After seeing the article "the eternal flame" in MAKE: Magazine, my first thought upos seeing the project was that it's not really "eternal" because you can't change the battery if it's glued shut! After reading, I saw that it used teflon tape to make it watertight and re-openable, but not before a project idea came about.
Several days later, I completed my new wirelessly charged, rechargable indestructable LED lantern. While it's not nearly as cheap or easy as the version in make, this one can stand some pretty tough abuse, as well as costing less after a few uses, depending on where you buy batteries. Don't forget to vote for me in the lamps and lighting and UP! Contests. If I win the UP! I will update with an even tougher 3d printed lamp, and more tests.
For a video and alternative guide, go to http://www.designnews.com/author.asp?section_id=1362&doc_id=245321 where this project is also published, along with a video.
Step 1: Materials
To make this you will need:
1- rechargable AA battery (per lantern)
1-1k resistor (per lantern)
1-High-power white LED (per lantern)
3-other-colored low power LED (3 per lantern)
1-AA battery case (per lantern)
1-4401 transistor (per lantern)
1-low inductance, center-tapped inductor (per lantern), (watch a youtube video on a "joule theif, this uses a similar circuit)
1-3.6V rechargable battery
1-High-power white LED
The first option usually lasts much longer or is cheaper (depending on the battery). But, it is more complex. Both options need:
2-1” PVC slip cap
1-1” diameter pipe, 3” long (cut from longer piece)
1 -470 ohm resistor
~20 x 2n3906 transistors (exact number depends on primary coil) (Jameco.com, part# 38375)
15 x feet magnet wire (7.5 or so per lantern)
1 x 12V power supply
Step 2: So... How Does It Work?
The sender circuit rapidly changes the power going to the coil, pulsing it thousands of times a second. This causes a strong, varying magnetic field to surround the coil, which also rapidly pulses. When the receiver enters this magnetic field, it pulls on the electrons in the coil, causing them to move into the rest of the circuit. (A permanent magnet will not do this unless physically moved about the coil, which is how generators make power.
Once the electrons are moving, current is flowing through the circuit, and once it is limited to a single direction and evened out with a capacitor, it charges the battery and powers the lights. This basic principle is actually how almost all wireless transfers work (though most use a single wire instead of a coil). Such transmitters and recievers cause current to flow without having anywhere to go. To do this, they oscillate so fast that before the elecrons going into the wire reach the end, the current reverses and they flow back. This circuit is too low frequency for that to work, and thus must use a coil. The positive of using coils is that much more power can be passed through them, and thus a stronger field can be produced. The downside is that the range is significantly reduced, and they're much less efficient. The coil method is most commonly used is from ac power supplies, which use two coils and a magnetic medium between the two to increase the efficiency of the transfer. This allows them to use a much lower frequency of oscillation.
Step 3: Circuits
There are two circuit for this, the sender and reciever.
For the sender, make sure to parallel several transistors (about 10), and run it for several minutes. If they get hot, turn it off, and replace ALL the transistors, and add a few extras. If you can get it to work, a high-power transistor would be better, but none of those that I used would turn fully on and off.
Make sure to test the circuits together before you put them in the case. The main LED should turn off, and the others on, when the coils are nearby. When separated, the main LED should light, and the others turn off.
The alternative circuit (for a 3.6V battery) is the third picture. Ignore the switch if you have no way of adding it, because it makes the circuit charge much faster, but it will work without it. The sender is the same for both.
Step 4: Case It.
Now, drill a hole in one of the PVC endcaps the same size as your white LED, and surround it with waterproof glue , then insert the LED. Glue the coil in place around the LED, with the turns perpendicular to the case. Make sure the rest of the circuit will fit in the piece of tube you cut, and if not, extend it until it does. Then glue the undrilled cap to the pipe with PVC cement (make sure to make a quarter turn to fully seal and secure it), and let it dry. For total indestructability, insert the circuit, and fill the rest of the tube with nonconductive epoxy before gluing on the end with the LED. Do not allow the epoxy to dry before you glue on the second end cap, or you run the risk of snapping off the LED leads when you turn the cap. to seal the glue.
If you don't use epoxy, just make sure the circuit is secured to the inside of the pipe, and put ~1" wires between the circuit and the LED, to avoid breaking off the leads when you turn it.
Test it one last time, and if it works, you're done! If not, good luck fixing this. Or breaking it. Or mildly damaging it.
Step 5: Break It. Destroy It. Vaporize It.
After a few days of playing with this, it turned out far tougher than I expected. An unepoxied light took repeated punts and throws into walls, as well as kicking, drowning, stomping, whacking, and being shot from an air cannon. It finally broke when a wire pulled loose from being shot at a cinder block at about three hundred miles per hour (and the case broke a little, too.
The epoxied one took being fired from an air cannon into a cinder block (LED at the back, not striking the block directly), being pressure-washed, hammered, dropped from a tree, launched into a lake and fished out half an hour later, an even a trip through the laundry. All with hardly a scratch. If finially gave up after a few whacks with a sledgehammer (it was a small sledgehammer, okay!)
I am yet to test the toddler-proof-ness or bullet-proof-ness of either. Though I doubt both, for both.
What other crazy electronics-tortures should be attempted?