Instructables

Small solar rechargeable led flash light with no solar panel/cell. (uses the led to charge!)

Picture of Small solar rechargeable led flash light with no solar panel/cell. (uses the led to charge!)
We see leds every where. what do they do? they light up. well yeah thats their main purpose but not a lot of people know that if you expose them to sunlight it can
produce a small amount of voltage. So in a way it can act as a solar panel. The flashlight I made uses one 10mm white led. The led charges the battery and with a flick of
a switch it allows the led to emit light. It is such a small circuit you can fit it into two pop bottle caps. For this small build I used a 10mm white led, small nickel battery
(20mah size of a super cap. good runtime for one led) a blocking diode and a spdt switch.
I will explain how all the componets go hand in hand in the 1st step.
if you want to see a video of this working please copy and paste this ;link into your browser. I didnt feel like uploading it to youtube just to emmbed it. its such a short
video so I hope you dont mind. here is the link https://www.facebook.com/photo.php?v=10200431925699995¬if_t=video_processed





In the comments section you will see there was a bit of confusion. I take full blame for that confusion. When I first measured the voltage output of the led it read as 1.2 volts. That was the foundation of the confusion. However I was measuring the voltage output when the sun was setting and being blocked by clouds and trees. I meaured it again today in absolute sunlight. And it read as high as 2.7 volts. The battery charged to 2.7. Now that is enough to charge the battery and enough to run the led, however it doesnt live up to its full potential. The led works best at 3 volts. I am sorry for the confusion and I thank everyone who commented and got me interested in taking it apart and re-testing the voltage. I will be making a newer better one in the future. The more leds you use in series the higher voltage output you will get. I plan on making a new instructable in a few days. Sorry for the confusion!
 
Remove these adsRemove these ads by Signing Up
Himers7 months ago
How long does it work? And how long does it take to charge?
yiego979 months ago

hi,,,can you help me to make a joule thief with solar charger with night detector...so it will turn on at night time,,thanks

i want to use it in the farm..thank you

out-of-the-box (author)  yiego979 months ago
Sure! Sounds pretty fun. I'll let you know when!
yajat12310 months ago

thanx man really helpful

ivanjacob1 year ago
Never knew this, awesome!
Macattacku1 year ago
I agree with him. U cant possible be charging this battery. This thing wouldnt charge if i put a 300 watt light bulb up to it. U said that 1.5 volts charges a 3.6 volt battery. Its impossible math and science says so. And i dont think u know what "trickle" charge means. A trickle charge still has to be above the battery voltage to charge. 3.8 or 3.7 would be a trickle charge. 1.5 volts - 0.5 volts from you diode = volt. Not even a 3rd of what you would need. No matter how you put it that led isnt charging that battery. If that battery is getting "charged" then there is other factors at work here.
out-of-the-box (author)  Macattacku1 year ago
hey. i just found out the problem as to why you think it doesnt work. when i tested the voltage output of the led the sun was setting giving me 1.2 volts, i just tested the led in absolute direct light and got 2.7 volts. Heres why i got 1.2, I was standing in my door way testing the led through glass (some times makes a difference) and the sun was setting in the distance. trees and a few clouds were messing with the results. Sorry for the misunder standing! The battery charges to 2.7-2.8 volts and still powers the led (not to its fullest potential). I will be making a better one of these using a double a battery and a few inductors and what not. Again sorry for the misunderstanding and thank you for your feedback.
Well 2.8 i wouldnt call a full or a good charge but i suppose it couple power an led for a few minutes. And i dont see the point of an inductor in a new design.
out-of-the-box (author)  Macattacku1 year ago
The new desing uses a 1.2 volt battery. Which by itself cant drive an led. Adding an inductor or a circuit around an inductor will drive that led. Its used everywhere. Solar yard lamps have one double a battery and an inductor.
I think rather than the yard light design simply make a joule theif. Its 1 transistor, a 1k resistor, and a toroid. It can driv an led on voltages down to 0.6 volts. Look it up.
out-of-the-box (author)  Macattacku1 year ago
I already began the design for using the inductor. I made a few jouel theifs before. I just want to try the inductor. Could make both http://www.youtube.com/watch?v=fcTwhqfUkxk&feature=c4-overview&list=UU4CMvhUs1LWVNhkUQmbldWg
That's more reasonable. However, if you have a meter handy, check the current flow into the battery during charging. If you use a 20mah battery, and you put back in about 2 to 3 hundred micro amps, you are talking about a VERY long time to charge back up. Days upon days in direct sunlight. And that's getting you to just barely the conduction voltage of that led (some conduct as low as 2.5v). You are only probably getting a 25% charge at its best.

I would recommend using at least two in series. Get a small double pole double throw to replace that single switch. Wire it so one pole is charging, while the other has the leds in parallel. Then you'll have something.
out-of-the-box (author)  efahrenholz1 year ago
I didnt bother measuring the current because I knew it would be very little. The idea behind this is to leave it in a sunny, weatherproof area until you need it. I already have a new one in the works. i have 3 leds in series in parallel with 3 more. The voltage out for those are close to 5 volts. im making it operate on one battery with the help of a few inductors
Unfortunately, I would have to see you perform this. You can't charge greater than the output voltage of the LED, which you measured out to be far less than the conduction voltage necessary to light up . Current will not flow into a voltage source higher than its own.

You would need at least two or three in series to charge the battery, then switch to them to parallel in order to work at the batteries voltage.
out-of-the-box (author)  efahrenholz1 year ago
hey. i just found out the problem as to why you think it doesnt work. when i tested the voltage output of the led the sun was setting giving me 1.2 volts, i just tested the led in absolute direct light and got 2.7 volts. Heres why i got 1.2, I was standing in my door way testing the led through glass (some times makes a difference) and the sun was setting in the distance. trees and a few clouds were messing with the results. Sorry for the misunder standing! The battery charges to 2.7-2.8 volts and still powers the led (not to its fullest potential). I will be making a better one of these using a double a battery and a few inductors and what not. Again sorry for the misunderstanding and thank you for your feedback.
out-of-the-box (author)  efahrenholz1 year ago
hey. i just found out the problem as to why you think it doesnt work. when i tested the voltage output of the led the sun was setting giving me 1.2 volts, i just tested the led in absolute direct light and got 2.7 volts. Heres why i got 1.2, I was standing in my door way testing the led through glass (some times makes a difference) and the sun was setting in the distance. trees and a few clouds were messing with the results. Sorry for the misunder standing! The battery charges to 2.7-2.8 volts and still powers the led (not to its fullest potential). I will be making a better one of these using a double a battery and a few inductors and what not. Again sorry for the misunderstanding and thank you for your feedback.
Also, you are witnessing the battery voltage climbing up due to an effect called hysteresis. The battery voltage potential has a plus or minus factor. When discharged, it will lower. But if you meter the battery after unloading, the voltage will climb as if it were being recharged. You keep doing this enough, and the battery voltage will fall rapidly under load, to the point of failure.
And it's not a theory, it's a law.
I'm sorry but this is flawed in so many ways. The most important problem is that you are charging a 3.6 volt source with a 1.5 volt source. The second problem is that white leds run on around 3 volts typically. So while it works in theory now, it will eventually run down below led conduction voltage. The third problem is trickle charging. Leds being used in reverse only put out micro amps of current. Any battery/capacitor will self drain almost as fast as it could charge, even if you supplied the correct charging voltage.
out-of-the-box (author)  efahrenholz1 year ago
And nicke based batteries are found in almost every hand crank flashlight/radio. you can crank the motor slow enough to output a voltage lower than rated on the battery and it will charge just fine. Yeah there will be unwanted discharge as there is in anything that uses batteries.
out-of-the-box (author)  efahrenholz1 year ago
I had it set in the sun a little bit ago and it charged just fine. Nickle based batteries can trickle charge just fine. I know that the led doesnt output a lot of current thats why the charging time is very slow but it still does charge. The diode helps direct the leds current.. If you want I can post a video of this actualy working. from drain to charge to use. Theory isn't always reality. Once the battery drops below say 2.5 volts the led will stop working or even get very dim, however that could be avoided. Say using a few white leds in series also in parallel with a few more to increase both voltage and amperage. you would see a better performance. this flash light isnt for ideal use. altough it fery well could be. The main purpose of this is to show that leds do more than what most think. I will design a better circuit in a few weeks and post it. Add more leds, inductors and a better battery. I dont see this as flawed... However I respect your opinion and feed back. If it was flawed it wouldnt charge or work. (im not saying it wont stop working, cant see into the future). Ill be working on another one. a bigger better one.
lgarza31 year ago
very good instructable
out-of-the-box (author)  lgarza31 year ago
thank you