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Vol. 18: LED Light Brick

The Little Glowing Friend uses a single-chip microcontroller to drive 20 LEDs in a variety of patterns.

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+ Downloads & Extras:

2. MAKE THE MOLD

The easiest way is to just buy a pre-made mold like the MC-7 (see bill of materials for sources), which at 3"x5"x1-1/16" is a reasonable fit. I prefer to make my own mold for a better fit and a more interesting finished piece.

2a. First, make the mold master - the shape you want your brick to be. The circuit board is designed to fit nicely into a volume 3" by 4-1/4" by 1" to 2-1/2" deep, but you can make the master any size and shape that will accommodate the board. Remember that the power connector needs to be flush against 1 side of the finished casting.

Surface textures are good, as they scatter the light in interesting ways. Be aware that it's difficult to achieve a mirror-smooth finish like that of Lucite "deal tombstones" as they use a different process that involves grinding and polishing.

My master was made from sheets of plate glass cut to size because I liked the smooth top and the textured sides. You can make a master from almost anything that will work with the RTV molding compound. Modeling clay and wax are common materials - hand-formed, machined, carved, or anything else you can think of.

I made some additional mold tops using a dollhouse tin ceiling and some model bricks to get some different textures.

2b. Make a mold box. The mold box should surround the master by about 1/2" on each side and be watertight. You can make a mold box using Lego, which is easy to build and easy to take apart to get the finished mold out.

Be sure to pack the Lego bricks tightly to prevent leaks; you might need to apply blue tape if the liquid seeps from the seams. For a useful video on other ways to make a mold box, see makezine.com/18/lightbrick.

2c. Make the mold. Stick the master into the mold box using hot glue or another removable glue so that its top (the front face of the brick) is facing upward.

Mix the RTV molding compound according to the manufacturer's instructions, mixing slowly and carefully to avoid air bubbles. Pour the compound into the mold box, covering the master by about 1/2". Allow for the recommended time before removing the master.

3. CAST THE PIECE

WARNING: Casting resins need to be treated with respect and handled carefully. Please observe the following cautions:

• Use only epoxy, polyester, or urethane resin. Other "industrial" resins are significantly more dangerous to handle. Read and follow all manufacturer's instruction when handling these materials.
• Use resins and molding compounds in a well-ventilated area with good airflow away from you, preferably outdoors or in an open garage or porch.
• Dispose of unused compounds and resins carefully, preferably adding them to your hazmat pile. Do not pour down the drain.
• Wear protective eyewear when soldering, molding, or casting. Wear rubber gloves when molding and casting.

Obviously, the casting must be done upside down in the mold. The casting is done in 3 steps. First you'll pour the top of the brick. Once that has firmed up, you'll insert the board with the LEDs facedown, toward the top. Your final pour will fill the rest of the mold, embedding the circuit board in the piece.

Some dimensions: you need about 1/2" from the tops of the LEDs to clear the circuit board, and then another 1/4" from the bottom of the board to clear the tilt switch - for a total allowance of about 3/4" to embed the circuit board. So if you want the brick to be 2" tall, the first pour should be 1-1/4" and the second pour should be 3/4".

The MC-7 mold is only 1-1/16" deep, so the first pour into it should be no more than about 1/4".

3a. Pre-measure the pours. To determine the amount of resin needed for the first pour, fill the mold with water to the height needed for that pour (e.g., 1-1/4" for a 2" mold, or 1/4" for the MC-7). Then transfer the water to a new, clean paper cup. Mark the level on the outside of the cup. Then pour the water into a measuring cup and note the volume in ounces; you'll need this to measure the catalyst later. Repeat this using a new cup for the second pour. If your mold is asymmetrical, you just need to estimate or do some liquids judo.

WARNING: It is very important to thoroughly dry the mold and the paper cups once you've measured both pours. Resin does not like water.

3b. Place the mold on a piece of wood or other solid base, in a well-ventilated area where no debris or dust will fall on the hardening casting. Tape the mold to the base surface.

3c. Prepare the circuit board by cutting a neat rectangle of blue tape and covering the DC connector opening. Make sure the tilt switch is angled and does not extend more than 1/4" from the bottom of the board.

3d. Do the first pour. Carefully pour the resin into the first cup, pouring down the side to avoid creating air bubbles. Allow a little more resin than your measured line.

Add 5 drops of catalyst per ounce of resin, or whatever the manufacturer recommends for a first layer. It's better to use too little catalyst than too much, because excess catalyst will cause internal cracking due to overheating. While this shouldn't affect the circuitry, it does create visual flaws.

Slowly stir the mixture for about 60 seconds, scraping the sides and bottom to get a thorough mix. Avoid creating air bubbles.

Pour the mixture into the mold and let it sit. Cover the mold to protect it from dust and debris.

3e. Partially cure the first pour. You want the resin to be firm enough to hold the board without sinking, but not so firm that the board doesn't stick to it. Test the mixture periodically by gradually lifting the base to tilt the mold. If the resin holds its shape and doesn't run, then it's ready to support the board.

Setup time can vary widely due to resin, catalyst, and temperature, and can take anywhere from 15 minutes to many hours. It is better to firm up too much than too little, as you'll need firmness for the next steps. You can also accelerate the cure using lights (see Step 4a).

3f. Place the board with the LEDs facing downward onto the resin so that the DC power connector is flush against the edge of the mold. Do this carefully, and ideally only once.

3g. Do the second pour. Mix the resin and catalyst as per the first pour, but using only 4 drops of catalyst per ounce instead of 5 drops (or the manufacturer's recommended amount for a second layer).

Carefully tilt the mold and begin a slow pour from a bottom corner to avoid catching air bubbles under the circuit board. Gradually reduce the angle as you fill. When you have filled up to the level of the board, tilt the mold in different directions trying to chase out any remaining air bubbles.

Finish up by covering the board with another 1/4" of resin - enough to cover the tilt switch. Cover the mold to keep out any dust.

Adjust the final position of the board once you're done pouring. You can use your stir stick to make some alignment adjustments. Be careful not to pull the power connector away from the side of the mold.

3h. Wait.

3i. Wait some more. Don't touch it - you can leave permanent fingerprints.

4. FINISHING UP

4a. Don't get impatient and demold the piece too early, as this can ruin it. Follow the manufacturer's recommended demold times. And then some. It's best to leave it overnight or even longer.

Remove the brick when it is truly cured. It should be fully cooled and hard. Resin hardens from the inside out, so the surface is the last part to harden. This can take well over a day depending on the mix and conditions. Don't judge by time; demold only when the surface is hard and no longer tacky. Test hardness using a stick, not your finger.

You can speed the surface cure by warming the brick under some lights, but be careful not to overheat the casting or the mold. Don't leave the lights on overnight or unattended.

4b. Remove the tape from the power connector using hemostats. You may also need a knife if it's gotten coated over.

4c. Even though the brick is hard at this point, the finish is still fragile. It will pick up fingerprints and will pit with dust. It's best to handle it only by the edges. You may want to "tent" it under wax paper and continue the cure. Don't let the wax paper touch the surface, or it will leave marks.

Optionally, you can spray on a surface coat of resin at this point to protect the finish, but be aware that this may cloud the surface. Use sparingly.

4d. Apply the bumper feet to the bottom and you're done.