Solar lanterns already exist, (for example, The Mpowrd Luci, found here) and they aren't too expensive, but most of them aren't very bright. I have one, and love it, but when I'm camping, I always end up using my propane lantern. I wanted something that could rival the propane lantern in brightness, or at least something my freinds and I could sit and play cards around without any trouble seeing .
So, I set out to build one myself. Besides being higher powered than the Luci, I wanted it to be similar in price, so I had to salvage as many parts for this project as possible. I was able to get the LEDs that I needed from a burnt out household LED lightbulb, and a couple of 18650 lithium ion batteries from an old laptop battery. In addition to this, I lucked out and found a solar panel from a non working solar security lamp in a dumpster.
The end result has around 400 lumens compared to the Luci's 80 lumens. This is not as bright as a propane lantern, but it is great for playing cards. The battery life is shorter than the luci, but it is around 3.5 hours which is more than enough time for a satisfying game of cards, and it cost me around $10. All in all, I'm pleased with the end result, but there is a few things I would change/may change in the future (mainly the enclosure).
To do this project, you need a basic understanding of electronics, how to use a multimeter, and some soldering skills. In this instructable, I will have detailed step by step instructions in the photos using the exact parts that I used. However, there is a ton of room for improvisation in this project as well.
If you like this instructable, please vote for it!
Also I would love to see modified versions of this in the comments!
Step 1: You Will Need
Here's a parts list:
-A "burnt out" (I guess it doesn't have to be burnt out) LED lightbulb. The one I used was a 60 watt Phillips Slimstyle. If you want to use the exact parts im using, you can find it here, however most LED bulbs will work probably work. I put "burnt out" in quotations because burnt out LED light bulbs almost always have plenty of perfectly good high powered LEDs in them! Get them away from the landfill!
-A pair (or more) of 18650 lithium batteries. You can easily salvage them from old laptop batteries (there's instructables on how to do this) or you can buy them on ebay for next to nothing.
-18650 battery holders like these, but be warned, on the the ones that I bought, the wires that came attached did not actually work. I had to solder on new ones.
-A 2s 18650 charge circuit/protection board. I used this one.
-A buck converter. I used one of these.
-A 12v Solar panel. You're kind of on your own for this one. You could maybe use a car float charger like this, but it might be kind of big. I was able to salvage mine from an old solar security light.
-A spdt switch. you could probably find one on ebay, I had one lying around.
-An enclosure of some kind. I used a quart Talenti gelato container. It's fine, but I know you can find something better!
-Scrap wood, solder, glue and hot glue, wire, etc. Pretty much the things a maker should have around at all times.
Hot glue gun
Tiny flat head screwdriver for adjusting the buck converter
Angle grinder or a dremel
Thats about it!
Step 2: Some Theory Real Quick...
If you reeeaallly want, you can skip this step, but i think its interesting!
When I started this project, I had some understanding of circuitry, but I didn't know much about LED's or how they worked, but through a lot of research, here's the info that I've gathered:
(Oh and to any LED "veterans", if any of this is wrong, please correct me in the comments.)
1. LED's have something called a "forward voltage". This is the "ideal" voltage that an LED will take. It is generally between 2 and 8 volts, but depends on which type of LED that you are using. Also, this voltage can differ slightly between two LED's of the same type and due to atmospheric conditions like temperature.
2. At an LED's ideal voltage, the LED will let an ideal current through it. For example, a 1 watt LED with a forward voltage of 3.2v will let around .313 Amps or 313mA through it. This is calculated by the power equation: Power=Current*Voltage. In this case the power is 1 watt and the voltage is 3.2V so the current must be .313A.
3. Small changes in voltage around an LED's forward voltage, result in a large change in current. This is dangerous for LED's as they can be burned out quite easily by too much current. This, combined with the fact that LED's each have a different forward voltage, can result in some frusterating times in designing LED arrays. Most people get around this by either:
a.) using a resistor in series with each LED or
b.) using something called a "constant current driver" and wiring the LED's in series(This is the option used for most high powered arrays, as resistors tend to burn up when a lot of current is put through them.) This type of regulator doesn't let more than a set amount of current through it ever, so that LEDs that have a higher voltage being applied to them than their forward voltage wont take on extra current.
In this instructable, I will be doing NIETHER, because im a REBEL!
(or maybe just lazy)
Step 3: LED Bulb Modification
When LED light bulbs "burn out", it is usually caused by one of the components on the PCB going bad. If it is an LED that is burned out, its usually just one, which means that The rest of them are perfectly fine and there for the taking!
We need to take apart the LED bulb. This takes some trial and error, but for my specific bulb, I ripped off the bottom with some pliers and pried the casing apart with a screwdriver.
In LED bulbs, the LEDs are conveniently packaged and soldered onto a board for you already. However they are wired in series, which is good in some cases but not in ours. This is because when LEDs are in series, their forward voltage adds. In the case of the light bulb im using, there is 30 LEDs. If each LED has a forward voltage of 3V, then the array would need around 96V to light up. Meanwhile, our poor little 18650 batteries in series can only supply around 7.2V. (Using boost convertors, you could potentially do it, but I digress...)
Long story short, we need to cut the traces between each LED wired in series and wire them in parallel. Use the dremel (or angle grinder) to cut the traces completely and the sandpaper to expose the traces for soldering. Be careful not to nick the LEDs when you're cutting.
After the traces are cut and sanded, solder one wire to each of the positive traces of the LEDs and another to each of the negative traces.
You can follow along in the pictures if you are using my exact bulb.
After you are done, MAKE SURE there is no shorts between the two parallel buses using your multimeter (infinite resistance).
Note: Technically, You are not "supposed" to wire LEDs in parallel without a resistor for each LED. This is especially the case when you're using a constant current regulator. It has to do with the stuff mentioned in the previous step. Basically, its because the LED with the lowest forward voltage will hog all the current and burn itself out, then the current that the burned out LED was using will be distributed to the LED with the next lowest forward voltage, which will cause that LED to burn out, and so on. However, because we are using a voltage regulated power supply instead of a current regulated one, if one LED burns out, each of the other LED's will have the exact same voltage and therefore the exact same current. Therefore the worst thing that could happen is you burn out one of the LEDs you have in your array (which, may I remind you, was free anyway).
Step 4: Wiring Up Batteries to the Charge/Protection Board
For this step you can follow along in my photos or just look at the schematic in the introduction. Basically you need to solder the negative of one battery holder and the positive of the other battery holder together in the spot on the board marked "BM". After that, just solder the other two wires from the battery holders to the B- and B+ spots on the board.
You will also need to solder the connector for the input and output voltage wires.
Note: Make sure you do this without the batteries installed in the battery holders! I accidentally let out some magic smoke the first time i ever soldered up one of these boards due to an exposed wire touching the wrong place!
After you get the battery holders soldered up, you can put in the batteries and test the output voltage of the board using your multimeter. (should be around 8V if batteries are charged.)
Note: In the specific board I'm using for this, you might have to "activate" it by hooking the charge cables briefly up to a DC voltage source of some kind (like a 9V or 12V AC adapter) while the batteries are installed.
Step 5: Finding the LED's Forward Voltage
My specific bulb is rated at 10.5 watts. However, I am only using half of the LEDs, which means I should only have half of the power consumption (around 5 watts). This information is vital to finding the forward voltage. It means that the voltage used to power my LEDs multiplied by the current the LEDs are using should never be greater than 5!
To do this, you will need to solder wires to the buck converter's four terminals. With the batteries charged and installed in the holders, connect the positive and negative outputs of the charging board to the positive and negative inputs on the buck converter. (don't solder them together yet, this is just for testing.) Measure the voltage output of the buck converter with the multimeter and using the flat head screwdriver, adjust the potentiometer on the buck converter until the output voltage is around 2.5V. This will be our starting point.
Now connect the parallel array of LEDs to the outputs of the buck converter. Turn the potentiometer until the LEDs light up. Note the voltage at which this happens. At this point, wire up the multimeter in series with the LEDs and measure the current they draw. Now multiply the voltage by the current. If the product is a lot under the rated power consumption for the LEDs, you can try turning up the voltage a bit more and re-measuring. Keep doing this until you have the brightness you desire (but dont get greedy or else you will have burned out LEDs).
I found the forward voltage for the LEDs that I was using to be around 3.1V.
Step 6: Finishing It Up
Now that you have the buck converter properly adjusted, solder the outputs of it to the LED array (checking polarity of course). Next, solder (one of) the ground wire(s) of the charge board to the middle terminal of the spdt switch. At this point, solder the negative input of the buck converter to one of the outer terminals of the switch and solder the negative terminal of the solar panel to the other outer terminal on the switch. Finally, solder the positive wire of the solar panel to the positive input of the charge board and solder the positive output of the charge board to the positive input on the buck converter.
If you get confused, you can refer back to my schematic the intro. Your circuit is complete!
Now just find a way to package it up all nicely and diffuse the light coming off the LEDs. I used a Talenti Gelato container that I sanded the outside of, to make it translucent. However it is a bit top heavy and I'm not totally satisfied with it. Let me know what you come up with!
Thanks for Reading! If you like my instructable, don't forget to vote for it!