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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

<p>I'm in the process of making one, I took a super cap from a micro chargers car, and I have a solar cell from a calculator. Also I added a blocking diode, and once the cap is done charging, I will try an LED</p>
<p>Did you get it working?</p>
When I tried mine it only worked for a few minutes, so I just revamped the project. it has a light detection circuit, a 600 mah aaa battery, joule thief, and can be charged through solar. it lasts the entire day, and much longer. And I can wear it as a necklace.
<p>Yeah, the capacitor has to be pretty high value for it to last any length of time, 0.5F (not uF). LED's often have a very narrow voltage range where they are high enough intensity we can see them. Adding the Jewel Thief was a great way to make it work over a wider voltage range. Thanks for sharing!</p>
<p>Sweet, post a picture and/or let us know how it goes. </p>
ok! and I will be using a joule thief so it can light up brighter longer.
Thanks for the great explanations. Have you tried this with any sort of light sensor switch?
<p>No, we haven't tried this circuit with a light sensor switch. That would be cool, did you have a use in mind?</p>
<p>OK <br>i can't get the capacitor of 2.7 v <br>i have a 16v 100f capacitor , can i use it ???<br>and if i can what is your advises ?<br>please help me as this will be my first project :D</p>
<p>Thank you for the picture. That capacitor has a maximum voltage rating of 16 volts. As that is higher than 2.7 volts, voltage won't cause a problem. The capacitance of that capacitor is a 100uF. Which is 1/5,000th the size of the capacitor used in this Instructable. The project will work if you use a 100uF capacitor, but it will only hold a charge for a few seconds.</p>
<p>then i can't use it :(</p>
<p>Nice simple idea, I like the use of the surface mounted LED to keep the project small. I'd like to make this as a necklace with the same configuration as shown, but with the solar cell horizontal across the top, and a loop on both leads of the capacitor. Do you think the leads of the capacitor are long enough for this?</p>
<p>Yes! The leads on the capacitor are about as long as the solar cell, so it should work. Post a picture when its done!</p>
<p>Looks awesome! Hey, just wondering how do you guys get by without using a diode to protect the photocells? Can't current from the supercapacitor damage them?</p><p>Thanks!</p>
<p>The reverse current blocking diode is most solar arrays isn't to protect the solar array from damage. The diode is there to keep the solar array from discharging the battery/super-cap the solar array charged up during the day. In this case, I've tested the reverse leakage current of the amorphous silicon solar cell at 2.7 volts and it's less than one micro-amp. One micro-amp is low enough that it doesn't significantly effect the run time. </p>
Hi! Is this ok if it is exposed to the weather? I'd like to use this in an installation that will be outside. Do I need to enclose it?
If it got wet it might stop working till it dried out. We recommend encasing it in clear epoxy, we've done this many times with great results. The epoxy won't hurt any of the components and it will still work beautifully. Good luck, post a picture if you get one.
Thanks for the tip, lumenjewelry! I have a few more questions :) My project is not for jewelry, but for a larger, <a href="http://cdr-artifacts.tumblr.com" rel="nofollow">outdoor installation</a>. For my application, I am integrating the components into tops of plastic bottle caps. They work great, but discharge quickly/stopped working. From the photo attached, can you tell me what I am doing wrong? I believe I have wired this in the correct sequence, but does the wiring sequence matter? And this will be outdoors, so you're saying I should encase in clear epoxy. Could I do silicone? I've twisted the wires pretty tightly together. Should I still solder? Thanks again!
Looks correctly wired. For longer charge life a larger super-capacitor will need to be used. (or several in parallel for testing) Looks like you can get suitable <a href="http://www.digikey.com/product-detail/en/BCAP0005%20P270%20T01/1182-1008-ND/3079272" rel="nofollow">5 Farad</a> or <a href="http://www.digikey.com/product-detail/en/BCAP0010%20P270%20T01/1182-1009-ND/3079273" rel="nofollow">10 Farad</a> caps without too much of a premium.&nbsp; For even more run time a <a href="http://www.digikey.com/product-detail/en/ML-621S%2FDN/P044-ND/278835" rel="nofollow">surface mount rechargeable lithium</a> could be used, but they die if discharged to zero volts.&nbsp; Either way, the charge time will also get much longer.&nbsp;<br> <br> Silicone will work as encapsulation as long as it's &quot;no smell&quot; and or &quot;low VOC&quot; type.&nbsp; Clear latex or polyurethane paint will also work, but they're not as tough as epoxy.&nbsp; If you solder the joints, be sure to clean ALL the flux off before sealing the parts.<br> <br> As tightly as those parts are twisted, I don't think they need to be soldered.&nbsp; I would recommend encapsulating the exposed metal leads to prevent corrosion between the twisted wires.
Now if only we could make it pulse in and out... <br> <br>Freak out a few neighbors.
Kiteman, good idea. <br> <br>Rockyt, the best analogy I can come up with is think of a bucket with a small hole in the bottom. The bucket is the capacitor, the hole is the LED. The solar cell pours water (current) into the bucket (capacitor). If the solar cell pours more in than drains out through the LED, the bucket fills up. If the solar cell stops pouring current into the bucket, the bucket will eventually drain out through the LED. Make sense?
How will the capacitor charge if it is dumping everything to the led WHILE charging? Does some of the charge make it back to the solar panel? No diode needed?
If you're going to hang it in the window, I would suggest facing the cell and LED in opposite directions - cell towards the light, LED towards the room. <br>

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Bio: We’re life-long tinkerers, siblings, and fourth generation engineers. We’re not quite sure which of us had the idea of putting LED’s on ... More »
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