3 Piece Solar Powered LED

This is an easy beginner project for anyone wanting to combine LED's and Solar cells. It can be a decorative piece you wear as earrings, pin, or necklace. Or just a fun thing to hang in a window. I'll link to components you can buy for the project, but upcycled solar cells and LED's will work great too. So lets begin!

Step 1: What You'll Need.

Here's what you'll need.

Soldering iron
Solder
Diagonal cutters

Helping hands are optional. They sure do make things easier.

Step 2: Schematic and Parts

For all you soldering geeks out there, here's the schematic. This circuit is super simple. It is 3 components soldered together in parallel. From left to right we have:

1. One amorphous silicone solar cell. Amorphous silicon solar cells have a different band gap than crystalline silicon and work best with visible wavelengths of light. I normally use an AM-1417CA solar cell from Digikey, but any solar cell with four or more elements upcycled from a cheap calculator should work just as well. The only geeky requirement is the upcycled solar cell needs an open circuit voltage of at least 2.7 volts in direct sunlight. Whip out your multimeter when the cell is in a sunny window to find this out.
2. One super high brightness LED. Ours is a surface mount blue LED because it makes the whole project look so darn cute. But any high brightness LED (like this one) will work just as well.
3. One 2.7 volt super capacitor, larger than 1/2 Farad. With this beauty fully charged the project will stay lit for several hours in pitch black. Bigger capacitors will extend running time but also extend charging time, whereas smaller capacitors will do the reverse.

Step 3: Solder It Together.

Solder it together exactly how the schematic shows. Positives to each other, negatives to each other. The final layout is up to you, our picture shows only one possibility. The LED could be on top, or on the bottom, etc. Experiment to find what you like.

Some of you may already know how to tell the difference between positive and negative, so you can skip the rest of this. The rest of you may be wondering, how DO you tell the difference?

1. For solar cells, they usually come with two wires attached, one red and one black. Red is always positive, black is always negative. We removed them from our solar cell to make the project more compact. If you do this I'd recommend writing a "+" and "-" on the back so you don't get confused.
2. Don't be afraid to try a surface mount LED, this is the perfect project to make the leap and I think makes the finished project more attractive. On surface mount LED's, the positive side should be marked. This may be hard to see without a magnifying glass, they are so darn small. In our picture you can see a green T on the back of the LED. The top of the T is the positive side, the base of the T points to the negative. The specs for your LED should say how the manufacturer has labeled them, there isn't a universal convention yet. However for through hole LED's, there are standards. The + side is the longer lead, - is the shorter lead. If the leads on your LED have been trimmed, you can also tell which is which because there is a flat on the base of the LED. The flat is on the negative lead.
3. Capacitors are easier to tell their polarity. They usually have a big minus sign printed on the side to mark the negative terminal. You can see it in the picture on the gray band. And capacitors have the same rule as through hole LED's, the longer lead is the positive.

Step 4: Enjoy!

We used one of the extra capacitor leads to make a loop for hanging, making it the perfect geeky earring or necklace. It can also be hung in a window or from a car rear view mirror. The possibilities are endless.

The operation of the circuit is super duper simple. Straight out of the box the supercap needs conditioning, so charge it all day in a window. After that the project will fully charge in about an hour under most bright lights. Once out of the light, the glow visibly dims in the first hour after a charge, but takes many hours to completely run out of juice. If the piece is exposed to a constant light source, even florescent office lighting (the least favorite light source for most solar cells) is enough to keep the LED happily glowing all day.

Happy soldering!

Robin and Marty
Lumen Electronic Jewelry