Refining Edge-Lit Holiday Cards

We've picked up a bunch of improvements on our Edge-Lit Holiday Cards since last year and we've collected them here for you to see. (Also, welcome Popular Science readers! This project is mentioned in the December 2009 issue.)

We've settled on 1/16" PETG polyester for this project. It is the right thickness for use with a 3 mm LED, and not so thick as to be unreasonable for a card. The polyester has a couple of advantages over the acrylic we used last year. It is soft enough to be indented with a push pin and is still optically clear. It is also inexpensive. (Part 85815K11 at McMaster-Carr is only $2 for a square foot.)

With care, it can be cut by hand with heavy duty scissors into rectangles about 2 1/2 to 3" on each side, which will fit into a card made by folding a letter sized sheet of paper (heavy weight paper or cardstock is best) in quarters.

Polyester sheets typically ship with a protective film on both sides (you can see that it makes the plastic look opaque in the photo above). You'll want to peel it off before you get started with your design.

We previously used a hobby knife to carve the plastic, but a pushpin can be used to make just about any pattern. Not that we've stopped liking hobby knives, but pushpins do work well and might be more suitable for kids who want to do this project.

You can work from a hand-drawn design, or something computer generated. When we released SymmetriSketch this summer, we also updated our Vector Snowflake Application, which is a great way to create a card design. Just print out your snowflake at the right scale and tape it to your plastic.

Press your pin in repeatedly around the edge of your pattern. You can flip it over to check and make sure your marks are getting through.

Once your pattern is complete, peel off your paper pattern. Next you'll need to tape over the edges of the plastic with electrical tape to prevent light leaks.

A piece of black construction paper behind the plastic will help provide contrast when the card is lit. The scratched side of the plastic should be against the black paper, and the smooth side toward the viewer.

You'll need a window in your card paper as well, just a bit bigger than your design, but small enough to hide the taped edges of the plastic.

For our LED setup, we're still using the classic "LED Throwie" arrangement-- an ultrabright LED is attached directly to the leads of a CR2032 lithium coin cell. When the bright LED is pointed at the edge of the plastic, it travels through cleanly but lights up the areas that we've scratched.

3 mm clear lens LEDs are ideal for this since they work well with the plastic thickness; they transmit lots of light through the edge of the plastic.

We recently did some throwie analysis which suggests that if you're going to use red, yellow or yellow-green LEDs, you may want to add a resistor to the circuit in order to extend the battery life. Blue, white, and pure green LEDs work very well driven straight off of the CR2032.

Another way to save the battery is to deliver the card with an "on" switch. A piece of freezer paper can be folded so that the slick side goes between the lead of the LED and the side of the battery. It can be pulled out by the recipient when they get it.

That's it!

If you make your own edge-lit cards, we'd love to see pictures in the Evil Mad Science Auxiliary!