Introduction: Auto On/Off LED Throwie
This is my version of the LED Throwie. The design isn't mine, this is just my mod of it. In the original design, the LED Throwie is on from the moment you put it together. This can be annoying if you're planning to make tons of these, or if you want to make them ahead of time but not have the battery run out. Someone has already posted a mod that adds an on/off tab to the throwie, however I've taken it one step further: in this design the throwie automatically turns on when it sticks to something, and turns off as soon as you remove it! It's just as simple to make as normal throwies, and doesn't require any extra materials either! These also work great as simple LED pinch lights. Credit goes to everythingdigital for the idea (it was mentioned in a comment), but the implementation is mine.
I'll point out that though this instructable may seem long or complicated at first, it's only because I wanted to make sure it was well documented. It is actually VERY easy and VERY quick to do. Making these throwies should be just as easy, and take no longer than the original design. Once you've done one and you understand how it works, you'll be whipping these out in no time.
From the original throwie:
Developed by the Graffiti Research Lab a division of the Eyebeam R&D OpenLab, LED Throwies are an inexpensive way to add color to any ferromagnetic surface in your neighborhood. A Throwie consists of a lithium battery, a 10mm diffused LED and a rare-earth magnet taped together. Throw it up high and in quantity to impress your friends and city officials.
Step 1: Parts and Materials
If you only intend to make a few of these, or just want to try them out, you can get all the parts you need at Radioshack. However if you plan to make a large quantity, you should buy all your parts in bulk online, as Radioshack would get kinda pricey for more than a few of these. You can go to the original throwie article (https://www.instructables.com/id/LED-Throwies) for info on where to buy the parts.
What You Need:
-5mm Diffused LED (the original uses 10mm, but these were cheap in an assorted pack at Radioshack, so I got them)
-CR2032 3V Button Cell
-3/8" x 1/16" Neodymium Disc Magnet
-Scotch Tape, or Strapping Tape (Strapping is better, but Scotch works fine)
-Double sided tape, or just about any glue
-Leftover cardboard from battery package/other packaging
Note: The magnets I used were 3/16" diameter (that's all Radioshack had at the time). They seem plenty strong for just sticking the throwie to things, however if you wanted to actually throw it at something, I think you'd want something a little stronger so that it would actually stick. I'd recommend 3/8" diameter by 1/16" thick, though you can try other sizes and see what works best. What is important is the magnet has to be smaller than the battery - half the size should work fine, two-thirds is too large. You'll see what I mean when you see how the design works.
As of March 2011, if you purchase all the parts at Radioshack, one throwie would cost you about $7 ($5.50 of that is the battery).
According to the original throwie article, if you purchase all your items bulk online, you can get the cost down to about a doller per throwie.
Step 2: Bend the LED Wire
The first step is to bend the LED's wire. You want it so that the space between the two wires is just barely wider than the width of the battery plus the magnet.
To start, test the LED by placing it so one wire touches each side of the battery. The longer wire typically goes to the positive side (top) of the battery, but as some LED's are different, if yours isn't working just flip it and try again.
Next you want to make your first bend. The wire to be bent will be the negative side. Using needle nosed pliers, make a perpendicular bend a little over 1/8" below the LED (see picture).
Now you need to bend the wire back to parallel with the other. Determine where to make the bend by placing your magnet on your battery and comparing to their width. Your next bend should put the spacing between the wires just barely thicker (think about one wire's width) than the battery and the magnet. Look at the images to see what the bend should look like.
Step 3: Make Your Separator Platform
The next step is to make a platform that will hold the LED wire away from the magnet.
To make this more clear I will describe how the final assembly will go together: the magnet will sit on the negative (bottom) side of the battery, pushed most of the way to one side. Directly opposite it will go a platform that prevents the LED's negative wire from contacting the magnet. There will be about 1/8" between the magnet and the platform. The LED wires will slip over the battery, magnet, and platform from the side of the magnet. The positive wire will simply touch the entire positive side. The negative wire will go all the way over the magnet, and sit on the platform, suspended just barely over the magnet, close but not touching. When the throwie sticks to something, the wire will be pressed into the magnet, completing the circuit. See the pictures for a better idea.
Cut a strip of cardboard from the packaging the battery came in, about 1" long to start, and about 3/8" wide.
Fold the cardboard back and forth, until you have a piece of cardboard about the thickness of the magnet (3 layers for me). Using double sided tape or glue, stick all the back and forth folds together to make one piece. Put a piece of double sided tape on the bottom of the platform to stick it to the battery.
Once the piece is glued or taped, it should be just thick enough to sit on the battery next to the magnet, and hold the LED wire less than a wire thickness above the magnet.
Note: if the LED is lighting up, before you assume your platform is too thin, try applying pressure to the very end of the negative wire, so as to lift the middle part off the magnet. This is usually all that is needed to make it work, and your platform is most likely the right size. When you perform the final wrap of tape around the assembly, the tape will apply this pressure so that the circuit remains broken.
Look at the image to understand more clearly.
Step 4: Wrap the Assembly
The final step is to wrap the entire assembly in tape. You can use either strapping tape or scotch tape for this. Strapping tape is more durable, but scotch tape worked fine for me.
Take about 2" of tape, and wrap it around the entire assembly. Pull it tightly over the end of the negative wire, until the middle of the wire no longer makes contact with the magnet and the LED stays off.
If the LED is not going off, try repositioning the magnet, or making the platform slightly thicker by adding a layer of tape. Also the space between the wires could be too little, so try adjusting the bend so that there is enough space for the wire to hang over the magnet without touching it. It may seem tricky at first, but after you get it once you'll get all of them correct right away from then on, without any adjusting needed.
That's it! Make a bunch, and go out and have a throwie party! Throw them at anything magnetic, and they should turn on as soon as they stick! Also useful as a pinch-on LED flashlight!
Disclaimer: The original design is not mine; this is just my modification of it. I take no responsibility for what you do with these.