all with the same orientation) and taped the side to keep them in place. I then made a couple cuts in the tape (one for each wire) to allow the wire through and placed red wire loop touching the top of the battery stack and the black wire loop touching the base. I then folded down the tape to keep both loops in place. Test that the battery stack works with the circuit. To improve the look of this configuration, I cut a piece of heat shrink, fed the wire/battery combo through it until the batteries were centered in the strip. Quick work with the heat gun and the batteries were firmly in place. Check the circuit using the power source just to double-check. The ends were then cut to match.

Step 9: Attach the Power Source

I removed the wires from the chain and pushed them under some of the wire frame to keep them in place. I then used additional jewelry wire to attach the power source to the pendant. A bit of soldering to connect the wires and add a switch, and the pendant was done.

Step 10: Additional Thoughts

The wire work in step 3 can be greatly reduced if you have or have access to circuit board etching equipment.
Heat shrink tubing comes in different colours. I recall seeing black, white, green, red, and yellow. I'm sure there were other colours as well. I'm not sure how receptive it is to paint.
One thing I pondered was cutting a hole in the heat shrink to expose the center of the battery top (which was one reason to use a wire loop) but I didn't have anything that would give me a clean circle so that idea was abandoned.
I have all the components flush against the board. However, they can be pushed higher if you want them closer to the front of the pendant.
Some additional ideas:
- Add the LEDs after the resin step. The LEDs can be soldered onto the board after the resin has cured. This will allow the LEDs to protrude from edge of the pendant.
- Break up the circuit into several different components, embedding each in its own resin mold, connected via wires. This is useful for larger circuits or if you want multiple small pieces.

Edited to add schematics. These schematics are assuming you're creating a 2 layer board. These are based on the wiring in step 4 (photo 2). I've added a couple spots for attaching wires for power (labeled with "+" and "-"). If these are used, the wires will be visible in the resin. If you don't use these holes, just attach the power wires to the back of the board using the leads of the components as done in the instructable.

NOTE: These schematics are untested. I have not created boards using these schematics so I can't guarantee they'll work.