It's great for high traffic kitchens and It's fun for kids and adults alike.
Step 1: What You Need
-Super Shield conductive nickel paint
This can be found at electronics parts stores. It's usually used to add RF shielding to plastic cases. We will be using it because it's electrically conductive.
-1/4" copper tape used for circuit board repair (optional)
If conductive paint can't be found, this may be a possible substitute. It may be a good idea to get some anyway as a way to repair any future scratches or chips in the conductive paint.
I used Krylon Fusion For Plastic because it sticks to almost anything, doesn't require a primer and has a nice finish.
-10mm LEDs in quantities and colors of choice
I used 20 LEDs of each Red, Green, Blue, Yellow and White. These can be bought online.
-330 Ohm surface-mount resistors
Get one for each 2.4 Volt LED (Typically red, orange, yellow and sometimes green LEDs are 2.4 Volts). The 3.6 Volt LEDs (typically blue, white, UV and true green) do not require resistors.
-One 4.5 Volt, 500 milliamp AC power supply
By using AC, the polarity of the LEDs won't matter. They will light up whichever way they are played onto the grid. This also reduces power consumption because the LEDs will run at a 50% duty cycle.
-1/8" diameter x 1/16" NdFeB Nickel plated disc magnets
Get two for each LED. These can be found online.
-1/4" diameter x 1/16" NdFeB Nickel plated disc magnets
I used six - two for attaching the power source to the fridge, and four more for making magnetic jumper wires to bridge the gap between the door and the side of the fridge.
-5 minute epoxy
Get the kind that you mix from clear and yellow tubes.
-1/4" Quilter's tape
This is just masking tape but 1/4 inch wide, the thinnest tape I could find. You can find this in craft stores. Ideally, you want tape that is just slightly wider than the diameter of the magnets used on the LEDs.
-Small wire cutters or fingernail clippers
-Soldering iron or gun
-Wire wrapping tool or other tool with a flat round 1/8" diameter tip
It's really the 1/8" diameter we're going to use so you could use a grinded down dollar store screwdriver if that's what's available.
-The cap from a cheap pen
This is primarily for holding LEDs in place while you work on them.
Step 2: Paint The Fridge
Apply one even coat of the base paint. I used this base coat to cover any existing scratches on the surface of the fridge and also to help ensure that the conductive paint will not peel off.
Once the base coat is touch dry start applying the 1/4" masking tape (Quilter's tape) to form two seperate traces that will power the grid, and then apply all the traces in the grid itself. I used a marker to mark out the vertical spacing between the horizontal lines, then followed the dots as I placed the tape by hand.
The grid should basically look like two interlocked combs. One power line will go to the left side and make "teeth" going back to the right, and the other power line will start at the right side, making teeth go to the left. Neither of the two power lines should ever touch at any point. The grid is basically an open circuit and closed only when the magnetic LEDs will be placed upon it - one magnet of the LED touching one power line and the other magnet touching the other.
As soon as the masking tape is applied, start painting with the conductive paint. This stuff is nasty - you want to make sure that the area is well ventilated. Open all windows, doors and turn on the vent over the stove.
Keep applying more layers until the can is nearly empty. You may want to save a little bit just in case you ever need to touch up the conductive traces. If you are painting a larger surface area, you may consider using more than one can.
Step 3: Remove The Masking Tape
Be very careful pulling up the masking tape because the conductive paint is so thick that it may pull off along with the tape. I used an X-acto knife to score the edges in the corners of the fine traces to prevent the paint from being lifted off. Should any conductive traces start lifting, you may be able to press them back down if the paint is still not completely dry. After it is dry, you may try using Crazy glue to glue down any lifted corners.
Step 4: Hook Up The Power
There is a gap in the circuit between the door and the side panel of the fridge. To complete the circuit, I used two small jumper wires with magnets attached to both ends. Make sure to leave enough slack in the wires to cover the gap even when the fridge door is opened all the way.
Now the grid has power. Since the LEDs are not constructed yet, you may test the circuit simply by placing some magnets onto the traces of the grid and lying some LEDs over the magnets. The pins/wires on LEDs are attracted to the magnets, so no special effort is required for this test.
I measure 15 Ohms of resistance on each trace going from the back of the fridge to the grid, and about another 15 Ohms from the side of the grid to the center. When I place a jumper wire in the center of the grid, I measure about 60 Ohms between both power terminals at the back of the fridge - so a full circuit between the power supply, crossing any given point of the grid and back to the power supply is about 60 Ohms.
I found that the conductivity of the paint increased as the paint dried, so don't be too worried if you're getting readings higher than that at first. If you get something much higher or much lower, you may opt to use a different voltage power supply to compensate or use copper tape to reduce resistance in the traces.
Step 5: Assemble The 3.6 Volt LEDs (blue, white and green)
Once the wires are bent, use nail clippers or small wire cutters to cut off excess wire. You only need one looping of the wire to form a circle with each LED pin. Next, use needle-nose pliers to adjust the loop so that the inside of the circle is just a little bit smaller than the diameter of the magnets. This will add a bit of spring to the wires so that they clamp over the magnets when you push them in. (Don't add the magnets just yet.)
Before attaching the magnets to the LEDs I used modeling clay to hold the LEDs upside-down. Get your magnets ready. Mix up a small batch of epoxy. Apply a small bit of epoxy into the center of each circle of wire. Apply just enough for the magnets to hold without having the epoxy overflow over the bottom of the LED.
Now place magnets onto one side of each LED. They will most likely just jump onto the top edge of the circle of wire as the wires will attract the magnets. Once one row is place, use the cap from a cheap pen to push the magnets into the center of the circle and press down on them until they "click" into place. If they don't "click", or are too hard to push into the circle, you may need to adjust the diameter of the looped wire on your next batch of LEDs.
Add magnets to the second row of pins in the same way. If any epoxy gets onto the pen cap, just wipe it clean with a tissue. This will prevent covering the tops of the magnets with epoxy.
For the next minute or two, use the pen cap to level off the tops of the magnets. Then let them sit for 20 minutes or so. You do not want to be moving the magnets now because it will potentially break it's contact with the looped wire.
After 20 minutes, you can use a toothpick to carefully scrape any epoxy that may have covered the top surface of the magnets. Don't try placing the magnets onto your fridge until several hours have passed as the metal surface of the fridge will pull at the magnets and break the contact with the LED wires.
You can test your LEDs before the epoxy has completely hardened with a 3V battery attached to small wires - simply touch the wires to the magnets to see the LEDs light up. Try both ways because LEDs will only light up if power is hooked up in the correct polarity.
Step 6: Assemble Magnets To The 2.4 Volt LEDs (red and yellow)
Bend the longer wire of the LED to lie flat and point straight out. Next bend the shorter wire to lie flat around 40 degrees below the first wire. Bend the first wire to come back towards the LED. Make the bend overhang the edge of the LED a bit to make it easier to cut later. Next, use the wire wrapping tool to curl the bent back wire into a circle. Do the same with the next wire. Try to keep the loops evenly apart, with the edges of the circles going right up to the edges of the LED.
Use nail clippers or a wire cutter to cut off excess wire. You only need the wire to make one loop around. Use needle-nose pliers to tighten the inside diameter of the circles to be just slightly smaller than the diameter of the magnets so that the wires will clamp around the magnets when they are pushed into place later.
Place the LEDs in the modeling clay and use epoxy to attach the magnets just like with the 3.6 Volt LEDs.
When it comes to testing the LEDs to check that they light up, add an extra resistor to the battery because 2.4 Volt LEDs will burn out without a current limiting resistor.
Let the epoxy harden for about two hours, but do not place these LEDs onto your fridge because without the resistor (next step), they will burn out.
Step 7: Add Resistors To The 2.4 Volt LEDs (red and yellow)
Cut and bend the wires to make room for a surface mount resistor. The resistor should rest about halfway between the edge of the magnets and the edge of the LED itself. Use an X-acto blade or toothpick to bend the wires down a bit so that they will clamp down on the resistor once it is wedged underneath them.
Attach a small piece of double sided tape to the end of your blade. This will make it easy to pick up and position the tiny resistors.
Now it's time to solder in the resistors. This takes a delicate touch. Make sure to clean the soldering gun's tip often and lightly touch the points you want to solder with both solder and pre-heated soldering gun. If the solder doesn't "take" the first time, wait until the wires cool and try again. Make sure not to touch the magnets with the soldering gun. The magnets will break down if exposed to excessive heat. If you accidentally solder a bridge over both wires, use the toothpick to push apart the solder as you reheat it.
For those who are unsure of their soldering ability, it might be possible to omit the soldering step altogether if you make sure the wires firmly clamp down on the resistors. Test the LEDs to make sure they're working and then put a dab of epoxy over the resistor to keep it from moving out of it's current (working) position.
Repeat steps 5, 6 and 7 until all the LEDs are assembled.
Step 8: The Finished Project
When not used in the grid, the magnetic LEDs can also be used as regular refrigerator magnets - although they won't light up unless they're placed in the grid.